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25 September 2015

CMS Phase II Upgrade Scope DocumentCMS Collaboration

Submitted to the CERN LHC Committee and the CERN Experiments Resource Review Board

September 2015

The High-Luminosity LHC (HL-LHC) has been identified as the highest priority pro-gram in High Energy Physics by both the European Strategy Group and the US Parti-cle Physics Project Prioritization Panel. To fulfil the full potential of this program, whichincludes the study of the nature of the Higgs boson, the investigation of the proper-ties of any newly discovered particles in the upcoming LHC runs, and the extensionof the mass reach for further discoveries, an integrated luminosity of 3000 fb−1 willhave to be accumulated by the end of the program. In preparation for operation at theHL-LHC, CMS has documented the necessary upgrades and their expected costs in aTechnical Proposal submitted to the CERN LHC Committee (LHCC) in mid-2015. The“Scope Document” provides additional information to assist the LHCC and the CERNResource Review Board (RRB) in their review of the CMS upgrade. The documentcommences with a summary of the process followed to develop the scope of the “ref-erence” design described in the Technical Proposal. The upgrades of reduced scopethat have been explored, along with two representative detector configurations thatlower the cost, from the estimate of 265 MCHF for the reference design to 242 MCHFand 208 MCHF, are then presented. The performance of all three configurations iscompared, along with the capability of the reference design to operate effectively ata potentially increased instantaneous luminosity, as recently introduced in projectionsfor the HL-LHC. It is shown that the CMS reference upgrade will ensure the successof the full scientific program at the HL-LHC, providing also the opportunity to exploitthe highest luminosity potential of the accelerator. An alternate configuration with lim-ited reduction of scope should sustain good performance, but would limit the ability toprofit from the highest luminosities for some fundamental and difficult measurements.Large scope reductions, as considered in the third configuration, will irrevocably haveadverse effect on major parts of the physics program.

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Contents1 Purpose and outline of the Scope Document . . . . . . . . . . . . . . . . . . . . . 12 The HL-LHC physics program and experimental challenges . . . . . . . . . . . . 2

2.1 The physics opportunities at the HL-LHC . . . . . . . . . . . . . . . . . . 22.2 HL-LHC beam conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Overview of the CMS Phase-II upgrades . . . . . . . . . . . . . . . . . . . . . . . 103.1 Performance considerations for the Phase II upgrades . . . . . . . . . . . 103.2 Design optimization of the upgrade elements . . . . . . . . . . . . . . . . 123.3 Upgrade cost estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4 Upgrade configurations of reduced cost . . . . . . . . . . . . . . . . . . . . . . . . 184.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.2 Options for cost reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.3 Upgrade configuration of 242 MCHF cost . . . . . . . . . . . . . . . . . . 204.4 Upgrade configuration of 208 MCHF cost . . . . . . . . . . . . . . . . . . 20

5 Comparative Performance studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.1 Performance implications of reduced cost configurations . . . . . . . . . 225.2 Combined performance studies . . . . . . . . . . . . . . . . . . . . . . . . 315.3 Phase II detector performance at pileup 140 and pileup 200 . . . . . . . . 32

6 Project organization and planning . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.1 Phase II organization in CMS . . . . . . . . . . . . . . . . . . . . . . . . . . 396.2 Project timeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3 R&D program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406.4 Cost profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42A Project planning and cost estimates . . . . . . . . . . . . . . . . . . . . . . . . . . 44

A.1 Tracker planning and cost estimate . . . . . . . . . . . . . . . . . . . . . . 44A.2 Barrel calorimeter planning and cost estimate . . . . . . . . . . . . . . . . 44A.3 Endcap calorimeter planning and cost estimate . . . . . . . . . . . . . . . 46A.4 Muon systems planning and cost estimates . . . . . . . . . . . . . . . . . 47A.5 Beam radiation instrumentation and luminosity planning and cost esti-

mate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49A.6 Trigger/DAQ planning and cost estimate . . . . . . . . . . . . . . . . . . 50A.7 Infrastructure upgrades and logistic of work cost estimate . . . . . . . . . 51

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1 Purpose and outline of the Scope DocumentThe Technical Proposal [1] for the Phase II upgrade of the CMS detector identifies the sub-systems that will either not survive the harsh radiation environment of the HL-LHC or notfunction efficiently because of the increased data rates. It also presents conceptual designs andtechnical solutions of upgrades that will address these issues in order to ensure that CMS fullyexploits the physics potential of the HL-LHC. In defining the scope and extent of these up-grades, the design choices were made based on considerations of both performance and cost.Nevertheless, the design and cost optimization process will continue with the preparation anddocumentation of technical designs on a time scale of two years. By the end of this period,substantial progress will have been made to finalize the techniques and narrow down the un-certainties in the cost estimates. Further adjustment of the detector configurations will also bepossible, based on new performance studies as well as on the first physics results obtained at13 TeV in Run II.

The estimated CERN CORE cost for the Phase II “reference” upgrades is 265 MCHF. As agreedwith CERN and the Resource Review Board, the CMS Phase II Upgrade “Scope Document”evaluates two detector upgrades of reduced scope targeting lower costs in the range of 235MCHF and 200 MCHF, respectively. Representative configurations for these two scenarios arederived from “descoping” or “downgrading” of upgrade elements of the reference design, inan attempt to preserve to the greatest possible extent the physics reach of the experiment.

The first part of the Scope Document reviews the scientific motivation of the HL-LHC researchprogram and discusses the requirements that the physics processes impose on several sub-systems of the experiment. This is illustrated with a few representative performance studiesthat are presented in the Technical Proposal. Although it is not discussed in this document, itshould be noted that the Phase II upgrades would also greatly benefit a program of Heavy Ionphysics during the HL-LHC era. The accelerator high-luminosity beam conditions that will en-able CMS to achieve the physics goals in the next two decades are then presented. The secondpart of the document summarizes the experimental challenges that arise from this operatingenvironment. All the resulting upgrades are briefly described, outlining the criteria used todevelop performant and cost-effective designs. A discussion of the costing methodology and apresentation of the cost estimates for each subproject of the upgrade conclude this part.

The third part of the document presents the considerations used to define the “reduced-scopedetector configurations that correspond to the two scenarios of lower total cost. The cost re-ductions resulting from each descope are then estimated. The comparison of the performanceof the two configurations with that of the reference design is carried out both at the level ofphysics objects reconstruction as well as for the full physics reach, as illustrated with somerepresentative benchmark signals. The performance is presented for the baseline HL-LHC lu-minosity of 5× 1034cm−2s−1. Also presented is the ability of the reference design to exploit7.5× 1034cm−2s−1, which is now believed to be the “ultimate” luminosity that the LHC canachieve.

The document ends with a discussion of the project organization and planning, and concludingremarks.

2 2 The HL-LHC physics program and experimental challenges

2 The HL-LHC physics program and experimental challenges2.1 The physics opportunities at the HL-LHC

The goal of the CMS experiment at the Large Hadron Collider (LHC) is to answer fundamentalquestions in particle physics. What is the origin of elementary particle masses? What is thenature of the dark matter we observe in the Universe? Are the fundamental forces unified?How does QCD behave under extreme conditions? What physics causes the dominance ofmatter over antimatter? In the first major physics run in 2011 and 2012, at center-of-mass en-ergies of 7 and 8 TeV, the collider reached a peak luminosity of 7.7× 1033cm−2s−1, more than75% of its design luminosity, and delivered an integrated luminosity of ∼25 fb−1 to each of itstwo general purpose experiments, ATLAS and CMS. These data have yielded a vast numberof physics results, summarized by the CMS collaboration in more than 400 publications. Themajor achievement of the run, and a milestone in humankind’s understanding of nature, hasbeen the observation in 2012 of a new particle of mass ∼125 GeV by the ATLAS and CMScollaborations [2, 3].

In addition to discovering the new particle, CMS was able to show that it behaved like a stan-dard model (SM) Higgs boson. Studies of the properties of this new particle decay have pro-vided compelling evidence that it is indeed of spin and parity 0+ establishing it as a Higgsboson [4]. By using a combination of theory predictions for the decays and production, thecouplings of the new boson to the known particles have been determined and are shown inFigure 1 to follow a mass dependence characteristic of the Higgs field.

The SM does not provide answers to the remaining questions. Those require new physics.Although the 125 GeV Higgs boson behaves like a SM Higgs boson, measurement of its prop-erties are still not very precise. Deviations from perfect SM behavior because of its interactionwith other forms of matter, including dark matter, could be a signature of this new physics.The detailed study of the 125 GeV Higgs boson is a scientific imperative that must be pursuedto a much higher level of statistical precision than is available today.

Many searches have been undertaken with the data taken in 2011 and 2012, but they have notyet revealed evidence of new physics “beyond the standard model” (BSM) . The theory knownas supersymmetry (SUSY) contains a partner for every SM particle, including a candidate fordark matter. Since no “superpartners” have yet been observed, if they exist, they would haveto have very specific, hard-to-detect decay chains or higher masses than have been accessibleso far at the LHC. With the present LHC results, in simplified models the SUSY partners ofthe gluons, the “gluinos”, and the partners of the two lighest generations of the quarks, the“squarks”, with masses below about 1 TeV are excluded, while scenarios with 3rd generationsquarks, the sbottoms and the stops, with masses below 1 TeV are still compatible with thedata. SUSY also predicts several more Higgs-type particles. Searches for these have also beenundertaken, but so far, no additional Higgs bosons have been found.

The direct search for exotic processes and particles is another approach to discovering newphysics. An example is the direct search for dark matter in a final state with two weakly inter-acting massive particles (WIMPS), characterized by large missing transverse energy, Emiss

T . Thesensitivity of CMS in certain ranges of the dark matter particle mass, for example at very lowmass, and cross section surpasses that of other search techniques, such as direct searches for in-teractions with bulk matter and indirect evidence from WIMP annihilation with accumulationsof darkmatter in the sky. Other examples are searches for new gauge bosons with SM cou-plings, leptoquarks, and quarks with vector-like couplings. Searches for these particles havebeen carried out with data from the 2011 and 2012 LHC run but none has so far been observed.

2.1 The physics opportunities at the HL-LHC 3

New particles are expected at the TeV scale but have not yet been seen. This could mean thatthey exist at masses above the current level of sensitivity. It could also mean that they could bepresent at lower masses but their cross sections are lower than expected or their experimentalsignatures are especially difficult to observe. In either case, the sensitivity for searches of newparticles grows with increased luminosity.

The CMS physics program at the HL-LHC will build on the experience acquired and the resultsobtained from more than 300 fb−1 of integrated luminosity accumulated in the first phase ofthe LHC operation. Independent of potential discoveries in this period, the physics programwill continue the quest to answer fundamental questions in particle physics, on one hand withprecision measurements and, on the other, by direct searches for new physics.

In the rest of this section, a few examples of important physics goals achievable with 3000 fb−1

accumulated at the end of the HL-LHC program are presented. The results are based on pro-jections or simulations that take into account the improvements to the detector planned to pre-serve its capabilities at the Phase II high luminosity. More complete and detailed descriptionsare given in the Technical Proposal.

The study of the Higgs boson will continue to be central to the program and provides a pow-erful argument for higher luminosity. It will include precise measurements of its couplings toother particles, determining if it has a tensor structure, and the search for rare SM and BSMdecays. The enormous dataset will give access to nearly all of the production processes anddecays of the Higgs boson. Figure 1 shows the current CMS results and a projection for themeasurement of Higgs boson couplings in a dataset of 3000 fb−1 at 14 TeV center-of-mass en-ergy as a function of the boson or fermion masses [5, 6]. Compared to a precision of about20% on Higgs boson couplings today, percent-level precision can be reached for most couplingmeasurements.

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preserve a linear mass dependence. The dashed lines indicate the predicted dependence on theparticle mass for the SM Higgs boson.

In order to achieve the full benefit of the HL-LHC, CMS must continue to be able to reconstructat the much higher luminosity all the standard physics analysis objects with high efficiency, lowfake rate, and high resolution. Excellent electron, photon, and muon reconstruction is neededfor Higgs decays to γγ, ZZ∗ and WW∗ and to observe Higgs decays to µ+µ−. The dominant

4 2 The HL-LHC physics program and experimental challenges

decay mode is H → bb̄, which requires b-quark tagging and, consequently, continued precisionreconstruction of primary and secondary vertices. The reconstruction of τ leptons also requirestracking of charged hadrons and the measurement of electromagnetic energy as well as muonand electron reconstruction. The goal of seeing Higgs bosons produced in association with at− t̄ pair requires jet reconstruction and b-quark tagging. Because of its relatively low mass, theHiggs decay products have also low energy or transverse momentum. For efficient event selec-tion of all processes, it is therefore mandatory that the thresholds on these variables are as lowas possible at the first level of the event selection (hardware trigger). This also applies to sev-eral other SM or new physics processes. For instance, sub-dominant production mechanismsof the Higgs boson are more easily triggerable or produce event samples with better signal-to-background ratios than the dominant gluon-gluon production mode. This is especially true forH → bb̄, which is hard to trigger on alone, but can be produced in association with W or Zboson or top quarks, all of which are much easier to trigger on. All these requirements explainwhy the full capabilities of the original CMS design must be retained.

Higgs boson production through Vector Boson Fusion (VBF) is a rare process in which theproton-proton collision is transformed into the collision of a pair of massive vector bosons.These collisions are characterized by two “tagging” jets travelling in opposite directions at rel-atively small angles with respect to the colliding beams, with the signal products going intothe more central regions of the detector. Most of the tagging jets emit significant amounts ofenergy at low angles beyond the present tracker acceptance. Due to the harsher conditionsat high luminosities, an extension of the tracker coverage to higher η is needed to efficientlytag these jets. This also applies to the similar vector boson scattering processes, which areimportant measurement of the role of the Higgs boson in the electroweak symmetry break-ing. These measurements could also be sensitive to new physics through the triple-gauge cou-plings (TGCs) and quartic-gauge couplings (QCGs). In general, precision measurements ofelectroweak observables have played a key role in validating the SM and in putting constraintson BSM physics.

Higgs boson coupling to charged leptons is a crucial measurement. The coupling to electronsis too small to measure even at the HL-LHC, but the coupling to τ’s will be well-measured bythe end of Phase II. This decay in the VBF production of a Higgs boson is one of the benchmarkmeasurements used to evaluate the performance of the CMS upgrades. The coupling to thesecond-generation fermions will be probed for the first time by measuring the Higgs bosondecay exclusively to two muons. The branching fraction in the SM of only ∼10−4, can bemeasured with a precision of about 5% with 3000 fb−1. This depends on the improvement inmass resolution (40%), because the upgraded detector has less mass in the tracking region, andin efficiency (20%), because of the extended η coverage, achieved with the upgraded detectoras shown in Figure 2.

Measurements of the di-Higgs production with a very low cross section estimated to about 40fb will allow the study of the Higgs boson self-coupling. This measurement is a unique wayto fully establish the Higgs field potential. Figure 3 shows the simulated mass distributionmeasurement with the CMS upgrade for the HH → bbγγ final state including the background.The ability to measure this process in the most promising final states will largely depend onthe identification and momentum resolution performance for b-jet, photons and τ.

In a major class of SUSY models, the lightest SUSY particle will be stable and interact veryweakly with ordinary matter. This will result in events with large missing transverse energy,Emis

T , which is taken as one of the main experimental signatures of SUSY. Search strategies havebeen refined to be more sensitive to hard-to-identify configurations. For example, attention is

2.1 The physics opportunities at the HL-LHC 5

Figure 2: Di-muon mass distributions for Higgs boson events simulated with the Phase I (nom-inal and after radiation damage from exposure to 1000 fb−1 of luminosity) and Phase II de-tectors. The distributions are normalized to take the relative selection efficiency of differentdetectors and conditions into account.

being paid to “stealth” SUSY, in which new particles look very similar to SM backgrounds, andto “compressed” SUSY, where particles have very similar masses, making their decay configu-rations very hard to observe. Generic approaches to the searches have been developed that aresomewhat insensitive to the details of specific production mechanisms or decay patterns. Thesensitivity of CMS to SUSY and many other new physics signals improves with increasing lu-minosity and are important goals for the future. Figure 4 shows the reach for selected searchesfor supersymmetry for a dataset of 300 fb−1 and 3000 fb−1 at 14 TeV. The W±H+Emiss

T mea-surement is another benchmark selected to assess the CMS upgrade physics reach. It should benoted that improvements in analysis and large samples of data will likely open the possibilityfor searches for rare signals with exceptionally low background that could outperform theseprojections.

Another approach to discovering new physics is to make precision measurements of rare de-cays that are well-predicted in the SM. If new physics is present it might either enhance orsuppress the rate of these decays. An example of such a precision measurement made by CMSis the first observation of the very rare decay Bs → µ+µ−. This decay is very highly sup-pressed in the SM, but it can receive additional contributions from new physics. The observedbranching fraction is 3.0+1.0

−0.9 × 10−9, consistent with the expectations of the SM. The compan-ion decay, Bd → µ+µ−, is predicted to have a branching fraction that is a factor of 30 lowerthan the Bs. At the HL-LHC, the decay Bs → µ+µ− will become a precision measurement, thedecay Bd → µ+µ− will be established and its branching fraction will be measured with reason-able accuracy. The projected mass distribution is shown in Fig. 5. The significance of the Bd,predicted to be 2.2σ after 300 fb−1, will improve to 6.8 σ with 3000 fb−1. This measurement isonly possible with the new trigger capabilities provided by the tracker upgrade.

The large luminosity sample collected at the HL-LHC will extend the reach of the search for

6 2 The HL-LHC physics program and experimental challenges

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new heavy gauge bosons, to 6 TeV or more for SM couplings or, in the case of very narrowwidth resonances, probe regions of 0.5-1 TeV. Similarly searches for extra dimensions, compos-iteness, leptoquarks etc. can be extended in range by a few TeV. Many signatures for exoticphenomena include the production of heavy semi-stable particles that will either traverse ordecay in the CMS detector. For these searches it is also imperative to keep the detector capabil-ities at least at the present quality level.

An exciting possibility for new physics that could be within the reach of the LHC is the dis-covery of an elementary particle that explains the existence of dark matter. If it is caused by aparticle, it is definitely not a member of our present standard model catalogue. The discovery

2.1 The physics opportunities at the HL-LHC 7

Figure 5: Bs,d → µ+µ− with integrated luminosity of 300 fb−1 (left) and 3000 fb−1(right).

of supersymmetric particles that respected R-parity would likely be a big step forward in ourunderstanding of dark matter. However several more generic techniques for looking for darkmatter have been developed over the last few years. These look for an excess of events withlarge missing ET, accompanied by a single SM object such as a jet, photon, or vector boson,which could come from initial state radiation and also provides a trigger. These searches turnout to be competitive with the direct search experiments in certain regions of comparison, andprojections indicate that the high luminosity upgrade of the LHC can pursue this search belowthe level of neutrino coherent scattering, which will be a concern, and possibly a limit, for thedirect experiments [7]. For this program to be successful in CMS, it is particularly essential thatthe quality of the missing ET measurement is kept at a similar level as for the present data.

Another exciting possibility is to use the Higgs boson as a search tool for dark matter. TheHiggs boson may well be a portal connecting the standard model with other new physics sec-tors, such as the dark sector. In that case, and if the dark matter particle is relatively light,the search for dark matter in the decay of Higgs particles, via the so called “invisible decay”channel will be an important channel. A new channel proposed for a dark matter search ismono-Higgs production [8] similar to e.g. the mono-jet signature, except that the Higgs isemitted in the final state from the produced dark matter particles. For this channel the highluminosity of the HL-LHC will be essential.

In the event of a discovery during the first phase of the LHC, the large dataset of the HL-LHCwill be critical to unveil the nature of the observed new particles. This will require precisemeasurement of their properties, such as production cross sections, masses, and spin-parity. Itwill also be essential to extend the searches of other related new physics signals.

In parallel to the searches for new physics and in support of these discovery topics, many mea-surements of SM phenomena will be made at the HL-LHC. In addition to high statistics mea-surements that can provide insight into these processes, they will also help define SM “back-grounds” that must be known and well-modelled to carry out the discovery portion of the pro-gram. For example, parton distribution functions (PDFs) of the proton are crucial ingredientsof measurements at the LHC. Future Higgs boson coupling measurements will be limited byPDF uncertainties unless significant progress is made. Other precision measurements, like themeasurement of the W boson mass, the effective lepton mixing angle, and the strong couplingconstant αS, have large uncertainties from PDFs. If new physics phenomena are discovered,their characterization will also suffer from PDF uncertainties, e.g. for gluino or squark produc-

8 2 The HL-LHC physics program and experimental challenges

tion in the few TeV range, uncertainties can be as large as 100% since they probe PDFs at verylarge values of the parton momentum fraction, x. Improvements are needed from experimentaldata, theoretical calculations, and methodological framework. With the high luminosity data,CMS will contribute to this program by precision measurements of inclusive, differential, anddouble-differential cross sections of events with jets, photons, W and Z bosons, and top quarks.This requires excellent trigger and pileup mitigation capabilities. The charm and strange PDFscan be constrained by measurements of charm-tagged jets in events with electroweak bosons.This will also require excellent vertex reconstruction capabilities. The search for new physicsbuilds on our knowledge of SM physics.

2.2 HL-LHC beam conditions

Figure 6: LHC schedule for long shutdowns and luminosity projections through HL-LHC.

To achieve the physics program CERN began planning an increase in the instantaneous andintegrated luminosities of the LHC above the original design even before the machine wentinto operation. Major revisions to the machine or the experiments require access to the acceler-ator tunnels and the experimental areas that can only be accomplished efficiently during longshutdown periods. The current plan calls for a series of long periods of data-taking, referred toas Run I, Run II, Run III etc., interleaved with long shutdowns, designated LS1, LS2 and LS3.

Run I is the completed data-taking period in 2011 and 2012. During the first long shutdown,LS1, in 2013 and 2014, modifications were made to the LHC to enable it to operate safely ata center-of-mass energy of 13 TeV for Run II. The bunch spacing has been reduced from 50 nsin Run I to 25 ns for all future runs. The original performance goal for the LHC, to operateat an instantaneous luminosity of 1 × 1034cm−2s−1 with 25 ns bunch spacing, is likely to beachieved relatively soon after the start of Run II. Under these conditions, CMS will experiencean average of about 25 inelastic interactions per bunch crossing, referred to as “pileup in the restof this document. This is the operating scenario for which the CMS experiment was originallydesigned.

A new scheme to form the bunch trains in the Proton Synchrotron (PS) should allow the lu-minosity to exceed the original design before the second long shutdown, LS2, planned for2019-2020. In LS2, the injector chain will be further improved and upgraded to deliver verybright bunches (high intensity and low emittance). It is anticipated that the peak luminositycould reach 2× 1034cm−2s−1 in Run 3, providing an integrated luminosity of over 300 fb−1 by

2.2 HL-LHC beam conditions 9

2024. To maintain its present performance in this period, the CMS detector will undergo aninitial series of staged upgrades in the period through LS2. This program, known as the CMSPhase I Upgrade, has been documented in a Phase I Technical Proposal [9] and three TechnicalDesign reports (TDRs) describing the upgrades of the Pixel detector, the Hadron calorimeterand the hardware trigger [10–12].

By 2024, the quadrupole magnets that focus the beams at the ATLAS and CMS collision re-gions are expected to be close to the end of their lives due to radiation exposure. There will beanother long shutdown, LS3, to replace them with new quadrupole triplets of larger apertureand higher field, and new insertion magnets will also be installed in the section preceding thisregion. With these changes, the focus of the beams at the interaction point will be substan-tially increased with a β∗ parameter as low as 10 cm to 15 cm. This will allow very high peakluminosities that can be tuned (leveled) to lower values along the beam fills. In addition, crab-cavities will be added to compensate the crossing angle of the beams, therefore extending theluminous region and reducing the density of the p-p collisions along the beam axis.

The schedule of beam operations and long shutdowns, together with projections of the peakand integrated luminosities, is shown in Fig. 6. The high luminosity period that follows LS3with the upgraded LHC is referred to here as HL-LHC or Phase II. The baseline operating sce-nario is to level the instantaneous luminosity at 5× 1034cm−2s−1 from a potential peak value of2× 1035cm−2s−1 at the beginning of fills. By design of the LHC upgrades, the leveled luminos-ity could, however, be tuned to the ultimate value of 7.5× 1034cm−2s−1 with slightly shorterfills. In this case, the integrated luminosity would be increased by 30% for the same operatingtime, but CMS will see an increased pileup of 200 instead of 140.

The ultimate instantaneous luminosity projection sets the particle occupancies, trigger require-ments, and data rates that the experiments must be prepared to handle to fully exploit thepotential of the accelerator. In this latter condition, an integrated luminosity of 4000 fb−1 couldbe delivered by the end of Phase II, compared to 3000 fb−1 at the baseline luminosity. Thisdefines the requirement for the radiation tolerance margin of the detectors.

10 3 Overview of the CMS Phase-II upgrades

3 Overview of the CMS Phase-II upgradesBecause of the constraints imposed by the physics program (see Section 2.1), a primary goal ofthe Phase II upgrade is to maintain the excellent performance of the Phase I detector throughoutthe extended operation of HL-LHC. Under the harsh operating conditions at the HL-LHC, themain challenges that must be overcome are the radiation damage to the CMS detector fromthe large integrated luminosity and the huge pileup that comes from the high instantaneousluminosity. In developing the upgrade scope, CMS has made a major effort of simulation tounderstand these effects on the current detector and to identify the mandatory upgrades theyrequire. Details of the issues for each of the CMS subsystems and the proposed remedies aregiven in the sub-detector chapters of the Technical Proposal. Here, only the main implicationsand features of the resulting upgrades are reviewed.

3.1 Performance considerations for the Phase II upgrades

In order to design a detector that will continue to perform well as the integrated luminosityapproaches 3000 - 4000 fb−1, predictions of the dose rate and particle fluence for each type ofparticle are needed. Simulations are used to predict the magnitude and composition of radi-ation as a function of luminosity. The information on the performance of the current detectorunder irradiation is obtained from test beam measurements, special radiation exposures, andthe beginnings of any radiation damage observed in Run I. All these measurements are usedto benchmark the simulations of the radiation damage anticipated at the HL-LHC doses. Fromthese studies, performed for all CMS sub-systems, it is very clear that the tracker and the end-cap calorimeters must be entirely replaced for Phase II.

With the replacement of these detectors, the performance issues associated with high pileup,that are also the most pronounced in the inner and forward detector regions can be addressed.Pileup effects refer to hits or energy deposits from the additional pp collisions in the currentbunch-crossing other than that from the collision containing the hard scatter of interest.

Pileup is the largest source of hits in the tracking system, it increases the combinatorial and thecomplexity of the track reconstruction and can increase the rate of fake tracks. It also adds extraenergy to the calorimeter measurements, such as jet energies, associated with the collision thatcontained a hard scatter. Pileup confuses the trigger and also the offline reconstruction andinterpretation of events. It increases the amount of data that has to be read out in each BX thatcontains a hard scatter and, in fact, at the HL-LHC, most of the data read out will be associatedwith the pileup collisions rather than the collision containing hard scatters. It also increasesthe execution time for the reconstruction of events in the High Level Trigger and the offlineanalysis.

Pileup can be observed in a single bunch-crossing by the many collision vertices that are re-constructed by the tracking system. The new tracking system can be designed with enoughsegmentation to associate charged particles with the correct interaction vertices (up to someefficiency and accompanied by some fake rate). The present calorimeters in CMS howeverdo not have capability to directly associate showers with particular vertices. This is likely tobe possible with the new endcap calorimeters in which accurate timing, and finer lateral andlongitudinal segmentation will be present. This can further improve pileup mitigation, partic-ularly for neutral particles.

In addition to the dominant “in-time” pileup, mentioned above, there is out-of-time pileup(OOT), which refers to energy left in calorimeters in the crossing of interest by particles in theprevious or later bunch crossings The degree of OOT depends on the intrinsic time spread and

3.1 Performance considerations for the Phase II upgrades 11

jitter of the pulses produced by particles in the detector, and by shaping times and other char-acteristics of the readout electronics. Using timing and pulse shape information, it is possibleto correct the energy deposition associated with the OOT pileup. With upgrades of the cal-orimeter readout electronics required for trigger or other purposes, proper design of the newfrontend chips will also allow implementing improved timing measurements to mitigate boththe OOT and the “in-time” pileup effects (see below).

To allow operation at the ultimate luminosity, the upgrades of all the readout electronics aredesigned for efficient data taking up to an average pileup of 200. With this possibility, CMS hasalso intensified a program of R&D into the use of precision timing to help solve the problemof vertex association for neutral particles. The colliding bunches at the LHC have an RMSlength of about 5 cm along the beamline and collisions are spread out by the time it takesfor the two colliding bunches to completely pass through one another. This results in a timedistribution for the individual collisions within a bunch with an RMS of about 150 ps. Withproper design of the barrel and endcap readout electronics shower energy deposits can betimed with a precision substantially lower than this spread, this will allow CMS to reduce theimpact of pileup by selecting only those energy deposits consistent with occurring at the sametime. A further step in the use of timing would be the addition to CMS of a dedicated “timinglayer” sensitive to minimum ionizing particles (MIPS). This layer should be able to achieve aresolution of 20-30 ps to measure the timing of the interaction vertex and in combination withthe calorimeter measurement would allow an “hermetic” and full determination of the neutralenergy associated with this vertex. The investigation of a timing layer is still at an early stageand is not included in the current scope of the CMS upgrades. However, the studies performedat 200 pileup presented in section 5.3 indicate that such a system could enhance the benefit ofoperating at the ultimate luminosity of the HL-LHC.

The ability to ensure efficient event selection for data acquisition is a key prerequisite to fullybenefit from increased luminosity. To achieve the required low transverse momentum, pT,or energy trigger thresholds, the hardware trigger must be upgraded. A sufficient reduction intrigger rate can only be accomplished by improving pT resolution to obtain lower rates withoutloss of efficiency, and by mitigating the effect of the combinatorial backgrounds arising frompileup. A new approach is therefore being developed that introduces tracking informationin the hardware trigger, providing capabilities similar to the current online software trigger(High Level Trigger or HLT). This is an integral part of the design of the Phase II tracker andit also requires a new hardware architecture to incorporate tracker information throughout thetrigger. While the addition of track information in the trigger provides mandatory gains inrate reduction with good efficiency, it is also necessary to increase the trigger acceptance ratein order to maintain the required acceptance for all of the important physics channels. This isparticularly the case for triggers involving hadrons and photons, for which the sensitivity topileup is higher and/or the track trigger is somewhat less efficient.

The measurement of rare processes is a major goal of the HL-LHC physics program. Thisrequires specific upgrades in the forward regions of the detector to maximize the physics ac-ceptance over the largest possible solid angle. To ensure proper trigger performance within thepresent coverage, the muon system will be augmented with the addition of new chambers. Thenew endcap calorimeter configuration also offers the opportunity to extend the muon coveragewith a tagging station up to |η| ≈ 3 or more, with significant acceptance gain for multi-muonfinal states. To also mitigate pileup effects in jet identification and energy measurement, thetracker will be extended up to |η| ≈ 4, thereby also covering the peak production region ofjets accompanying Vector Boson Fusion (VBF) and Vector Boson Scattering (VBS) processes.With this extension, the measurements of total and missing energies, which are critical to new

12 3 Overview of the CMS Phase-II upgrades

physics studies, will be greatly improved. The b-tagging acceptance will also be increased.

3.2 Design optimization of the upgrade elements

Tracker system: to ensure adequate track reconstruction performance at the much higherpileup levels of the HL-LHC, the granularities of the outer tracker and of the pixel detectorare respectively increased by roughly a factor 4 and 6 compared to the present systems. Thisproduces similar level of occupancies as at a luminosity of 1034cm−2s−1.

In the outer tracker, this is achieved by shortening the lengths of the silicon sensor strips with-out changing the pitch significantly. The overall configuration of the detector has been opti-mized with a standalone simulation program that allowed the comparison of several configu-rations with a performance benchmark established by the current detector. The system that waschosen has barrel layers to cover the central rapidity region and disks in the endcaps, just likethe current system. The number of layers in the new configuration has been reduced from 10to 6 in the barrel and from 9 to 5 disks in the endcaps. This is possible thanks to the implemen-tation of a fourth pixel layer (a solution already adopted for the Phase I upgrade) compared tothe original design with three layers. It has to be noted that some of the configurations inves-tigated were dropped for cost reasons. In section 2.1 it is shown that improved measurementresolution has significant benefits on important physics channels such as the H → µµ, howeverthe strip pitch in the outer layers of the detector has only been reduced to 90 µm. This valuewas identified as the threshold to a more expensive frontend hybrid technology. On the con-trary, increasing the pitch by 20%, leading to a similar value as in the current detector, wouldresult in a marginal cost-saving of less than 0.5% of the total tracker cost since it will only re-duce the number of frontend chips. To implement track information at the hardware triggerlevel, a specific module concept has been invented. Each module is made of two sensors, andthe on-detector readout electronics measure the direction of the tracks, bent by the high mag-netic field of the solenoid. This allows the selection of only those hits associated with tracks oftransverse momentum greater than 2 GeV to be sent to the trigger at the bunch crossing rate of40 MHz. The spacings between the sensors have been optimized with the standalone softwareto ensure that this selection remains similar as the radius (bending) increases. In the backendelectronic boards, tracks will be fully reconstructed and fitted before being sent to the centraltrigger. With this scheme, CMS has a strong plan to ensure powerful background rejection atthe earliest stage of the event selection.

To measure the z-coordinate of the tracks, one of the sensors in the modules of the three innerlayers has ministrips that are only 1.5 mm long. Similarly to the strip pitch of the outer layers,the length of the ministrips is chosen to allow lower power consumption and less expensivetechnology for connection between the frontend chips and the sensors. This limits the resolu-tion in the reconstruction of tracks for the trigger. It has to be noted that since the chips coverthe full area of these sensors the number of wafers needed (which drives the cost) is indepen-dent of the number of channels.

In the optimization process, the minimization of the material in the tracker has been a majorgoal. Thanks to configuration improvements, special module design, and new techniques de-veloped for the cooling and power distribution systems, the mass in the tracking volume isgreatly reduced, resulting in a rate of photon conversions that is lower by a factor 2 in the cen-tral region and by a factor up to 6 in the forward regions compared to the existing detector.To reach the high radiation tolerance required, the sensors will be thinner and produced in theplanar n-in-p technology. The R&D program is now focusing on working with potential sensorvendors to develop final specifications that will minimize costs.

3.2 Design optimization of the upgrade elements 13

The pixel system will implement smaller pixels and thinner sensors. Different configurationsdescribed in the Technical Proposal are still being investigated. With up to 10 pixel disks ineach of the forward regions, the system coverage will be extended to |η| ' 3.8, to cover asmuch as possible the calorimeter range.

Barrel electromagnetic calorimeter: the lead tungstate crystals of the barrel electromagneticcalorimeter (ECAL) will remain performant for the entirety of the Phase II running period andwill not be replaced. However, a substantial upgrade to the front-end electronics is required.This is mandatory in order to satisfy the upgraded hardware trigger requirements, to main-tain the ability to trigger efficiently on electrons and photons via the improved rejection ofanomalous signals in the photodetectors, and to mitigate radiation-induced effects that woulddegrade the energy resolution. The barrel supermodules will be removed from CMS duringLS3 and the front-end electronics will be replaced. Following the upgrade, the data will betransferred off-detector at 40 MHz, simultaneously overcoming present limitations in triggerlatency and acceptance rate. The full ECAL granularity will be made available to the hardwaretrigger, allowing more performant algorithms with improved pileup rejection to be developed.The supermodules will be operated at a lower temperature during Phase II. The predictedaging-induced noise increase in the avalanche photodiodes (APD), which would otherwisedominate the electron and photon energy resolution, will be significantly reduced by coolingthe APDs from 18 ◦C to 8 ◦C. A new front-end chip will be designed with a shorter shapingtime to further mitigate the APD noise and to provide better OOT pileup rejection. In addition,the design of the front-end chip will incorporate significantly improved rejection of anomaloussignals in the APDs. The capability for precision timing is also considered in the specificationsand design of the new front-end electronics.

Calorimeter endcaps: the detector that will replace the current endcap calorimeters is calledthe High Granularity Calorimeter (HGCAL). It has electromagnetic and hadronic sections withexcellent transverse and longitudinal segmentation for 3D measurement of shower topologies.The electromagnetic section consists of ∼ 28 tungsten and copper plates interleaved with sili-con sensors as the active material. This provides 25 radiation lengths and 1.5 interaction lengths(λ). The hadronic part has a front section of 12 brass and copper plates interleaved with siliconsensors for a depth of 3.5 λ. This section measures the hadronic shower maximum measure-ment. With this design, the fine granularity will also allow precise timing measurements, thatwill further help mitigating the pileup effects (see section 5.1.2).

The silicon sensor technology is n-in-p, as for the outer tracker, with three different active thick-nesses depending on radius/radiation doses. Each sensor has a pad of ∼ 0.5 cm2 or 1.0 cm2

depending on its thickness. The choice of the pad size is mostly driven by the input capacitanceat the entry of the frontend chip, to ensure sufficient signal to noise ratio for a Minimum Ioniz-ing Particle (MIP) measurement. This allows proper calibration of the system, and at the sametime, this range of transverse size is well suited to eliminate pileup energy along the longitu-dinal development of the shower. Similarly to the tracker, and particularly because the pitchand number of channels per chip is relatively small, reducing the transverse granularity wouldhave little impact on the overall cost of the detector (∼ 3% of the total endcap calorimetry costfor a pad size increase by a factor 2). To cover the large dynamic range required, the frontendASIC chip will feature a Time over Threshold TDC that provides the energy measurement anda time measurement with a precision of ∼50 ps for each pad when the energy deposit exceeds60 fC (' 30 MIPs). This latter feature will enable the precise timing of showers.

The design of the HGCAL draws upon the ILC/CALICE[13] concept. It has been optimizedwith a standalone simulation benchmarked to the ILC/CALICE performance predictions and

14 3 Overview of the CMS Phase-II upgrades

incorporating pileup interactions. The optimization process is progressing with the implemen-tation of an entirely new reconstruction software needed in the framework of the CMS particleflow reconstruction.

Several developments for the HGCAL have synergies with, and benefit from, those requiredfor the tracker or other systems. Particularly, this includes the approach to the sensor radiationtolerance and the commercial discussions with potential vendors. The R&D focuses on engi-neering of the complex frontend ASIC chip, of the module and absorber mechanics, and of thecooling system and services.

The HGCAL part of the endcap calorimeter is followed by a new “backing hadron calorime-ter” of similar design to the current HE detector, with brass plates interleaved with plasticscintillating tiles readout with a wavelength shifting fiber (WLS), to provide an overall depthof ∼10λ for the full calorimeter. The required radiation tolerance will be achieved with a newdesign of the tiles reducing the light path to the WLS and use of new scintillating material. Thetransverse granularity is also slightly increased to match the HGCAL geometry.

Muon system: the various muon systems in CMS, Drift Tubes (DT) and Resistive Plate Cham-bers (RPC) in the barrel, and Cathode Strip Chambers (CSC) and RPCs in the endcaps, areexpected to tolerate the increased radiation levels during Phase II without major degradation.Therefore there is no plan to replace these detectors, but further measurements are underwayto confirm their radiation tolerance margins. The one exception is the readout electronics ofthe DTs, which will suffer radiation damage and will therefore be replaced. This change willalso remove the current trigger rate limitation of the system at 300 kHz. It has to be noted thatalso because of the new trigger specifications, some CSC chambers readout electronics will beupgraded, as explained in the trigger paragraph below.

In addition to possible radiation-induced failures, the chambers will suffer failures from normalaging because of the long time (>30 years) during which they will operate. The long term rateof these failures has been estimated based on current operational experience. Although it maybe possible to repair some failed units during annual shutdowns, it is likely that some will notbe recoverable. This risk has been considered in defining the scope of the upgrade, in additionto other performance requirements.

In particular, the muon system in the region 1.5 ≤ |η| ≤ 2.4 currently consists of four stationsof CSCs. It is the only region of the muon detector that lacks redundant coverage despitethe fact that it is a challenging region for muons in terms of backgrounds and momentumresolution. To maintain good efficiency for the muon trigger in this region, these four stationsare complemented with additional chambers that make use of new detector technologies withhigher rate capability. The first two stations (named GE11 and GE21) are in a region where themagnetic field is still reasonably high and so will use Gas Electron Multiplier (GEM) chambersfor their high granularity and good position resolution. The two last stations (named RE31and RE41) will use low-resistivity RPCs with lower granularity but good timing resolutionto mitigate background effects and complete the redundancy of the system. This upgrade isalong the lines of what was planned in the original design of CMS. The configurations of thedetectors have been selected to provide adequate position resolution according to the levelof magnetic field and of multiple scattering specific to the location of each station. Since theGEM technology is mature and ready for large scale production and because the first stationwill have substantial benefits to the muon trigger in Run III, it is planned to install it duringLS2. More details are given in a dedicated Technical Design Report recently endorsed by theLHCC [14].

3.2 Design optimization of the upgrade elements 15

In addition to the previous upgrade, the extension of the muon coverage for muon detectionto |η| ≈ 3 is now possible in the space that becomes free behind the new endcap calorimeters,which are more longitudinally compact than the current calorimeters. The insertion of a GEMstation in this region will allow muon tagging with a matching track in the tracker extension.The number of layers and the granularity in this station is being optimized for backgroundrejection.

Beam radiation protection and luminosity measurement: the systems that provide protectionagainst beam background and measurement of the luminosity will require work in several ar-eas to survive and function properly in the high radiation levels of the HL-LHC. The protectionsystems will be upgraded with new poly-crystalline diamond sensors that will be read out us-ing the standard LHC Beam Loss Monitor hardware and software and fully integrated into theLHC control system. The machine induced background and luminosity measuring systems inthe Pixel volume must also be replaced.

Trigger: the new hardware trigger scheme implementing track information is mandatory tomaintain the present physics acceptance. It is demonstrated in the Technical Proposal thatwithout this feature, the bandwidth needed for the full data readout would reach 4 MHz at 200pileup. Such a data transfer rate is beyond the capabilities of foreseeable technologies, giventhe constraints on the space available for data transfer links. It will also imply a huge increasein computing power and DAQ bandwidth for the online event selection. To allow sufficienttime for the hardware track reconstruction and matching of tracks to muons and calorimeterinformation, the latency of the trigger must be increased to 12.5 µs. This change requires anupgrade of the readout electronics in the Barrel Electromagnetic Calorimeter. A proper designof the frontend electronics will overcome the latency limitations and also eliminate hardwaretrigger rate restrictions.

Based on the expected performance of the trigger with track information, a trigger acceptancerate of 750 kHz for beam conditions yielding 200 pileup is needed to maintain similar physicsacceptance as in Phase I. This specification can be easily accommodated in the design of allnew detector readout electronics. It only requires an additional upgrade in the inner rings ofthe CSC stations 2 to 4, to overcome data losses that will appear at trigger rates beyond 500kHz.

Data acquisition and trigger control: the Data Acquisition (DAQ) system is upgraded to im-plement the increase of bandwidth and computing power required to accommodate the largerevent size and trigger rate, and the greater complexity of the reconstruction at high pileup.Compared to Phase I, the bandwidth and the computing power requirements are respectivelyincreased by factors of about 10 (15) and 15 (30) for operation at pileup of 140 (200). This is wellwithin the projected network technology capabilities expected at the time of Phase II. Assum-ing an online event selection of 1/100 event at the HLT, as is the case in the current system, thesubsequent rate of recorded data will increase at pileup of 140 (200) to 5 (7.5) kHz from LHCRun I levels of about 1 kHz.

Software and computing: assuming only technological improvements and maintaining exist-ing techniques, the offline software and computing areas would fall short by a factor of 4 (12) ofthe resources needed for the challenging conditions expected in Phase II at 140 (200) pileup. Tominimize the offline computing needs, a significant R&D program has started as part of the up-grade effort to improve the algorithms and approaches used for data reconstruction, analysis,storage, and access; and to adapt the CMS software and computing model to new technologiesand resources. The online reconstruction will also benefit from these improvements. It has tobe noted that the computing costs are not part of the CORE cost discussed in the Technical

16 3 Overview of the CMS Phase-II upgrades

Proposal and in this document.

Experimental area and shutdown considerations: during long shutdowns CMS is highly con-figurable to allow access to the various sub-systems, but access to different areas must often besequential because of the limited overall size of the experimental cavern. Shutdown planningfor Phase II is still at an early stage, but an initial evaluation of the work sequence and timeestimates indicates that the full scope of work can be accomplished in a shutdown of approxi-mately 30 months duration, from end to re-start of beam operations. In order to gain flexibilityin scheduling the work during LS3, consideration is being given to advancing some specifictasks to LS2, as described in the Technical Proposal. Radiation protection and dose to per-sonnel is a primary concern in planning the upgrades and the shutdown work. This requiresdevelopment of special shielding, tooling, and work procedures.

3.3 Upgrade cost estimates

The cost estimates for the Phase II upgrades follow the CERN CORE costing rules. They rep-resent the material replacement value of the installed equipment, in terms of the M&S (ma-terials and services) for the production phase of the project. They also include engineeringcosts incurred during production at a vendor or contractor, and specialized equipment costsfor production fabrication, QA and system testing during the assembly process, transporta-tion, integration and installation at the experiment including costs associated with technicallabour supplied at CERN for these purposes. The transition of funding from R&D to COREbudget occurs with the final design validation or vendor qualification costs, typically involv-ing small-scale orders prior to full production. Costs are reported in 2014 CHF (using yearlymean of currency exchange), with no correction for inflation in future years. Contingency isnot included in the estimates.

The cost estimates are based on the conceptual designs presented in the Technical Proposal.For each sub-detector they are made at the individual component or board level. For someprojects, the dominant items are based on information provided by vendors. Other costs arebased on the original construction of CMS or the current Phase I upgrades. Wherever possible,standardized unit costs are used in all subprojects. This is the case for instance for backendelectronic boards, optical interfaces and ASIC chips. The estimates for these components arebased on the information provided by CERN. The cost of computing for the HLT/DAQ up-grade is estimated according to the CERN IT Division projection, assuming that CPU powerincreases by 25% per year at constant cost.

Every item whose cost is estimated is ranked in one of four standard “unit cost” quality cat-egories: (1) non-binding vendor quote or current catalog price; (2) similar past or recent pur-chases (correcting for inflation as appropriate); (3) engineering designs where sub-componentsare well known; (4) engineering concept or scaling from similar systems. In some cases thenumber of components (boards, wafers etc) is well known, while in others there is uncertaintyat this stage of the design. This is reflected with four additional “quantity of units” qualityflags, ranging in steps of 10% from a knowledge accuracy of better than 10% to more than 30%.Table 1 below provides a summary per project, while a mid-level cost breakdown for each canbe found in Appendix A. The entire cost of the upgrade is estimated to be 265 MCHF.

The complete component-level cost breakdown has been reviewed internally by CMS and sub-mitted for review by the CERN Upgrade Cost Group. The fractions of the total cost brokendown by “unit cost” quality flags, as described above, are presented in Figure 7. It indicatesthat the quality of the individual estimates for a large fraction of the overall cost is advancedfor this early stage of the project.

3.3 Upgrade cost estimates 17

Table 1: Summary of CORE costs for the CMS Phase II Upgrade.

CORE cost estimate MCHF (2014) Further Detailsin Table

Pixel Detector 23 8Outer tracker 89 8

Tracking System 112EB electronics 10 9HB scintillators 1 9Endcap HGC+BHE 64 10

Calorimeters 75DT and CSC electronics 10 11Muon stations: GE11, GE21, RE31 and RE41 10 11Muon extension ME0 5 11

Muon Systems 25Beam Monitors and Luminosity 4 12

Hardware trigger 7 13HLT 11 14DAQ 6 14

Trigger and DAQ 24Infrastructure, Systems and Support, Installation 25 15Total 265

15%  

26%  

24%  

35%  

Phase  II  CORE:  Fraction  of  total  cost  by  Unit  Cost  Quality  Flag  

(Total  is  265  MCHF(2014))  

1  2  3  4  

Figure 7: Breakdown of the total cost of 265 MCHF (2014) by unit cost quality flag (as describedin the text).

18 4 Upgrade configurations of reduced cost

4 Upgrade configurations of reduced cost4.1 General considerations

One of the most important goals of the physics program of the HL-LHC is the precise study ofthe properties of the Higgs boson and any other particles that may be discovered. The efficientmeasurement of all the Higgs decay properties and production mechanisms requires the recon-struction of all the standard physics objects and imposes stringent requirements on the solidangle coverage and sensitivity to low transverse momenta to reduce statistical and systematicuncertainties. This requirements also apply to a large extent to the search of new particles. Eachsub-detector, therefore, plays an important role, and reduced coverage or poor performance byany of them will diminish the physics output of CMS. For these reasons, the cost reductionsinvestigated have been distributed across all sub-systems, rather than concentrated in one ortwo of them, to maintain the combined detector performance at the highest possible level. Theoptions considered have been established from the reference designs, since they were alreadysubject to some level of performance and cost optimizations. The two “descoped” configura-tions, targeting the cost scenarios proposed by CERN, are therefore combining fewer of theupgrade elements or downgrading their performance, in an attempt to preserve the physicsreach in all areas to the greatest extent. At this stage, no re-optimization of the downgradedelements, individually or jointly, has been possible, as this will require substantial new designefforts and reconstruction software. We believe, nevertheless, that the two configurations pre-sented below are representative of the reduced performance and increased operational risksthat scope reductions of this magnitude would entail.

In selecting the two configurations, it was assumed that all upgrades are installed during LS3preceding the start of Phase II. However, it is clear that the full computing capability for theHigh Level Trigger and Data Acquisition Systems can be completed gradually to keep up withthe increasing luminosity of the accelerator. For other descope candidates it is also indicatedwhether the incurred performance reduction and operation risk can be mitigated by a possible“recovery, that is, if it is technically feasible to install them in a Long Shutdown (post LS3) afterthe start of Phase II.

4.2 Options for cost reductions

Following the general considerations outlined above, options to reduce costs have been in-vestigated for all sub-systems, assessing the balance between the anticipated loss of intrinsicperformance, the increased operational risks, and the associated cost reductions. While the out-come is specific to each sub-system, it is generally found that reducing the granularity of thesensitive components while maintaining the same geometrical coverage, even when technicallyfeasible, does not produce significant savings but results in severe degradation of performance.This was part of the upgrade optimization and it is explained in section 3.2. The cost reduc-tions have instead been sought in the reduction of the number of components, up to removingdetection layers. Reducing the number of layers better preserves the performance, but the lossof redundancy almost always increases operational risks in case of unexpected component fail-ures. It also reduces the protection against possible harsher beam conditions than foreseen.For each sub-system, technical implications and anticipated performance degradations of thereduced scope are briefly described below relative to the reference design. The detailed studiesof the performance impact of each de-scope are presented in Section 5.

The tracker system, the outer tracker and the pixel detector, are the most important systems formitigating the effects of pileup in the online event selection and in the offline analyses. Precisetracking and momentum reconstruction are also necessary for the particle flow technique that

4.2 Options for cost reductions 19

is used for the reconstruction of nearly every physics object. The tracker performance musttherefore be preserved as much as possible. Nevertheless, several descoping options have beenproposed and studied. A new geometry has been investigated where the modules in both endsof the three inner layers of the outer tracker are tilted towards the collision point. This reducesthe number of modules by about 8%. While this has some beneficial effect on the amount ofmaterial crossed by the incident particles, the precision on the origin of the particles along thebeam axis will be degraded at the hardware trigger level. This effect, however, is not expectedto substantially reduce the background rejection power of the track-trigger implementation.A second studied descoping option is to remove one layer of the outer barrel. Layer 4 hasbeen chosen for this exercise since it minimizes the track reconstruction performance degrada-tion. This will however reduce efficiency of the track reconstruction for the trigger since thisreconstruction does not benefit from the pixel seeding. The loss of redundancy will also causea serious risk of irrecoverable performance degradation in case of unexpected failures in theremaining layers.

A third descoping option for the tracker would be to limit the pixel coverage to η ' 2.4. Thiswill eliminate the ability to use the particle flow techniques in the very forward region, withdramatic impact on background rejection for key physics channels.

The endcap calorimeters will have an enhanced role in the HL-LHC physics program. Thehigh transverse granularity and depth segmentation of the silicon/tungsten calorimeter offersgreat potential to mitigate effects of pileup through the 4-dimensional measurement (includ-ing timing) of the electromagnetic and hadronic shower developments. The current design hasbeen optimized to provide good intrinsic energy resolution, but flexibility may exists to furtheroptimize the segmentation. Two configurations of decreasing cost have been considered. Theless severe one, denoted as 24-11 in Table 2, reduces the electromagnetic section by 4 layers,from 28 to 24, and the hadronic section by 1 layer, from 12 to 11. The more severe variant,denoted as 18-9 in Table 2, reduces the number of layers in the electromagnetic section from28 to 18 layers and in the hadronic section from 12 to 9 layers. Since, at this stage, it is notpossible to develop an optimized geometry of the downgraded detectors using the full CMSsimulation, their performance is derived by simply ignoring the concerned layers at the recon-struction level. A standalone simulation optimizing the mass resolution of the Higgs bosondecay to two photons was used to identify the layers to remove. A more sophisticated designre-optimization would consider the balance between the degradation of the energy resolution,the pileup mitigation, the shower pointing, and time measurement abilities. In addition, it isalso possible to reduce the silicon sensor cost by using n-type wafers in the region of lowerirradiation, instead of the p-type technology presently proposed for the full coverage of thedetector. This will substantially complicate the system, requiring the design of two frontendASICs with different polarities and voltage distributions.

The muon system cost reductions can be sought mainly through descoping of redundancy,descoping of the hardware trigger performance, and limiting the solid angle coverage. Oneapproach would be to eliminate some of the three new muon stations proposed (GE21, RE31,RE41). This is being studied by eliminating the chambers in succession starting with the one far-thest from the interaction point. This would lead to progressive reduction in efficiency, chargeidentification capability and resolution for high momentum muons, particularly sensitive inthe trigger performance. Another descoping option is to eliminate the replacement of the read-out electronics in the first rings of the two last CSC stations. It would result in substantialdegradation of the hit readout efficiency when the trigger rates exceed 500 kHz. The extensionof the muon coverage up to the edge of the calorimeters at η = 3 could also be descoped, reduc-ing the acceptance for important physics channels. While the DT readout must be replaced, it

20 4 Upgrade configurations of reduced cost

is possible that the replacement could be staged over many years (i.e. beyond LS3) if the failurerate observed in the coming years is substantially lower than anticipated. This would howeverrepresent a significant performance risk and would also limit the trigger rate to 300 kHz untilthe replacement of all the current electronics is completed.

The hardware trigger event selection efficiency is crucial to achieve the goals of the physicsprogram. The two main upgrades of the system consist of implementing the track informa-tion at the 40 MHz selection level and allowing an higher output rate, up to 750 kHz, for thefull readout of all detectors. As the second requirement mainly involves additional computingand data acquisition capacity, denoted as HLT/DAQ power in Table 2 and Table 3, the optionconsidered here is to limit the rate to 300 kHz at the beginning of Phase II and then to progres-sively increase the computing/DAQ capacity according to real operating needs and triggerperformance. A “permanent” limit of 300 kHz would present a substantial risk of acceptanceloss at 140 pileup and will definitely prevent the experiment from fully benefiting from thehigher luminosity potential, corresponding to 200 pileup.

4.3 Upgrade configuration of 242 MCHF cost

In developing a configuration of limited cost reduction, some flexibility will exist to finalizechoices and designs at a later stage, typically on the time scale of Technical Design Reports,when performance, costs and funding are further established.

Considering the anticipated operational risks, the potential performance degradation, and thecurrent level of optimizations, a representative configuration of intermediate cost comprisesthe tilting of the OT layer modules; a limitation of the trigger rate to 300 kHz at the beginningof the program, including both the HLT/DAQ power and the CSC stations 3 and 4 readoutdescoping; and a limited decrease of the number of layers in the HGCAL (typically the 24/11layers scenario). This would reduce the cost by about 23 MCHF. The cost reductions for eachupgrade element are presented in Table 2. They are established from the full component levelcost breakdown. The performance of this intermediate cost configuration, referred to as sce-nario 1, is presented in section 5.

Table 2: Upgrade scope reductions for scenario 1.

Upgrade configuration of ' 242 MCHF costDe-scoped item Operation and performance im-

pactCost reduc-tion (MCHF)

Recoverability

Tilted modules in the outer tracker Track-trigger resolution 3.9 NoNo muon endcap stations 3 and 4 Redundancy, efficiency, resolution 2.0 YesNo replacement of CSC stations 3and 4 readout

Efficiency at trigger rate ≥ 500 kHz 2.5 Yes

Reduced HLT/DAQ power Trigger rate ≤ 300 kHz 8.0 YesHGCAL with 24/11 layers Energy resolution, pileup mitiga-

tion, shower pointing, timing7.0 No

TOTAL cost reduction 23.4

4.4 Upgrade configuration of 208 MCHF cost

For this configuration of much reduced cost, it is necessary to implement all the cost reduc-tions discussed in Section 4.2. This reduces the cost by about 57 MCHF, as shown in Table 3,

4.4 Upgrade configuration of 208 MCHF cost 21

again based on the full component level cost breakdown. The subsequent degradation of per-formance of this configuration, referred to as scenario 2, is presented in section 5.

Table 3: Upgrade scope reduction for scenario 2.

Upgrade configuration of ' 208 MCHF costDe-scoped item Operation and performance im-

pactCost reduc-tion (MCHF)

recoverability

Tilted modules in the outer tracker Track-trigger resolution 3.9 NoNo muon endcap stations 3 and 4 Redundancy, efficiency, resolution 2.0 YesNo replacement of CSC stations 3and 4 readout

Efficiency at trigger rate≥ 500 kHz 2.5 Yes

Reduced HLT/DAQ power Trigger rate ≤ 300 kHz 8.0 YesNo muon endcap stations 2 Redundancy, efficiency, resolution 4.0 YesNo muon extension to η ' 3 Muon acceptance 4.5 NoHGCAL with 18/9 layers Energy resolution, pileup mitiga-

tion, shower pointing, timing13.0 No

No Pixel extension to η ' 3.8 Pileup, jet tagging, Missing ET 7.7 YesNo replacement of Muon DT read-out

Efficiency and trigger rate ≤ 300kHz

6.1 Yes

One less layer in outer tracker bar-rel

Track-trigger efficiency 5.0 No

TOTAL cost reduction 56.7

22 5 Comparative Performance studies

5 Comparative Performance studiesThe performance of the reference detector is documented in the Technical Proposal for an in-stantaneous luminosity of 5.0×1034cm−2s−1; which corresponds to a mean of 140 interactionsper beam crossing (pileup). The performance is compared to that of the Phase I detector at amean pileup of 50 and of the same, but aged, detector operated at a mean of 140 pileup. Theaged Phase I detector models the radiation damage after exposure to 1000 fb−1 of integratedluminosity for the outer tracker, hadron and electromagnetic calorimeters. The former config-uration provides a reference for the performance that CMS wants to maintain, while the latteridentifies the upgrades required.

In this section, the performance is evaluated for the two configurations of Scenario 1 and Sce-nario 2 (see Tables 2 and 3) operated at pileup 140, and then for the reference detector at aninstantaneous luminosity of 7.5×1034cm−2s−1, corresponding to a pileup of 200 events. The re-construction performance of all physics objects is established with a complete simulation of thedetector based on GEANT4 [15] and CMSSW software [16]. The impact on the physics reachis illustrated using selected physics benchmarks. The benchmarks have relevance to the mainareas of the physics program and are representative of the much wider range of processes thatwill be studied at the HL-LHC. They involve several decay products, referred to as physicsobjects, and are therefore sensitive to the performance of the various elements of the referenceupgrade. The study of the physics reach uses a parametrization of the object performance in asimplified simulation. This reduces the computing resources and allows the generation of themuch larger Monte Carlo samples needed to estimate backgrounds.

5.1 Performance implications of reduced cost configurations

In this section, the performance implications of the individual elements of the descope in Sce-nario 1 and Scenario 2 (see Tables 2 and 3) are discussed, by comparing to the reference scopeat a pileup of 140.

5.1.1 Tracker

Three descale options have been considered for performance studies: fewer outer tracker mod-ules, implemented by using a tilted configuration, a limitation in the tracking coverage, andthe removal of a layer in the outer tracker.

Tracker module tilting: tilting modules in the outer part of the tracking system slightly de-grades the z-resolution of online reconstructed tracks. However, this degradation has limitedeffect on the performance of the final track trigger. While the resolution on the vertex positionis degraded by 30%, the isolation of leptons and the total or missing transverse energy trig-gers do not show substantial degradation in performance with the relatively loose selectioncuts presently applied. No significant adverse impact on the online and offline reconstruc-tion performance is observed. A reduction of the material budget even leads to a moderateimprovement in the offline momentum resolution.

Tracker extended coverage: the removal of the forward pixel extension has significant implica-tions for the performance of the reference upgrade. Figure 18 shows the tracking efficiency andmisreconstruction rate of the tracking detector with limited pseudorapidity coverage. Chargedparticles from pileup outside the tracking acceptance can not be associated with the primaryvertex. This has a critical effect on the ability of the reconstruction and identification algorithmsto mitigate the effects of pileup. Figure 8 shows that the rate of Drell-Yan events with misre-constructed missing transverse energy is significantly increased without the tracker extension.

5.1 Performance implications of reduced cost configurations 23

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high-η tracking extension and the ratio of the number of events observed without and with thetracking extension vs Emiss

T for events at <PU>= 140 and <PU>= 200.

Figure 9 (left) shows that the missing transverse energy resolution is also degraded by about25% at 140 pileup. Jet counting and the identification of vector boson fusion or vector bosonscattering processes are diluted by the presence of misreconstructed jets in the forward regionof the detector. Figure 9 (right) shows the relative rate of additional misidentified jets as afunction of pseudorapidity.

A substantial increase in additional jets that come from pileup is observed and the performancedegradation is severe even at a pileup of 140 when the tracker extension is not present. Theimpact on the measurement of Higgs boson properties in events produced via the vector bosonfusion process is included in the discussion in Section 5.2. In addition, the limited coverage forcharged particle reconstruction reduces the acceptances for taus, muons, electrons, photons,and b-jet tagging. For the Higgs boson to four muons analysis, one of the key measurementsfor the HL-LHC, the acceptance is reduced by 20% as shown in Figure 10.

Tracker without layer 4: the performance of the tracking system with a reduced number oflayers has been studied. To simplify the performance studies, layer 4 of the outer tracker hasbeen removed without changing the other layers configuration. This layer is selected since itminimizes the impact on the track reconstruction performance. The offline track reconstructionperformance is mostly driven by the precise space-point measurements in the pixel detectorand the lever arm of the tracking volume is not modified by the removal of layer 4 and thereforethe momentum resolution is neither not affected. However, the track reconstruction for thehardware trigger does not include the pixel detector and it is therefore sensitive to the numberof layers in the outer tracker.

In order to estimate the impact of the loss of redundancy, realistic defects to the system havebeen studied. A first scenario assumes that 5% of the modules, randomly distributed, are not

24 5 Comparative Performance studies

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Figure 9: EmissT distribution for the perpendicular component of hadronic recoil to Z boson,

measured in Z → µµ, at <PU>= 140 with the high-η tracking extension (red) and without(blue). Results for Phase I at <PU>= 50 (black) are also included (left). Relative rate of addi-tional misidentified jets as a function of pseudorapidity for different pileup configuration. Re-sults are shown relative to the performance of the reference reference detector at <PU>= 140for the reference detector without tracking extension at <PU>= 140 (blue) and Phase I at<PU>= 50 (black) (right).

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Figure 10: Four lepton mass distributions obtained with 3000 fb−1 for the signal sample, H →ZZ → 4µ, and for the irreducible ZZ → 4µ background. The mass distributions are shown fora muon pseudorapidity coverage of 2.4 and 3.0 (right).

5.1 Performance implications of reduced cost configurations 25

functional. Based on the experience with the current tracking system, a second scenario sim-ulates the failure of a cooling loop in layer 5 (1/4 of the layer) and an additional 1% of themodules become inoperable. The defect scenarios are studied for both reference and descopedgeometries. Figure 11 shows the efficiency of the online track reconstruction as a functionof transverse momentum and pseudorapidity at 140 pileup for different types of particles:charged particles in top-pair events and electrons. Table 4 summarizes the integrated efficien-cies for the different configurations. Comparing the drop in efficiency for the 6 and 5 layergeometries clearly shows the effect of losing redundancy in the tracking system. Particularlya 9% drop is seen for the electrons with one less layer and this drop increases to 17% whena cooling loops fails. This directly translates into a decreased trigger efficiency for all triggerobjects. The performance loss at 200 pileup leads to the same conclusion.

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Figure 11: Track trigger efficiency as function of pseudo-rapidity and transverse momentumfor different detector configurations and charged particles (top-pair, electrons). The red dotsshow the efficiency in the limited angular region of the missing cooling loop.

Table 4: Summary of the online track reconstruction efficiency for different detector configura-tions and types of particles in the central part of the detector.

geometry tracks in top-pair events , |η| < 1.0 electronspT > 2 GeV pT > 10 GeV pT = 10 GeV

Reference6L, ideal 88% 93% 85%6L, 5% loss 87% 92% 84%6L, cooling+1% loss (−1 < η < 0) 87% 92% 85%

Scenario 2, without layer 45L, ideal 82% 91% 76%5L, 5% loss 77% 87% 73%5L, cooling+1% loss (−1 < η < 0) 72% 84% 68%

26 5 Comparative Performance studies

5.1.2 Endcap calorimetry

Electromagnetic energy resolution, electron identification and reconstruction: the energy res-olution for isolated objects such as electrons and photons is expected to worsen roughly as√

Nb/Na, where Nb and Na are the number of layers before and after descoping, i.e. by about10% and 25% respectively for Scenario 1 (HGCAL 24/11) and Scenario 2 (HGCAL 18/9) (seeFigure 12). In Scenario 2, the stochastic term becomes quite large and reaches values above30%/

√E, (with E in GeV). The reduced resolution has direct impact on measurements includ-

ing electrons and photons, for example Higgs boson to diphoton or four-electron measure-ments.

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Figure 12: Relative degradation in electromagnetic energy resolution as a function of the num-ber of layers removed (left). Relative increase in pointing resolution as function of showerenergy (right), as seen in standalone simulation which includes the effect of the reduced depthof the electromagnetic compartment in the descoped configurations.

The other parameters of interest are the decrease in the efficiency of reconstruction and identi-fication and an increase in the misidentification rate. Figure 13 shows, using Drell-Yan events,the performance of the electron identification in the endcap calorimeter for electrons with trans-verse momenta larger than 10 GeV. In Scenario 2, for an efficiency of 90% the misidentificationrate increases by a factor of three for low pT electrons and by a factor of two for high pT elec-trons. The impact of the degraded electron identification efficiency on the physics performancehas been studied in the context of the H → ZZ∗ → 4e analysis and is discussed in Section 5.2.

Single hadron, jet energy resolution, jet identification and reconstruction: the single (charged)pion energy resolution worsens by 6% and 15% in Scenarios 1 and 2 respectively, with the con-stant term increasing in Scenario 2. The identification and measurement of the energy of jetshas also been studied. Preliminary studies show little loss in performance, since in particleflow algorithms the jet momentum measurement is determined essentially by inner trackingand is not expected to be affected by calorimetric energy resolution. However, since the res-olution is much affected in the presence of pileup, techniques to mitigate the effects of pileupare very important. The high lateral and longitudinal granularity will allow use of precisiontiming of low-energy deposits and clearer association of individual particle showers with thetracking information from the hard interaction vertex. A redundant system is important forgood calorimetric measurements of energy in order to draw maximum benefit from particle

5.1 Performance implications of reduced cost configurations 27

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Figure 13: Operating characteristic for electron identification in the endcap calorimeter, inDrell-Yan events, for <PU>= 140 for electrons with transverse momentum between 10 and20 GeV (left) and larger than 20 GeV (right).

flow algorithms.

Measurement of the direction of electromagnetic showers and jets: the possibility of deter-mining the angle of incidence for high energy electron/photon showers was mentioned in theTP. In the reference design, with further optimization, measurement of the direction should beachieved with a precision of≈ 2 mrad for photons at |η| = 1.7 from H → γγ decays. This reso-lution allows the determination of their vertex of origin to within 2 cm, compared to the widthof the luminous region of about 5 cm, thus potentially decreasing the pileup for these events bya factor of about three. There is also interest in searching for displaced electrons and photonsand jets arising from new particles that are predicted in some models of physics beyond theSM. This is possible if the direction of electromagnetic showers can be determined with suffi-cient accuracy. The angular resolution degrades faster than

√Nb/Na because, in addition to

the reduction in number of measured points, the lever arm is shortened, see Figure 12. It de-grades by≈ 20% and 60% in Scenarios 1 and 2, respectively. Thus, in Scenario 2, the sensitivityto off-pointing electrons and photons resulting from models of physics beyond the SM wouldbe significantly degraded, and a potentially useful means for mitigating the effects of pileupthrough pointing would likely be rendered largely ineffective.

Measurement of the relative time of showers for electrons, photons and jets: the possibilityof precisely determining the time of showers was briefly mentioned in the TP, where it wasstated that cells with an energy deposit corresponding to a charge > 60 fC (a few tens of MIPs)potentially provide a single cell timing resolution of 50 ps or better, corresponding to the jitterin the present reference design for the HGCAL front-end chip. Since then, test beam measure-ments have confirmed that the time jitter in the intrinsic signal formation for signals above 10MIPs or so is ∼ 30 ps, well below the electronic jitter, and decreases with increasing signalamplitude.

In energetic electromagnetic showers there are dozens of cells with energy above the 60 fCthreshold, and the π0 component of hadron showers also results in several cells above thethreshold. Assuming that the system effects can be kept sufficiently low, this means that ashower timing of order 10-20 ps may be achievable. This precision can help considerably inorder to localize the hard interaction vertex and to mitigate the effect of pileup. In the case oflow-pT H→ γγ decays, where the ability to use the kinematics of charged tracks in the H→ γγto tag the production vertex is degraded significantly in the presence of very high pile-up, the

28 5 Comparative Performance studies

time of the charged hadron showers can be correlated with that of the photon showers in orderto aid the correct identification of the production vertex. This would then remove the largestcontribution to the mass resolution of di-photon events.

Similarly, a substantial amount of the energy in a jet is carried by lower energy particles (<20 GeV). Precise time determination of such deposits would also help in the removal of pileupfrom low-pT neutral particle energy deposits, in addition to that from charged particles, eveninside jet cones.

Precision timing of high energy electrons, photons, or jets is also affected by the reduction in thenumber of silicon layers. For high energy electromagnetic showers, e.g. photons from Higgsboson decays, the number of cells above the 60 fC threshold for 50 ps timing remains high, anda relative timing precision of 10-20 ps should be possible in all the scenarios considered here.But for lower energy electromagnetic showers, and especially hadronic showers, the number ofcells above the 50 ps timing threshold is small and the efficiency for precise time tagging of lowpT charged pions is substantially reduced by decreasing the number of layers in the HGCAL.Figure 14 shows that the probability that five or more cells have an energy deposit > 60 fC,at low charged hadron transverse momentum, decreases by 15% and 35% in Scenario 1 and 2,respectively, with a consequent loss of efficiency for vertex localization and pileup mitigation.

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Decrease in redundancy and robustness: with the reduction in the number of planes, the lossof topological information is expected to degrade the power of the pattern recognition andthe ability of the clustering algorithms to separate showers. The reconstruction algorithms arenot sophisticated enough at this stage to quantify the impact. However, in both scenarios,the substantial performance loss on almost all fronts would also leads to a loss of redundancyand robustness, particularly if unexpected failures occur in sensitive regions. For instance thenumber of layers in the FH is already minimal since the sampling is only every 0.33 λ (3.5 X0).In this part of the hadronic shower (at hadronic shower maximum) it is important to keep thesampling small, both in terms of λ as well as X0. This part could well play an important rolein precision timing of hadronic showers, and for the identification and measurement of theenergy of boosted jets, or VBF jets.

5.1 Performance implications of reduced cost configurations 29

5.1.3 Muon system

Muon redundancy: descoping elements of the reference upgrades of the muon system willreduce the redundancy required for the long term stability and efficiency of the system. Tostudy the impact of the limited redundancy, defect scenarios have been developed for the DT,CSC, and RPC systems. The deterioration is estimated using the experience with the currentdetector systems and the expected radiation damage. The following rate of failures have beenimplemented in the simulation: 28% of the DT chambers (representing expected degradationof the readout), 15% of CSC chambers, and 20% of the RPC chambers in the barrel and 5% inthe endcap.

Figure 15 shows the resulting efficiencies for a loose and tight muon identification. The impactof the muon system deterioration is substantial, resulting in a degradation of the tight muonidentification. In particular the impact of not replacing the DT readout and reducing the re-dundancy in the endcaps leads to large losses in efficiency, particularly at the transition regionbetween the barrel and the endcaps. This results in efficiency losses of 2% for Scenario 1 and20% for Scenario 2 for studies of the H → µµ process, the latter translates into a factor 1.25 ofadditional luminosity needed to achieve the same physics reach as with the reference design.

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Figure 15: Loose Muon identification efficiency (left) and Tight Muon Identification efficiency(right) in Drell-Yan events as a function of the |η| of the simulated muon for different detectorscenarios

Signatures of displaced muons are proposed by many theories beyond the standard model.Figure 16 shows the standalone muon reconstruction efficiency based on quality requirementsas function of pseudorapidity. The drop in efficiency is enhanced when the reconstruction andidentification is not assisted by the tracking system. This will significantly affect the ability totrigger on the displaced muons.

5.1.4 Trigger system

Limiting the hardware trigger accept rate reduces the acceptance for important physics signa-tures and compromises the CMS physics program of the HL-LHC. This is demonstrated in theTechnical Proposal, where the energy thresholds for several trigger objects implemented in anexample trigger menu are presented as a function of the total trigger rate. Here, the effect ontwo important physics processes is shown.

30 5 Comparative Performance studies

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Figure 16: Standalone muon reconstruction efficiency as function of pseudorapidity. Require-ments on the number of hits used to reconstruct the muon track are applied to ensure a goodmeasurement of the muon momentum.

The first example considers a SUSY signal, where top squarks are pair-produced and decay toa top quark and a neutralino. In the dark matter coannihilation scenarios, in which a secondSUSY particle is nearly degenerate in mass with the χ̃0

1, these events look very much like top-pair events. The acceptance is tested using an HT trigger and a relative drop of 14% and 30%is observed respectively for the signal at 140 and 200 pileup. The second example investigatesH → ττ decays in the fully hadronic tau final state. The combination of the single-tau anddouble-tau trigger is studied. The relative acceptance drop for the H → ττ signal is 20% and70% respectively at 140 and 200 pileup. The signal acceptances for different rates and pileupconditions are summarized in Table 5.

Table 5: Summary of the trigger acceptances for a top squark and a H → ττ signals. Triggerthesholds for the stop coannihilation scenario are given for an HT trigger and the thresholdsfor the H → ττ are given for the single-tau and double-tau trigger.

Scenario Thresholds [GeV] Acceptance [%]Stop coannihilation scenario

Reference, <PU>= 140 350 51Descoped, <PU>= 140 370 44Descoped, <PU>= 200 412 36

H → ττReference, <PU>= 140 88, 56 20Descoped, <PU>= 140 94, 59 16Descoped, <PU>= 200 136, 66 6

5.2 Combined performance studies 31

5.2 Combined performance studies

To illustrate the combined effect of the scope reductions in the two cost configurations, theimpact on five benchmark measurements has been studied. Tables 6 and 7 summarize theperformance degradation at 140 pileup as a factor of additional luminosity needed to achievethe same physics reach as with the reference design.

The coupling of Higgs bosons to taus has been established in Run I of the LHC. The precisionmeasurement of this coupling will shed light on the nature of the Higgs boson and has largepotential for discovery for new physics. The implications of limited performance in the trigger,missing transverse energy resolution, and the ability to identify the vector boson fusion pro-duction signature have been studied. Descoping the trigger system reduces the acceptance forthe important fully hadronic channel by 20% at <PU>= 140. The ττ invariant mass, calculatedfrom the visible tau decay products and the missing transverse energy, is used to discriminatethe signal from the irreducible Drell-Yan background. The performance of the analysis scaleslinearly with the ττ mass resolution. The worsened missing transverse energy resolution dueto the removal of the forward tracking extension degrades the ττ mass resolution by about 25%for Scenario 2 at <PU>= 140. The rate of jets reconstructed from pileup energy depositionsreduces the signal yield and increases the background from Drell-Yan production degradingthe analysis performance by 27% in Scenario 2, dominated by the descoping of the forwardtracking extension. Figure 17 shows the rate of wrongly identified jets as a function of pseu-dorapidity for the two pileup conditions relative to the performance of the reference detectorat <PU>= 140. Assuming that these three effects are uncorrelated, the overall performanceof the measurement of the Higgs to tau coupling is reduced by 20% and 50%, translating intoa factor 1.25 of additional luminosity needed to achieve the same physics reach for Scenario 1and a factor 4.2 for Scenario 2 as compared to the reference detector.

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Figure 17: Relative rate of additional misidentified jets as a function of pseudorapidity fordifferent pileup configuration. Results are shown relative to the performance of the referencedetector at <PU>= 140 for the Scenario 2 configuration at <PU>= 200 (green) and <PU>=140 (magenta) and for the Phase I detector at <PU>= 50 (black).

The coupling of the Higgs boson to second generation fermions can be tested via decays tomuons. Descoping limits the redundancy of the muon system, leading to a loss in perfor-mance. For the H → µµ channel, this translates into a reduction in signal efficiency of 20(2)% for Scenario 2 (1). The study of the Higgs boson using the four-lepton channel will drive

32 5 Comparative Performance studies

our understanding of this new particle. Descoping the extension of the rapidity coverage ofthe tracking and muon system leads to 20% loss in acceptance, which directly translates to anadditional 20% of luminosity required to record the equivalent number of Higgs events. Theimpact of a degraded electron identification efficiency and resolution on the physics perfor-mance has been studied in the context of the H → 4e analysis. For the reference and the twodescoped scenarios, a selection following the Run I analysis (without re-optimization) has beenapplied to simulated events. The degraded identification efficiency and resolution in HGCALhave been parametrized and the barrel performance is kept unchanged. The fiducial region forelectron identification of the default and Scenario 1 detectors is |η| < 3, while the Scenario 2detector allows electron identification only up to |η| = 2.5. The leading backgrounds in themeasurement are ZZ and Z+jets production, each contributing about 50% in the Run I analy-sis. In order to allow for a performant measurement and to profit from the HL-LHC dataset,the misidentification rate needs to be controlled while achieving high efficiencies for low-pTelectrons. In the comparison electron identification working points with a constant efficiencyof 90% are selected. The study shows significant impact from the degraded HGCAL perfor-mance and the reduced rapidity coverage of the tracking system. A loss in significance (S/

√B)

of 13% in Scenario 1 and 29% in Scenario 2 is observed. This corresponds to a factor 1.3 and 2of additional luminosity to achieve the same analysis performance with the descoped Scenario1 and Scenario 2 detector, compared to the reference detector.

The last example considered here is the search for the W±HEmissT final state. The degraded

missing transverse energy resolution, as for the H → ττ measurement, limits the mass reachfor discovery dramatically. The discovery of a model with χ̃0

2 = 950 GeV and χ̃01 = 100 GeV

would be possible with the reference detector and a dataset of 3 ab−1. The same analysis usinga detector with the Scenario 2 configuration would require more than 20 ab−1 to discover thesame model.

Table 6: Summary of physics performance studies for Scenario 1. The impact of the scopelimitation is given as a factor of additional luminosity needed in order to achieve the sameresult as with the reference detector at 140 pileup.

Process Descoped items Effects considered ImpactVBF H → ττ trigger trigger acceptance 1.25H → µµ RPC muon ID 1.02H → 4e HGCAL (24/11) electron resolution and ID 1.3

Table 7: Summary of physics performance studies for Scenario 2. The impact of the scopelimitation is given as a factor of additional luminosity needed in order to achieve the sameresult as with the reference detector at 140 pileup.

Process Descoped items Effects considered ImpactVBF H → ττ trigger, tracker ext. trigger acc., Emiss

T res., jet counting 4.2H → µµ muon system muon ID 1.25H → 4µ tracker ext. muon acceptance 1.25H → 4e HGCAL (18/9), tracker ext. electron res., ID, and acc. 2.0W±HEmiss

T tracker ext. EmissT resolution 6.7

5.3 Phase II detector performance at pileup 140 and pileup 200

In this section, the performance of the reference detector of the Technical Proposal is comparedfor pileup 140 and pileup 200. It is also benchmarked to the performance of the Phase I detectorat a pileup of 50.

5.3 Phase II detector performance at pileup 140 and pileup 200 33

The reconstruction and identification algorithms have not been fully optimized for these ex-treme pileup conditions. The main tool that can be used to mitigate the effects of pileup is theassociation of charged particles to the primary event vertex. The PUPPI [17] algorithm miti-gates pileup by utilizing additional information, such as shower shapes, to compute a weightfor each particle candidate. This weight describes the probability that the particle originatedfrom pileup and is used to rescale the particle’s four-momentum. After rescaling, particles withvery small weight or very small transverse momentum are discarded. Further mitigation of thepileup effect, particularly for neutral particles, can be achieved using the pointing ability in thehigh granularity calorimeter and the new precision timing in the barrel electromagnetic andhigh granularity calorimeters. Techniques using these detector features have not yet been fullydeveloped and implemented. The results presented are therefore considered to be conserva-tive.

Track reconstruction: efficient track reconstruction with low misreconstruction rate is crucialto the particle flow technique used for the reconstruction of all physics objects. Figure 18 showsthe tracking efficiencies for tt̄ events and the corresponding misreconstruction rates as a func-tion of the track η for tracks with pT > 0.9 GeV. The performance of the reference detectorat 140 and 200 pileup events is compared to that of the Phase I detector at 50 pileup events.The benefit from the extension of tracker coverage is also shown independently. It can be seenthat the track reconstruction efficiency at a pileup of 200 events is maintained at the level ofthe Phase I detector at 50 pileup. As the exact design of the pixel detector is not yet defined,the simulation uses the Phase I detector configuration with additional disks in the forward re-gion. This does not incorporate expected resolution improvements and therefore the resultspresented are likely conservative.

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Tagging of b-jets: the ability to distinguish the jets arising from the hadronization of b-quarks(b-jets) from the ones coming from the light partons is key to identify several physics processes.The decay of the top quark is a typical process that serves as a reference to estimate the per-formance. Figure 19 compares the b-jet tagging efficiency and misidentification rates for top-pair events simulated with 140 and 200 pileup events using the reference detector and with 50pileup events using the Phase I detector. It can be seen that the performance slightly degradeswith the increased level of pileup. Comparing the two pileup conditions with the reference de-tector, a relative drop in efficiency of about 5%, for a fixed misidentification probability of the

34 5 Comparative Performance studies

light-quarks, is observed. The efficiency is also improved by the forward tracking extension,which provides b-tagging in the high η range. The b-jet tagging performance is driven by theprecision of the pixel detector and the distance of the first measurement to the primary vertex.As previously mentioned it is also expected that the results will improve with an optimizedpixel detector design.

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Figure 19: b-jet tagging performance studied with top-pair events at <PU>= 140 (red) and<PU>= 200 (green) for different pseudorapidity ranges. Results based on Phase I detectorat <PU>= 50 (open black) are also shown. Performance is expressed as misidentificationprobability for udsg-jets (left) and c-jets (right) as a function of b-jet tagging efficiency. Eventsare selected only if the correct primary interaction vertex is reconstructed. Good b-jet taggingcapability extends to high pseudorapidity.

τ-lepton identification: the visible products of hadronic tau decays, namely π± and π0 mesons,are reconstructed from charged and neutral particle candidates. The classification of hadronictau decays is degraded by the presence of pileup because additional neutral particles frompileup events are more likely to be incorrectly associated with single charged pions in the iden-tification of decays. The identification of τ-leptons through isolation criteria is driven by theperformance of the charged particle measurement. Figure 20 shows that comparable misiden-tification rates can be achieved for the 140 and 200 pileup environments, with only a moderaterelative drop of about 10% in efficiency for simulated Z → τ+τ− events.

Electron and photon performance: the reconstruction efficiency for electrons and photonsslowly degrades with increasing pileup. Figure 21 shows the reconstruction efficiency forelectrons as a function of the number of simultaneous interactions in the barrel and endcapcalorimeters separately for simulated Drell-Yan events. A moderate drop in efficiency of about5% is observed in the barrel and about 8% in the endcap.

Figure 22 (left) shows the identification efficiency in the endcap calorimeter for electrons fromDrell-Yan events. In this region, the rate of misidentified electrons increases by about 60 % at90 % efficiency, while for constant misidentification rate, a drop of 5% in efficiency is observed.The performance for photons is similar to the electron performance. Figure 22 (right) showsthe photon selection efficiency and corresponding misidentification rate in the endcaps as afunction of |η| for simulated events with a photon and a jet. The working point is chosen suchthat the average efficiency across the |η| range 1.6-2.5 is approximately 85% for all samples.The effect of the limited rapidity coverage of the Phase I detector is visible in the plot. Pileup

5.3 Phase II detector performance at pileup 140 and pileup 200 35

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also degrades the resolution of the measurement of the energy of electrons and photons.

Jet and missing transverse energy (EmissT ) reconstruction: calorimetric measurements at low

transverse momentum are affected by the presence of pileup. Figure 23 shows, for jets fromQCD process, the increase in the offset and response for uncorrected jet energies as a function ofthe number of interactions per crossing. As expected, the additional energy in the reconstructedjets degrades the resolution. After applying PUPPI jet energy corrections, a degradation of 13(10)% for central jets with transverse momentum of 30 (100) GeV remains when the pileupincreases from 140 to 200.

Figure 24 (left) shows the EmissT distribution for the perpendicular component of the hadronic

recoil from a Z boson, measured in Z → µµ events. A degradation in EmissT resolution of

more than 15% is observed when the mean pileup increases from 140 to 200 interactions percrossing. Figure 24 (right) shows the relative increase in wrongly identified jets as a functionof pseudorapidity for these two pileup conditions. A substantial increase in additional jets is

36 5 Comparative Performance studies

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5.3 Phase II detector performance at pileup 140 and pileup 200 37

observed and the performance degrades at 200 pileup. These fake or “pileup jets” interferewith accurate counting of the number of jets in the event of interest, which is an importantaspect of many measurements and searches at the LHC.

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measured in Z → µµ, at <PU>= 140 with the high-η tracking extension (red) and <PU>= 200(green). Results for Phase I at <PU>= 50 (black) are also included (left). Relative rate ofadditional misidentified jets as a function of pseudorapidity for different pileup configuration.Results are shown relative to the performance of the reference detector at <PU>= 140 for<PU>= 200 (green), and Phase I at <PU>= 50 (black) (right).

The degradation in the reconstruction of jets and EmissT has significant impact on several anal-

yses. Figure 25 shows the effect of the degraded EmissT resolution on W±H+Emiss

T searches. Tocompensate for the increased background level, the event selection has been tightened withrespect to the reference in the TP. A significant reduction in the discovery potential is observed.In a dataset of 3000 fb−1 the mass reach for discovery is reduced from 940 GeV to about 800GeV. For measurements of the VBF Higgs to ττ final state, where the ττ mass is reconstructedusing the missing transverse energy, the performance of the analysis expressed as signal overthe square-root of background is degraded by about 15%. This translates into a 40% increasein the luminosity needed to achieve the equivalent result at 200 PU. In addition the rate ofjets reconstructed from pileup energy depositions reduces the signal yield and increases thebackground from Drell-Yan production, degrading the analysis performance by 25%.

Summary: The performance of the reference upgrade reference detector has been studied forpileup of 140 and 200 collisions per crossing and compared to the Phase I detector performanceat 50 collisions per crossing. The performance of the Phase I detector can be achieved with thereference detector in the presence of 140 collisions per crossing. The studies presented heredemonstrate that resolutions, efficiencies, and misidentification rates are degraded in eventswith 200 collisions per crossing. Objects measured predominantly with the tracker and thoseat large transverse momentum are less affected than objects measured using mostly calorimet-ric information or with low transverse momentum. As a result, crucial measurements of theH → 4µ, H → µµ, and searches or measurements of heavy resonance show limited sensitivityto pileup. Measurements relying on the missing transverse energy resolution or jet countingare significantly affected. The physics analyses that are very sensitive to these observablesindicate that improvements are needed to fully exploit the data collected at higher luminosi-ties. For the endcap calorimeter, this is addressed by the new timing and pointing capabilitiesof the High Granularity Calorimeter. It is expected that these new features will substantiallyimprove the mitigation of pileup. As well, the improved timing resolution in barrel electro-magnetic calorimeter is anticipated to provide improve photon energy resolution. Overall, the

38 5 Comparative Performance studies

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final states. The estimated effect of the degradation of EmissT resolution has been applied.

current studies demonstrates that for physics channels relying mostly on charged particles, theincreased luminosity at 200 pileup can be exploited. For other physics signatures, the studiesare continuing.

39

6 Project organization and planning6.1 Phase II organization in CMS

Each subsystem upgrade is organized and managed within the corresponding subsystem project.To this extent, there is a new subsystem, the Endcap Calorimeter, which includes the electro-magnetic and hadronic calorimeters in the endcap region.

The responsibility for the upgrade of each subsystem rests with the corresponding subsystemProject Manager (PM) with oversight from the corresponding Institution Board (IB). The re-sponsibilities of the PM cover all aspects of the project including operations and upgrades ofthe detector, as well as management of the allocated resources.

Responsibility for the full CMS detector rests with the CMS Spokesperson who delegates themonitoring and follow up of the various upgrades, along with their overall coherence, to theUpgrade Program Coordinator and the CMS Technical Coordinator. The Upgrade ProgramCoordinator and the Technical Coordinator will call, on a regular basis, reviews of the specificscope, the conceptual designs, the project planning and the resources for each upgrade project.These reviews will include at least one Annual Comprehensive Review where all UpgradeProjects will be reviewed at the same time, to establish the overall status and plans for the fullCMS upgrade.

A major task for the Upgrade Program Coordination is the supervision and coordination of thetimely production of the TDRs for the different parts of the CMS upgrade.

The start of construction will be marked by the approval of the TDRs. Following this, and inline with the construction of the initial CMS detector, oversight of the execution of the vari-ous upgrades will rest with the Technical Coordinator, who will organize all the correspondingreviews (Engineering Feasibility Review, Production Readiness Review, Engineering DesignReview, Electronics System Review, Manufacturing Progress Review, Engineering Change Re-view, Installation Readiness Review).

The CMS Resource Manager maintains overall oversight of the resources.

The CMS Management Board, Finance Board and Collaboration Board oversee the upgradeprogram along with all other aspects of the experiment. This overall organization ensurescommon oversight of priorities and resources across all activities of the experiment.

6.2 Project timeline

An outline of the project timeline is illustrated in Figure 26. The tracking system, endcapcalorimetry and track-trigger upgrades require significant R&D. The timeline for the projectsanticipate about 2 years for R&D to finalize the designs ahead of the TDRs, and a further 2-3 years for prototyping to develop production grade components. The construction is thenplanned for 4-5 years with installation during LS3, starting in 2024. Individual project timelinesare presented in the appendix. Compared to the Technical Proposal, they have been updatedaccording to the more recent HL-LHC schedule.

Technical Design Reports are anticipated from early 2017 to late 2017 except for the Trigger andDAQ upgrades. In order to maximize the benefit from advances in computing technology andcost reduction, it is expected that these TDRs and project plans will be finalized nearer to LS2,in 2020.

By the time of the TDRs, with progress in performance studies, technical developments andknowledge of cost and funding, it is anticipated that the overall CMS upgrade scope and the

40 6 Project organization and planning

specific project technical choices will be mostly finalized. This will proceed through the re-view process outlined above. The planning and resourcing of the projects will be established,allowing flexibility wherever possible, to fulfill the entire needs of the experiment.

6.3 R&D program

The CMS Technical Proposal describes the necessary R&D to develop technical solutions forthe Phase II upgrades. This is presented for each project at the component level, includingthe current state of progress and the planning of major milestones towards the preparation ofTechnical Design Reports and up to the start of the construction.

The R&D program is centrally approved and coordinated by CMS, however, the efforts of eachinstitution are typically resourced through the support that it directly receives from its fund-ing agency. To help planning these activities the estimate of the necessary personnel, and theinterests of institution in each area have been summarized in a document submitted to theRRB [18].

In developing the technical solutions CMS capitalizes as far as possible on common R&D withother projects and with CERN. Examples of this include the R&D on ASICs and fast opticaldata transmission, radiation-tolerant silicon sensors, cooling systems, powering schemes, lightmechanical structures. The collaborations are established through dedicated workshops suchas the ECFA High Luminosity LHC Experiments Workshop on Physics and Technology Devel-opments (in 2013 and 2014), and generic R&D programs at CERN (RD42, RD50, RD51, RD52and RD53). As for previous R&D, CMS works closely with industrial partners to develop costeffective specifications for the component production, and to ensure proper technology trans-fers.

Through this process, progress on several developments have been achieved in recent years.The baseline technologies for components are mostly identified and the work is evolving tofinalize their specifications and engineering designs. This effort needs to intensify, with con-trolled iterations for qualification and validation of the final grade components, individuallyand in system tests. While the overall required resources appear well covered, it is impor-tant that the associated funding becomes available on a relatively short time scale to fulfill thepresent project planning constraints.

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6.4 Cost profile 41

6.4 Cost profile

The estimated profile for CORE expenditures is illustrated in Figure 27. It has been updatedwith the new HL-LHC schedule, compared to the one presented in the Technical Proposal.Expenditures on construction will start in 2017 with a ramp-up through 2019 and a broad dis-tribution through 2024. With further development of designs for the TDRs, the cost estimateand profile for each project will be updated.

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Figure 27: Anticipated CORE cost profile with the installation occurring during LS3 in 2024-2026.

42 7 Concluding remarks

7 Concluding remarksCMS has carried out an extensive series of design studies and physics simulations to define thedetector upgrades needed to fully exploit the High Luminosity LHC. The proposed detectordesign, referred to as the reference design, is documented in a Technical Proposal [1].

In the Technical Proposal it is demonstrated that the tracker and the endcap calorimeters, thedetectors most exposed to radiation, must be replaced. Additionally, the hardware trigger mustbe improved and the front-end and data acquisition electronics must be upgraded to handlethe much higher data rates. The muon system does not need to be replaced but additionaldetector stations will be required to maintain the performance and provide redundancy, sincethe current chambers, being more than 20 years old at the time of the upgrade, will begin tofail.

In addition to ensuring the necessary performance of the upgraded detector, a major consider-ation in the development of the designs has been the containment of the associated costs. Theproposed detector configurations are therefore based on cost-effective technical solutions thatprovide sufficient rather than maximal performance. The Technical Proposal, which documentthe solutions chosen, demonstrates that the goal of maintaining the excellent performance ofthe Phase-I CMS detector at the baseline HL-LHC luminosity of 5× 1034cm−2s−1 can be met bythe reference design.

In all studies, the performance for reconstructing physics objects was obtained from the fullsimulation of the detector, and the results were used for the analyses of physics benchmarksin a simplified simulation allowing the generation of the required large background samples.With this procedure, CMS has now thoroughly evaluated the benefit of each upgrade, bothindividually and in several overall configurations of the detector, including some in whichupgrade elements are dropped or downgraded.

These studies demonstrate that all the upgrades are necessary and, equally importantly, thatthe reference designs are not over-performing. In particular, it is shown that the performancedegrades relatively smoothly when the instantaneous luminosity of the LHC is raised, from apileup of 140 interactions per beam crossing to 200 interactions per beam crossing. It is alsofound that some physics measurements are more sensitive than others to this degradation. Tosome extent, this will be addressed by new features of the calorimeter upgrades that are notyet implemented or evaluated in the simulations. Overall, the reference upgrade has sufficientperformance margin to benefit from the ultimate luminosity of 7.5× 1034cm−2s−1 achievable atthe HL-LHC, albeit with some loss of efficiency.

To address possible shortfalls in funding, an upgrade configuration of intermediate cost reduc-tion from 265 MCHF to about 242 MCHF has been prepared, based on the reference designand the performance studies available at the time of the Technical Proposal. In this representa-tive configuration, scope reductions are distributed across sub-detectors with a relatively moresubstantial downgrading in the High Granularity Calorimeter, for which the design is at anearlier stage of optimization. This configuration also descopes the computing power availableto online event selection, limiting the trigger rate capability. The studies at the baseline lumi-nosity show that compensating the performance degradation, of such a configuration, wouldrequire about 20% more LHC operation time, for important sections of the physics program.Partial recovery of the losses may be possible with further design and descope optimizations,and implementing additional computing power as the luminosity increases. Nevertheless, it isvery likely that, for some crucial signals that are limited by statistics, it will not be possible tobenefit significantly or even marginally from the higher luminosity potential of the accelerator.

43

A configuration costing about 208 MCHF has also been evaluated. It implements incrementsin scope reduction of the upgrade elements. Simulations of this detector configuration demon-strate that already at the baseline luminosity the physics program will be adversely affected inall thematic areas, with some major physics goals becoming unachievable.

It is also demonstrated that, in addition to the expected loss of performance, substantial de-scoping or downgrading of the upgrade elements, will also create important operational risksarising from the loss of redundancy in the detector measurements.

The next step for CMS, on the time scale of the next two years, is to prepare and documentTechnical Designs for the upgrades that require large construction projects. To optimize theallocation of the resources available to the CMS upgrade, the Technical Designs will not com-plete individually. Instead, they will be finalized only after reviewing all systems as part ofone upgrade program. To this end, planning and preparations for the upgrades will proceedwith central oversight of all projects through regular reviews that will monitor design progress,technical feasibility, and cost estimates. With this process, CMS intends to allow sufficient flex-ibility to make scope decisions, as needed, and at the appropriate point in time, particularlywhen more accurate estimates of the costs and the available funding are established.

It is expected that CMS institutions will contribute to the upgrades in their areas of interest.The process for determining the sharing of responsibilities will proceed within the generalrequirements to cover all needs and, to this end, several preliminary discussions have alreadybeen held within the collaboration. A model of the contributions to the various projects isemerging and these discussions will intensify once the upgrade project is endorsed and CMSproceeds to the preparation of the TDRs.

In summary, the continuation of the physics program at the LHC is necessary to exploit theearly and extraordinary discovery of a Higgs boson, which is now a tool for searching for newphysics through precision studies. Moreover, there are compelling reasons to expect furtherdiscoveries at the higher energy and luminosity that will be accessible over the next decadewith the LHC. In the decade after that, the High-Luminosity LHC will be the unique facilityto uncover all properties of the new particles that will have been discovered and to exploreall possible corners of the physics parameter phase space for additional particles and forces.CMS is fully committed to preparing an upgraded detector that will meet the unprecedentedexperimental challenges in this second phase of the LHC physics program. The collaboration isproceeding with the necessary studies to optimize the upgrade configurations and to developthe technical solutions through a solid R&D effort. In this process, the cost-effectiveness of theupgrades has been and remains a major concern.

CMS warmly thanks all the funding agencies for their sustained support and for the stronginterest they show in contributing to the unique scientific endeavor offered by the HL-LHC.

44 A Project planning and cost estimates

A Project planning and cost estimatesA.1 Tracker planning and cost estimate

Production

Prototypes

Module

Sensor and Component

ProductionPre−production

ED

RPrototyping

Pre−production

Mech/Systems

TD

R

Design andPrototyping

Prototyping

LS3

Commissioning

Installation and

CommissioningInstallation and

Preparation in UXC

Preparation in UXC

Testing

Detector Assembly

Off−detector electronics & systems

Mechanics Production

Module Assembly

Tracker AssemblyTesting

Off−detector electronics & systems

Mechanics Production

Module Assembly

Sensor and Component Production

ED

R

2016 2017 20182015 2019 2020 2021 2022 2023 2024 2025

Outer

Tracker

Inner

Pixel

Conceptual Schedule (v2)

2026

Figure 28: Timeline of the CMS Phase II Tracker upgrade

Table 8: CORE cost estimates for the Phase II Upgrades of the Pixel Detector and Outer Tracker.Estimated CORE cost in MCHF (2014)Outer Tracker Pixel Detector

Modules 2S 27.9 11.8PS 22.8

Mechanics 6.6 3.6BE electronics 12.4 2.0Power system 6.1 1.4Services cables and pipes 5.5 1.1Cooling system 4.2 0.6Infrastructure 3.5 2.7and installation

TOTAL 89 23

A.2 Barrel calorimeter planning and cost estimate

TD

R

LS3

reinstall SMs

ED

R

MegatileProduction

On−detectorElectronics

ES

R

Off−detectorElectronics

ReplacementMegatile

2016 2017 20182015 2019 2020 2021 2022 2023 2024 2025

Design andECAL

Barrel

HCAL

Barrel

Prototyping

Design

Conceptual Schedule (v2)

2026

Remove/refurbish/developmentEngineering

Figure 29: Timeline of the CMS Phase II barrel Calorimeters upgrades

A.2 Barrel calorimeter planning and cost estimate 45

Table 9: CORE cost estimate for the ECAL and HCAL barrel calorimeter upgrades.Estimated CORE cost in MCHF (2014)

Front end electronics 5.7Off-detector Services 4.3Total EB 10Rebuild tiles 1.1Optical distribution and installation 0.3Total HB 1.4Total Barrel Calorimeters 11.4

46 A Project planning and cost estimates

A.3 Endcap calorimeter planning and cost estimate

Prototyping

Prototyping

TD

R

Engineering and

Pre−production

Engineering and

Pre−production

LS3

Installatio

n an

d

Co

mm

ission

ing

Preparation in UXC

AssemblyEndcap

AbsorberAssembly

Absorber Production

Megatile Production

ED

R

Mechanics Production

Module Assembly

Cassette Assembly

Sensor Production

Endcap Assembly Test

Off−detector electronics

Electronics and Photo−sensor Production

ED

R

On−detector Electronics

Test

2016 2017 20182015 2019 2020 2021 2022 2023 2024 2025

BackingHE

HGCAL

2026

Conceptual Schedule (v2)

Figure 30: Timeline of the CMS Phase II Endcap Calorimeter upgrades

Table 10: CORE cost estimate for the endcap calorimetry system.Estimated CORE cost in MCHF (2014)

Mechanical Structures 5.4Silicon Modules 28.7On-detector electronics and services 3.4BE Electronics and Controls 2.5Power System 5.0Cooling System 7.5Assembly and Installation 1.4Total HGCAL 54Absorber 6.6Scintillating Tiles and WLS fibers 1.3Readout and on-detector electronics 1.2BE Electronics and Controls 0.3Power System 0.1Assembly and Installation 0.3Total Back HE 10Total Endcap Calorimeters 64

A.4 Muon systems planning and cost estimates 47

A.4 Muon systems planning and cost estimates

GE

1/1

TD

R

GE 1/1 Pre−production

GE 1/1 Production

Design and Prototyping

Design and Prototyping

Mu

on

TD

R

Design and Prototyping

Design and Prototyping

LS3

Test GE 1/1 Install

ED

R

ME0 Production

GE 2/1 Production

TestED

R

RE 3/1 and RE 4/1 Production

Prototyping

Prototyping Engineering andPre−production

Engineering andPre−production

Prototyping

Off−Det Elect. Prod

On−Det Elect. Prod

ES

RPre−productionEngineering and

Test

ES

RElectronics Production

Test

Test

Engineering andPre−production

2016 2017 20182015 2019 2020 2021 2022 2023 2024 2025

GEM

CSC

RPC

DT

2026

Installation and

Commissioning

Conceptual Schedule (v2)

LS2

Figure 31: Timeline of the CMS Phase II Muon system upgrades

48 A Project planning and cost estimates

Table 11: CORE Cost estimates for the Phase II Upgrades of the Muon System.Estimated CORE cost in MCHF (2014)Minicrate electronics 3.3Trigger electronics 2.1Opto-links 0.7Total DT electronics 6.1Electronics boards 3.1Low voltage system 0.2Opto-links 0.4Total CSC electronics 3.7Chamber assembly 2.5Electronics 3.2Power system 1.2Services 0.7Installation 0.3Total GE11 and GE21 7.9Chamber assembly 1.2Electronics 0.3Power system 0.5Services 0.2Installation 0.2Total RE31 and RE41 2.3Chamber assembly 1.2Electronics 2.0Power system 0.9Services 0.3Installation 0.2Total ME0 Muon extension 4.5Total Muons 25

A.5 Beam radiation instrumentation and luminosity planning and cost estimate 49

A.5 Beam radiation instrumentation and luminosity planning and cost estimate

TD

R

Production

Production

ED

R

ED

R

Integration

2016 2017 2018 2019 2020 2021 2022 2023 2024 2025

Luminosity

radiation

Beam and

monitors

EngineeringDesign and Prototyping

Design and Prototyping

2026

LS3

commissionInstall and

commission

Install and

development

development

Engineering

Conceptual Schedule (v2)

Figure 32: Timeline of the CMS Phase II BRIL upgrades

Table 12: Cost estimates for the Phase II Upgrades of the BRIL systems.Estimated CORE cost in MCHF (2014)

Beam Monitoring for Protection 0.7Machine-Induced Background 1.1Radiation Monitoring 0.3Luminosity Measurement 1.9Total Beam Monitors and Luminosity 4.0

50 A Project planning and cost estimates

A.6 Trigger/DAQ planning and cost estimate

TD

R

Pre−production

Prototyping and

Demonstrator

Specifications and

technology R&D

Installation and

Commissioning

Test

ES

R

Production

2016 2017 20182015 2019 2020 2021 2022 2023 2024 2025

LS2

Conceptual Schedule (v2)

2026

LS3

Figure 33: Timeline of the CMS Phase II Trigger and DAQ upgrades

Table 13: Estimated CORE cost for the L1 Trigger Upgrade.Estimated CORE cost in MCHF (2014)

Track Correlation Trigger 1.6Calorimeter Trigger 3.5Muon Track Finder (Barrel and Overlap) 0.7Muon Track Finder (Endcap) 0.9Global Trigger 0.6Total Trigger 7.3

Table 14: Estimated CORE cost for TTC/DAQ/HLT systems, for a 200 PU operating condition.

Estimated CORE cost in MCHF (2014)TTC system 1DAQ read out, network and storage 5HLT computing nodes 11Total 17

A.7 Infrastructure upgrades and logistic of work cost estimate 51

A.7 Infrastructure upgrades and logistic of work cost estimate

Table 15: CORE Cost estimates for the Phase II Infrastructure upgrades, common systems andsupport, and installation.

Estimated CORE cost in MCHF (2014)Magnet power and cryogenics 1.6Beampipe 1.7Infrastructure 6.1Test Facilities 0.4Surface facilities 0.3Safety Systems 2.3Electronics Integration 0.9Engineering Integration 3.2Technical support 8.9Total infrastructure, common systems and support, and installation 25

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