RESEARCH ARTICLE

Journey in the Search for the HiggsBoson: The ATLAS and CMS Experimentsat the Large Hadron ColliderM. Della Negra,1 P. Jenni,2 T. S. Virdee1*

The search for the standard model Higgs boson at the Large Hadron Collider (LHC)started more than two decades ago. Much innovation was required and diversechallenges had to be overcome during the conception and construction of the LHCand its experiments. The ATLAS and CMS Collaboration experiments at the LHChave discovered a heavy boson that could complete the standard model ofparticle physics.

One of the remarkable achievements of20th-century science is the revelation thata large number of natural phenomena

that characterize the world around us can bedescribed in terms of underlying principles ofgreat simplicity and beauty. The standard model(SM) of particle physics is built upon those prin-ciples. The SM comprises quarks and leptons asthe building blocks of matter, and describes theirinteractions through the exchange of force car-riers: the photon for electromagnetic interactions,the Wand Z bosons for weak interactions, and thegluons for strong interactions. Quarks are boundby the strong interaction into protons and neu-trons; protons and neutrons bind together intonuclei; electrons bind to nuclei by electromagneticinteraction to form atoms, molecules, and matter.The electromagnetic and weak interactions areunified in the electroweak theory (1–3). Althoughintegrating gravity into the model remains a fun-damental challenge, the SM provides a beautifulexplanation, from first principles, of much of Na-ture that we observe directly.

The SM has been tested by many experimentsover the past four decades and has been shownto successfully describe high-energy particle in-teractions. The simplest and most elegant wayto construct the SM would be to assert that allfundamental particles are massless. However,we know this to be untrue, otherwise atomswould not have formed and we would not exist.The question of the origin of mass of funda-mental particles is the same as the one that isposed in the unified electroweak theory: Whydoes the photon remain strictly massless whileits close cousins, the W and Z bosons, acquire a

mass some 100 times that of the proton? Togenerate mass of the Wand Z bosons, the electro-weak gauge symmetry must somehow be broken.For protons or neutrons, which are compositeparticles, most of the mass arises from the massequivalent of the internal energy due to theintense binding field of the strong interactions.

Nearly 50 years ago, a mechanism was pro-posed for spontaneously breaking this sym-metry (4–9) involving a complex scalar quantumfield that permeates the entire universe. Thisnew field has the same quantum numbers asthe vacuum. The quantum of this field is calledthe Higgs boson. In addition to imparting massto the W and Z bosons, this scalar field wouldalso impart masses to the quarks and leptons,all in proportion to their coupling to this field.After the discovery of the W and Z bosons inthe early 1980s, the search for the Higgs boson,considered to be an integral part of the SM,became a central theme in particle physics. Thediscovery of the Higgs boson would establishthe existence of this field, leading to a revolu-tionary step in our understanding of how Natureworks at the fundamental level. The elucidationof this mass-generating mechanism became oneof the primary scientific goals of the Large HadronCollider (LHC).

The mass of the Higgs boson (mH) is not pre-dicted by theory. Below mH = 600 GeV, previousdirect searches at the Large Electron Positroncollider (LEP), the Tevatron, and the LHC wereunable to exclude mass regions between 114 and130GeV (10–14). Furthermore, inDecember 2011,the AToroidal LHCApparatus (ATLAS) Collab-oration and Compact Muon Solenoid (CMS) Col-laboration experiments reported an excess of eventsnear a mass of 125 GeV (13, 14). The Tevatronexperiments, CDF and D0, recently reported anexcess of events in the range 120 to 135GeV (15).

In July 2012, the discovery of a new heavyboson with a mass around 125 GeV was an-

nounced at CERN by the ATLAS and CMS ex-periments (16, 17), and the current data areconsistent with the expectation for a Higgsboson. Here, we present an overview of thesearch for the Higgs boson and highlight someof the exciting implications. Despite its incrediblesuccess the SM is still considered incomplete. Anumber of questions remain: Why would themass of the Higgs boson be only ~102 GeV?What is dark matter? How does the matter-antimatter asymmetry arise? How is gravity to beincluded? Physics beyond the SM has been muchdiscussed over the past few decades, and suchphysics might manifest itself via the productionof exotic particles such as superparticles from anew symmetry (supersymmetry), heavy Z-likebosons in grand unified theories or theories withextra space-time dimensions.

Design challenges. In the 1980s, it was clearthat new accelerators were needed that couldreach energies beyond those that had allowedthe discovery of many of the subnuclear parti-cles within the SM. Several ideas were vigor-ously discussed concerning the accelerators anddetector concepts most capable of tackling themajor open questions in particle physics, oftenparaphrased as the “known unknowns,” and pos-sibly discovering new physics beyond the SM,the “unknown unknowns.” Finding the Higgsboson was clearly a priority in the first cate-gory and was expected to be challenging. Themass of the Higgs boson (mH) is not predictedby theory and could be as high as 1000 GeV(1 TeV). This required a search over a broadrange of mass, ideally suiting an exploratory ma-chine such as a high-energy proton-proton (pp)collider. The suitability arises from the fact thatthe energy carried by the protons is distributedamong its constituent partons (quarks and glu-ons), allowing the entire range of masses to be“scanned” at the same time. The main mecha-nisms predicted to produce the Higgs boson in-volve the combination of these subnuclear particlesand force carriers.

The accelerator favored at CERN to probethe TeV energy scale was a pp collider. The re-quired energy of the collider can be estimatedby considering the reaction in which a Higgsboson is produced with a mass, mH, of 1 TeV.This happens via the WW fusion productionmechanism. A quark from each of the protonsradiates a W boson with an energy of ~0.5 TeV,implying that the radiating quark should carryan energy of ~1 TeV so as to have a reasonableprobability of emitting such a W boson. Hence,the proton should have an energy of ~6 × 1 TeV,as the average energy carried by a quark insidethe proton is about one-sixth of the proton en-ergy. The LHC (18, 19) was designed to accel-erate protons to 7 TeV, an order of magnitudehigher than the most powerful available acceler-ator, with an instantaneous pp collision rate of800 million per second.

The Higgs Boson

1Imperial College, London SW7 2AZ, UK, and CMS, Geneva,Switzerland. 2CERN, Geneva, Switzerland, and ATLAS, Geneva,Switzerland.

*To whom correspondence should be addressed. E-mail:tejinder.virdee@cern.ch

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It was proposed to build the new acceler-ator in the existing underground tunnel ofLEP. Because the radius of the tunnel is fixed,the value of the magnetic field of the dipole-bending magnets had to be ~8.5 T to hold inorbit the 7-TeV protons. The main challengesfor the accelerator were to build ~1200 super-conducting dipoles, each 15 m in length, ableto reach this magnetic field; the large distrib-uted cryogenic plant to cool the magnets andother accelerator structures; and control sys-tems for the beams. The stored energy in eachof the beams at nominal intensity and energyis 350 MJ, equivalent to more than 80 kg ofTNT. Hence, if the beam is lost in an uncon-trolled way, it can do considerable damage tothe machine components, which would resultin months of down time [see (18, 19) for fur-ther details].

The LHC was approved in 1994 and con-struction began in 1998. A parallel attempt tobuild an accelerator that could reach even higherenergies was made with the SuperconductingSuperCollider (SSC) in Texas in the late 1980sbut was canceled in 1993 during the early stagesof construction.

This search for the Higgs boson also pro-vided a stringent benchmark for evaluating thephysics performance of various experiment de-signs under consideration some 20 years ago.The predicted rate of production ( = L⋅s, whereL is the instantaneous luminosity of the col-liding beams, measured in units of cm−2 s−1,and s is the cross section of the production re-action, measured in units of cm2) and naturalwidth (G = ħ/t, where t is the lifetime, and ħis Planck’s constant divided by 2p) of the SMHiggs boson vary widely over the allowed massrange (100 to 1000 GeV). Once produced, theHiggs boson disintegrates immediately in oneof several ways (decay modes) into known SMparticles, depending on its mass. A search hadto be envisaged not only over a large range ofmasses but also many possible decay modes: inpairs of photons, Z bosons, W bosons, t lep-tons, and b quarks. Not only is the putative SMHiggs boson rarely produced in the proton col-lisions, it also rarely decays into particles thatare the best identifiable signatures of its produc-tion at the LHC: photons, electrons, and muons.The rarity is illustrated by the fact that Higgsboson production and decay to one such dis-tinguishable signature (H → ZZ → ‘‘‘‘, where[[[‘]]] is a charged lepton, either a muon or anelectron) happens roughly only once in 10 tril-lion pp collisions. This means that a large num-ber of pp collisions per second must be studied;the current operating number is around 600 mil-lion per second, corresponding to an instanta-neous luminosity of 7 × 1033 cm−2 s−1. Hence,the ATLAS and CMS detectors operate in theharsh environment created by this huge rate ofpp collisions.

A saying prevalent in the late 1980s andearly 1990s captured the challenge that layahead: “We think we know how to build a high-energy, high-luminosity hadron collider—butwe don’t have the technology to build a detec-tor for it.” The role of the search for the Higgsboson at the LHC in influencing the design ofthe ATLAS and CMS experiments, and the ex-perimental challenges associated with operatingthem at very high instantaneous collision rates,are described in (20).

Different values of mH place differing de-mands on the detectors, none more stringentthan in the mass range mH < 2 × mZ (where themass of the Z boson mZ = 90 GeV). In design-ing the LHC experiments, considerable empha-sis was placed on this region. At masses below~140 GeV, the SM Higgs boson is producedwith a spread (natural width) of mass valuesof only a few MeV (i.e., a few parts in 105)such that the width of any observed mass peakwould be entirely dominated by instrumentalmass resolution. The Higgs boson decay modesgenerally accepted to be most promising in thisregion were those into two photons, occurringa few times in every thousand Higgs boson de-cays, and those, occurring even less often, intoa pair of Z bosons, each of which in turn de-cays into a pair of two oppositely charged lep-tons (electrons or muons). However, in a detectorwith a very good muon momentum resolutionand electron/photon energy resolution, the in-variant mass of the parent Higgs bosons couldbe measured precisely enough with the pros-pect of seeing a narrow peak, over background,in the distribution of the invariant masses ofthe decay particles (e.g., two photons or fourcharged leptons). Early detailed studies can befound in (21, 22).

Other signatures are associated with a Higgsboson. Most of these signatures are plagued bylarger backgrounds, as the signal characteristicsare less distinctive. For example, some of thesesignatures include narrow sprays of particles,known as “jets,” resulting from the fragmenta-tion of quarks. These represent the most likelyfinal states from the decay of a SM Higgs boson,but in a hadron collider they are overwhelmed bythe copious production from known SM pro-cesses. Among these jets are b quark jets char-acterized by a short (submillimeter) flight path inthe detector before disintegrating. Finally, neu-trinos can be produced, which, as neutral weaklyinteracting leptons, leave the detector without di-rect trace. Energy balance in the transverse planeof the colliding protons appears to be violated, asthe neutrino energy is not measured, leading tothe signature of missing transverse energy (Emiss

T ).For example, when a Higgs boson decays intotwo Z bosons, one can decay into a charged lep-ton pair and the other into a pair of neutrinos,leaving as final state an oppositely charged leptonpair and Emiss

T .

The conditions in hadron colliders are moreferocious than in electron-positron colliders. Forexample, at the LHC ~1000 charged particlesfrom ~20 pp interactions emerge from the inter-action region in every crossing of the protonbunches. Highly granular detectors (to give lowprobability of a given cell or channel registeringa hit or energy), with fast response and good timeresolution, are required. Tens of millions of de-tector electronic channels are hence required, andthese must be synchronized to cleanly separatethe different “bursts” of particles emerging fromthe interaction point every 25 ns. The enormousflux of particles emerging from the interactionregion leads to high radiation levels in the de-tecting elements and the associated front-endelectronics. This presented challenges for detec-tors and electronics not previously encounteredin particle physics.

The counterrotating LHC beams are orga-nized in 2808 bunches comprising some 1011

protons per bunch separated by 25 ns, leadingto a bunch crossing rate of ~40 MHz (the LHCaccelerator currently operates at 50-ns bunchspacing with 1380 bunches). The event selec-tion process (“trigger”) must reduce this rateto ~0.5 kHz for storage and subsequent de-tailed offline analysis. New collisions occur ineach crossing, and a trigger decision must bemade for every crossing. It is not feasible tomake a trigger decision in 25 ns; it takes about3 ms. During this time the data must be storedin pipelines integrated into onboard (front-end)electronics associated with almost every one of100 million electronic channels in each experi-ment. This online event selection process pro-ceeds in two steps: The first step reduces therate from ~40 MHz to a maximum of 100 kHzand comprises hardware processors that usecoarse information from the detectors; uponreceipt of a positive trigger signal (set by high-momentum muons or high-energy deposits incalorimeters; see below), the data from theseevents are transferred to a processor farm, whichuses event reconstruction algorithms operat-ing in real time to decrease the event rate to~0.5 kHz before data storage. The tens ofpetabytes that are generated per year per ex-periment are distributed to scientists locatedacross the globe and motivated the develop-ment of the so-called Worldwide LHC Com-puting Grid (WLCG) (23).

Timeline and general features of the ATLASand CMS experiments. To accomplish the phys-ics goals, new detector technologies had to beinvented and most of the existing technologieshad to be pushed to their limits. Several detectorconcepts were proposed; two complementaryones, ATLAS and CMS, were selected in 1993after peer review to proceed to detailed design(24–27). These designs were fully developed, andall elements prototyped and tested, over manyyears before construction commenced around

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1997. Today each experiment comprises morethan 3000 scientists and engineers from around180 universities and laboratories in around 40countries. Table 1 provides a timeline of thesedevelopments.

The typical form of a collider detector is a“cylindrical onion” containing four principal lay-ers. A particle emerging from the collision andtraveling outward will first encounter the innertracking system, immersed in a uniform mag-netic field, comprising an array of pixels andmicrostrip detectors. These measure preciselythe trajectory of the spiraling charged particlesand the curvature of their paths, revealing theirmomenta. The stronger the magnetic field, thehigher the curvature of the paths, and the moreprecise the measurement of each particle’s mo-mentum. The energies of particles are measuredin the next two layers of the detector, the elec-tromagnetic (em) and hadronic calorimeters. Elec-trons and photons will be stopped by the emcalorimeter; jets will be stopped by both the emand hadronic calorimeters. The only known par-ticles that penetrate beyond the hadron calori-meter are muons and neutrinos. Muons, beingcharged particles, are tracked in dedicated muonchambers. Their momenta are also measured fromthe curvature of their paths in a magnetic field.Neutrinos escape detection, and their presencegives rise to Emiss

T .ATLAS and CMS have differing but com-

plementary designs (28, 29). The single mostimportant aspect of the overall design is thechoice of the magnetic field configuration formeasuring the muons. The two basic configura-tions are solenoidal and toroidal, in which themagnetic field is parallel or azimuthal to the beamaxis, respectively. The CMS has a superconduct-ing high-field solenoid with a large ratio of lengthto inside diameter; ATLAS has a superconduct-ing air-core toroid. These are the two largestmagnets of their kind and hold a stored energy ofup to 3 GJ. In both magnets a current of ~20 kAflows through the superconductor. The CMS so-lenoid additionally provides the magnetic fieldfor the inner tracking system, whereas ATLAShas an additional solenoid magnet to carry outthe same function.

At the nominal pp collision rate, the particleflux varies from 108 cm−2 s−1 (at a radius of r =4 cm) to 2 × 106 cm−2 s−1 (at r = 50 cm), requir-ing small detection cells (channels) of typicalsize varying from 100 mm × 100 mm (pixels) to10 cm × 100 mm (microstrips). The more chan-nels there are, the easier it is to recognize thetrajectories of all the charged particles produced.In practice, the number of channels is limitedby the cost of the associated electronics, by thepower they dissipate (which in turn requires cool-ing fluids), and by the need to minimize theamount of material in front of the em calorim-eter. The inner tracker detectors, comprising sil-icon sensors and gaseous “straw” chambers, were

challenging to develop because of the need tooperate in a harsh radiation environment, espe-cially when close to the beam pipe. Radiation-hard electronics associated with each cell, with ahigh degree of functionality, needed to be packedinto as small a space as possible, using as littlematerial as possible.

In the early 1990s there were only twocomplementary possibilities for the em calo-rimeters that could perform in a high-radiationenvironment and had good enough electron andphoton energy resolution to cleanly detect thetwo-photon decay of the SM Higgs boson atlow mass: a lead–liquid argon sampling calo-rimeter, chosen by ATLAS, and fully sensitivedense lead tungstate scintillating crystals, chosenby CMS. Both are novel techniques, and eachwas tested and developed over many years be-fore mass production could commence. Theelectrons and positrons in the electromagneticshowers excite atoms of lead tungstate or ionizeatoms of liquid argon, respectively. The amountof light emitted, or charge collected, is propor-tional to the energy of the incoming electronsor photons.

The hadron calorimeters in each detector aresimilar and are based on known technologies:alternating layers of iron or brass absorber inwhich the particles interact, producing showersof secondary particles, and plastic scintillatorplates that sample the energy of these showers.The total amount of scintillation light detectedby the photodetectors is proportional to the in-cident energy.

The muon detectors used complementarytechnologies based on gaseous chambers: driftchambers and cathode strip chambers that pro-vide precise position measurement (and also

provide a trigger signal in the case of CMS), andthin-gap chambers and/or resistive plate cham-bers that provide precise timing information aswell as a fast trigger signal.

The electronics on the detectors, much ofwhich was manufactured in radiation-hard tech-nology, represented a substantial part of thematerials cost of the LHC experiments. The re-quirement of radiation hardness was previouslyfound only in military and space applications.

The construction of the various componentsof the detectors took place over about 10 yearsin universities, national laboratories, and indus-tries, from which they were sent to CERN inGeneva. This paper can do only partial justiceto the technological challenges that had to beovercome in developing, constructing, and in-stalling all the components in the large under-ground caverns. All the detector elements wereconnected to the off-detector electronics, anddata were fed to computers housed in a neigh-boring service cavern. Each experiment hasmore than 50,000 cables with a total length ex-ceeding 3000 km, and more than 10,000 pipesand tubes for services (cooling, ventilation, pow-er, signal transmission, etc.). Access to repairany substantial fault, or faulty connection, buriedinside the experiment would require months justto open the experiments. Hence, a high degreeof long-term operational reliability, which is usu-ally associated with space-bound systems, hadto be attained.

The design of the ATLAS experiment. Thedesign of the ATLAS detector (28) was basedon a superconducting air-core toroid magnet sys-tem containing ~80 km of superconductor cablein eight separate barrel coils (each 25 m × 5 m ina “racetrack” shape) and two matching end-cap

Table 1. The timeline of the LHC project.

1984 Workshop on a Large Hadron Collider in the LEP tunnel, Lausanne.1987 Workshop on Physics at Future Accelerators, La Thuile, Italy; the Rubbia “Long-Range Planning

Committee” recommends the LHC as the right choice for CERN’s future1990 European Committee for Future Accelerators (ECFA) LHC Workshop, Aachen, Germany

(discussion of physics, technologies, and designs for LHC experiments)1992 General Meeting on LHC Physics and Detectors, Évian-les-Bains, France (four general-purpose

experiment designs presented along with their physics performance)1993 Three letters of intent evaluated by the CERN peer review committee LHCC; ATLAS and CMS

selected to proceed to a detailed technical proposal1995 The LHC accelerator approved for construction1996 ATLAS and CMS technical proposals approved1997 Formal approval for ATLAS and CMS to move to construction (materials cost ceiling of 475 million

Swiss francs)1997 Construction commences [after approval of detailed engineering design of subdetectors

(magnets, inner tracker, calorimeters, muon system, trigger, and data acquisition)]2000 Assembly of experiments commences; LEP accelerator is closed down to make way for the LHC2008 LHC experiments ready for pp collisions; LHC starts operation; an incident stops LHC operation2009 LHC restarts operation; pp collisions recorded by LHC detectors2010 LHC collides protons at high energy (center-of-mass energy of 7 TeV)2012 LHC operates at 8 TeV; discovery of a Higgs-like boson

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toroid systems. A field of ~0.5 T is generatedover a large volume. The toroids are comple-mented with a smaller solenoid (diameter 2.5 m,length 6 m) at the center, which provides a mag-netic field of 2 T.

The detector includes an em calorimetercomplemented by a full-coverage hadronic cal-orimeter for jet and Emiss

T measurements. Theem calorimeter is a cryogenic lead–liquid argonsampling calorimeter in a novel “accordion” ge-ometry allowing fine granularity, both laterallyand in depth, and full coverage without any un-instrumented regions. A plastic scintillator–ironsampling hadronic calorimeter, also with a novelgeometry, is used in the barrel part of the ex-periment. Liquid argon hadronic calorimetersare used in the end-cap regions near the beamaxis. The em and hadronic calorimeters have200,000 and 10,000 cells, respectively, and arein an almost field-free region between the to-roids and the solenoid. They provide fine lateraland longitudinal segmentation.

The momentum of the muons can be pre-cisely measured as they travel unperturbed bymaterial for more than ~5 m in the air-coretoroid field. About 1200 large muon chambers

of various shapes, with a total area of 5000 m2,measure the impact position with an accuracybetter than 0.1 mm. Another set of about 4200fast chambers are used to provide the “trigger.”The chambers were built in about 20 collab-orating institutes on three continents. (Thiswas also the case for other components of theexperiment.)

The reconstruction of all charged particles,including that of displaced vertices, is achievedin the inner detector, which combines highly gran-ular pixel (50 mm × 400 mm elements leadingto 80 million channels) and microstrip (13 cm ×80 mm elements leading to 6 million channels)silicon semiconductor sensors placed close to thebeam axis, and a “straw tube” gaseous detector(350,000 channels), which provides about 30 to40 signal hits per track. The latter also helps in theidentification of electrons using information fromthe effects of transition radiation.

The air-core magnet system allows a rela-tively lightweight overall structure leading to adetector weighing 7000 tonnes. The muon spec-trometer defines the overall dimensions of theATLAS detector: a diameter of 22 m and alength of 46 m. Given its size and structure, the

ATLAS detector had to be assembled directlyin the underground cavern. Figure 1 shows oneend of the cylindrical barrel detector after about4 years of installation work, 1.5 years beforecompletion. The ends of four of the barrel to-roid coils are visible, illustrating the eightfoldsymmetry of the structure.

The design of the CMS experiment. Thedesign of the CMS detector (29) was based ona superconducting high-field solenoid, whichfirst reached the design field of 4 T in 2006.The CMS design was first optimized to detectmuons from the H → ZZ → 4m decay. Toidentify these muons and measure their momen-ta, the interaction region of the CMS detec-tor is surrounded with enough absorber material,equivalent to about 2 m of iron, to stop all theparticles produced except muons and neutrinos.The muons have spiral trajectories in the mag-netic field, which are reconstructed in the sur-rounding drift chambers. The CMS solenoidwas designed to have the maximum magneticfield considered feasible at the time, 4 T. This isproduced by a current of 20 kA flowing througha reinforced Nb-Ti superconducting coil builtin four layers. Economic and transportation

Fig. 1. Photograph of one end of the ATLAS detector barrel with the calorimeter end cap still retracted before its insertion into the barrel toroidmagnet structure (February 2007 during the installation phase).

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constraints limited the outer radius of the coil to3 m and its length to 13 m. The field is returnedthrough an iron yoke, 1.5 m thick, which housesfour muon stations to ensure robustness of mea-surement and full geometric coverage. The ironyoke is sectioned into five barrel wheels andthree end-cap disks at each end, for a total weightof 12,500 tonnes. The sectioning enabled thedetector to be assembled and tested in a largesurface hall while the underground cavern wasbeing prepared. The sections, weighing 350 to2000 tonnes, were then lowered sequentially be-tween October 2006 and January 2008, using adedicated gantry system equipped with strandjacks; this represented the first use of this tech-nology to simplify the underground assembly oflarge experiments.

The next design priority was driven by thesearch for the decay of the SM Higgs bosoninto two photons. This called for an em cal-orimeter with the best possible energy resolu-tion. A new type of crystal was selected: leadtungstate (PbWO4) scintillating crystal. Fiveyears of research and development (1993–1998)were necessary to improve the transparency andthe radiation hardness of these crystals, and it

took more than 10 years (1998–2008) of round-the-clock production to manufacture the 75,848crystals—more crystals than were used in allprevious particle physics experiments put to-gether. The last of the crystals was delivered inMarch 2008.

The solution to charged particle tracking wasto opt for a small number of precise positionmeasurements of each charged track (~13 eachwith a position resolution of ~15 mm per mea-surement), leading to a large number of cellsdistributed inside a cylindrical volume 5.8 m inlength and 2.5 m in diameter: 66 million siliconpixels, each 100 mm × 150 mm, and 9.3 millionsilicon microstrips ranging from ~10 cm × 80 mmto ~20 cm × 180 mm. With 198 m2 of active sil-icon area, the CMS tracker is by far the largestsilicon tracker ever built.

Finally, the hadron calorimeter, comprising~3000 small solid angle projective towers cov-ering almost the full solid angle, is built fromalternate plates of ~5 cm brass absorber and ~4-mm-thick scintillator plates that sample the energy.The scintillation light is detected by photodetec-tors (hybrid photodiodes) that can operate in thestrong magnetic field. Figure 2 shows the trans-

verse view of the barrel part of CMS in late 2007during the installation phase in the undergroundcavern.

Preparation of the experiments. All detec-tor components were tested at their productionsites, after delivery to CERN, and again aftertheir installation in the underground caverns.The experiments made use of the constant flowof cosmic rays impinging on Earth. Even atdepths of 100 m, there is still a small flux ofmuons—a few hundred per second traversingeach of the experiments. The muons were usedto check the whole chain from the hardware tothe analysis programs of the experiments, andalso to align the detector elements and calibratetheir response prior to pp collisions (30, 31).

The ATLAS and CMS experiments wouldgenerate huge amounts of data (tens of petabytesof data per year; 1 PB = 106 GB), requiring afully distributed computing model. The LHCComputing Grid allows any user anywhere ac-cess to any data recorded or calculated duringthe lifetime of the experiments. The computingsystem consists of a hierarchical architectureof tiered centers, with one large Tier-0 center atCERN, about 10 large Tier-1 centers at national

Fig. 2. Transverse section of the barrel part of CMS illustrating the successive layers of detection starting from the center where the collisions occur: the innertracker, the crystal calorimeter, the hadron calorimeter, the superconducting coil, and the iron yoke instrumented with the four muon stations. The last muonstation is at a radius of 7.4 m.

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computing facilities, and about 100 Tier-2 cen-ters at various institutes. The center at CERNreceives the raw data, carries out prompt recon-struction almost in real time, and exports theraw and reconstructed data to the Tier-1 centersand also to Tier-2 centers for physics analysis.The Tier-0 centers must keep pace with theevent rate of 0.5 kHz (~1 MB of raw data perevent) from each experiment. The large Tier-1centers provide long-term storage of raw dataand reconstructed data outside of CERN (a sec-ond copy). They carry out second-pass recon-struction, when better calibration constants areavailable. The large number of events simulatedby Monte Carlo methods and necessary for quan-tifying the expectations are produced mainly inTier-2 centers.

The operation of the LHC accelerator and theexperiments. The LHC accelerator began to op-erate on 10 September 2008. On 19 September2008, during the final powering tests of the maindipole circuit in the last sector (3-4) to be pow-ered up, an electrical fault in one of the tens ofthousands of connections generated a powerfulspark that punctured the vessel of a magnet, re-sulting in a large release of helium from the mag-net cold mass and leading to mechanical damageto ~50 magnets. These were repaired in 2009,and it was decided to run the accelerator at anenergy of 3.5 TeV per beam (i.e., at half the nomi-nal energy).

The first pp collisions (at an energy of 450 GeVper beam) occurred on 23 November 2009; thefirst high-energy collisions (at 3.5 TeV per beam)were recorded on 30 March 2010, and since then

the collider has operated smoothly, providing thetwo general-purpose experiments, ATLAS andCMS, with data samples corresponding to anintegrated luminosity of close to 5 fb−1 (fb,femtobarn) during 2011, and another 5 fb−1, at theslightly higher energy of 4 TeV per beam, up toJune 2012. In total, these data (~10 fb−1) corre-spond to the examination of some 1015 pp collisions.Typically, there are 20 overlapping pp interac-tions (“pile-up”) in the same crossing of protonbunches as the interaction of interest. ATLAS andCMS have recorded ~95% of the collision datadelivered with the LHC operating in stable condi-tions. In all, 98% of the roughly 100 million elec-tronic readout channels in each experiment havebeen performing at design specification. This out-standing achievement is the result of a constant anddedicated effort by the teams of physicists, engi-neers, and technicians responsible for the hard-ware, software, and maintenance of the detectors.This efficient operation of the accelerator and theexperiments has led to the discovery of the Higgs-like boson soon after the first pp collisions at highenergy.

The ATLAS and CMS experiments startedrecording high-energy pp collisions in March2010 after a preliminary low-energy run in theautumn of 2009. Many SM processes, includ-ing inclusive production of quarks (seen ashadronic jets), bottom quarks, top quarks, andW and Z bosons, have been measured with highprecision. These measurements, in a previouslyunexplored energy region, confirm the predic-tions of the SM. It is essential to establish thisagreement before any claims for new physics can

be made, as SM processes constitute large back-grounds to new physics.

Extensive searches for new physics beyondthe SM have also been performed. New limitshave been set on quark substructure, supersym-metric particles (e.g., disfavoring at 95% CLgluino masses below 1 TeV in simple models ofsupersymmetry), potential new bosons (e.g.,disfavoring at 95% CL new heavy W-like W´and Z-like Z´ bosons with masses below 2 TeVfor couplings similar to the ones for the knownW and Z bosons), and even signs of TeV-scalegravity (e.g., disfavoring at 95% CL black holeswith masses below 4 TeV).

Undoubtedly, the most striking result toemerge from the ATLAS and CMS experimentsis the discovery of a new heavy boson with amass of ~125 GeV. The analysis was carriedout in the context of the search for the SMHiggs boson.

For mH around 125 GeV, and from thenumber of collisions examined, some 200,000Higgs bosons would have been produced ineach experiment. Folding in the branching frac-tion, each experiment expected to identify acomparatively tiny number of signal events (e.g.,a few hundred two-photon events or tens offour-lepton events) from a hypothetical Higgsboson, before including factors of efficiency.The four–charged-lepton mode (H → ZZ →‘‘‘‘) offers the promise of the purest signal(S/B ~ 1, where S is the number of expectedsignal events and B is the number of expectedbackground events) and has therefore beencalled the “golden channel.”

Fig. 3. Event recorded with theCMS detector on 13 May 2012 ata proton-proton center-of-mass en-ergy of 8 TeV. The event shows char-acteristics expected from the decayof the SM Higgs boson to a pair ofphotons (dashed yellow lines andgreen towers). Solid yellow lines rep-resent the reconstructed trajectoriesof the charged particles produced inaddition to the two photons in thesame collision. The event could alsobe due to known SM backgroundprocesses.

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The search for the Higgs boson is carried outin a variety of modes. Below, we give some de-tails of the two modes that have the best invariantmass resolution and had played a particularlyimportant role in the design of the ATLAS andCMS experiments.

H → gg. In our detectors, the signature ofthe H→ gg decaymode is a pair of isolated photonseachwith a high transversemomentumof ~30GeVor higher. Transversemomentum is the componentof the momentum vector projected onto the planeperpendicular to the beams. Figure 3 shows suchan event recorded with the CMS detector.

Events containing two isolated photon can-didates were selected with the goal of identi-fying a narrow peak in the diphoton invariantmass distribution superimposed on a large back-ground. This background arises from two sources:the dominant and irreducible one from a varietyof SM processes, and a reducible backgroundwhere one or both of the reconstructed photoncandidates originate from misidentification ofjet fragments.

The criteria to distinguish real photons fromthose coming from jet fragmentation (labeled“fake photons”) depend on the detector technol-

ogies of the two experiments. Both experimentsare able to reject fake photons such that theircontribution to the background is only 25% ofthe total. The size of this contribution was thesubject of much debate in the 1990s, and thelow value has been attained through the designof the em calorimeters and the rejection powerof the associated analyses.

To enhance the sensitivity of the analysis,candidate two-photon events were separated in-to many mutually exclusive categories of dif-ferent expected S/B ratios. These categories aredefined on the basis of the expected properties

Fig. 4. (A) The two-photon invar-iant mass distribution in ATLAS ofselected candidate events weightedby the S/B value of the category inwhich it falls. The 7-TeV and 8-TeVdata are combined and correspondto a total integrated luminosity of10.7 fb–1. (B) The two-photon in-variant mass distribution in CMS ofselected candidate events weightedby the S/(S + B) value of the cat-egory in which it falls. The 7-TeVand 8-TeV data are combined andcorrespond to a total integrated lu-minosity of 10.4 fb–1.

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of the reconstructed photons and on the pres-ence of two jets expected to accompany a Higgsboson produced through the vector-boson fu-sion process, a particularly sensitive category.The analysis of events in each category rep-resented a separate measurement, with a specificmass resolution and background, and the re-sults from each category were statistically com-bined through a procedure that used likelihoodanalysis.

The distributions of the two-photon invar-iant masses, weighted by category, are shownin Fig. 4 for ATLAS and CMS, respectively,along with the best fit of a signal peak on topof a continuum background. The weight chosenwas proportional to the expected S/B in therespective category.

An excess of events, over the background,was observed at a mass of ~125 GeV by bothexperiments, corresponding to a local signifi-cance of 4.5 standard deviations (s) for ATLASand 4.1s for CMS.

H → ZZ → ℓℓℓℓ. The signature of the H →ZZ→ ‘‘‘‘ decay mode is two pairs of oppositely

charged isolated leptons (electrons or muons). Themain background arises from a small continuumof known and nonresonant production of Z bosonpairs. Figure 5 shows an event recorded with theATLAS detector with the characteristics expectedfrom the decay of the SM Higgs boson to a pairof Z bosons, one of which subsequently decaysinto a pair of electrons and the other into a pairof muons.

For a Higgs boson with a mass below twicethe mass of the Z boson, one of the lepton pairswill typically have an invariant mass com-patible with the Z boson mass (~91 GeV),whereas the other one will have a considera-bly lower mass, called “off-mass shell.” Be-cause there are differences in the instrumentalbackgrounds and in the mass resolutions forthe three possible combinations of electron andmuon pairs (4e, 4m, and 2e2m), the searcheswere made in these independent subchannelsand then combined statistically with a likelihoodprocedure. In the case of CMS, the angular dis-tribution of the four leptons is included in thelikelihood.

Figure 6 shows the four-lepton invariant massdistribution for the ATLAS and CMS experi-ments, in each case for the combination of allthe channels (4e, 4m, and 2e2m). The peak near90 GeV corresponds to the expected but raredecay of Z bosons to four leptons (Z → ‘‘‘‘).The rate is higher in the CMS experiment than inATLAS because of differing kinematic criteriaapplied to the four leptons. Both experimentsobserve a small but significant excess of eventsaround an invariant mass of about 125 GeVabovethe expected continuum background, with a spreadas expected from the mass resolution and statis-tical fluctuations corresponding to a local signif-icance of 3.4s and 3.2s in ATLAS and CMS,respectively.

Combined results. The ATLAS and CMSexperiments have both studied more Higgs bos-on decay modes than described in this paper,as discussed in the associated papers in thisissue and (16, 17). Figure 7 shows the com-bined statistical significance observed for thedifferent Higgs mass hypotheses by the ATLASand CMS experiments, respectively. The largest

Fig. 6. (A and B) The distribution ofthe four-lepton invariant mass, mℓℓℓℓ, inATLAS (A) and CMS (B) for the selectedcandidates relative to the backgroundexpectation. The signal expectation fora SM Higgs boson with mH = 125 GeVis also shown. The 7-TeV and 8-TeV dataare combined and correspond to a totalintegrated luminosity of 10.7 fb–1 forATLAS and 10.4 fb–1 for CMS.

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local significance is observed for a SM Higgsboson mass of mH = 126.5 and 125.5 GeV,where it reaches 6.0s and 5.0s, correspondingto a background fluctuation probability of 2 ×10−9 and 5 × 10−7 for the ATLAS and CMSexperiments, respectively. The expected local sig-nificance in the presence of a SM Higgs bosonsignal at these masses is found to be 4.9s forthe ATLAS experiment and 5.8s for the CMSexperiment. The evidence for a new particle isstrengthened by the observation in two differentexperiments, comprising complementary detec-tors, operating independently.

In both experiments the excess was mostsignificant in the two decay modes gg and ZZ.These two decay modes indicate that the newparticle is a boson; the two-photon decay im-plies that its spin (J ) is different from 1 (32, 33).Because the Higgs field is scalar, the spin of theSM Higgs boson is predicted to be zero.

Furthermore, the number of observed eventsis roughly equal to the number of events ex-pected from the production of a SM Higgsboson for all the decay modes analyzed, with-in the errors, in both experiments. The mea-sured value of the observed/expected ratio, forthe combined data from all the decay modes,was found to be 1.4 ± 0.3 and 0.87 ± 0.23 forthe ATLAS and CMS experiments, respective-ly. The best estimates of the masses measuredby the ATLAS and CMS experiments are alsoconsistent: 126.0 ± 0.6 GeVand 125.3 ± 0.6GeV,respectively.

Outlook. The results from the two exper-iments are consistent, within uncertainties, withthe expectations for the SM Higgs boson, afundamental spin-0 (scalar) boson. Much moredata need to be collected to enable rigoroustesting of the compatibility of the new bosonwith the SM and to establish whether the prop-erties of the new particle imply the existenceof physics beyond the SM. For this boson, at amass of ~125 GeV, almost all the decay modesare detectable, and hence comprehensive testscan be made. Among the remaining questionsare whether the bosons have spin J = 0 or J =2, whether their parity is positive or negative,whether they are elementary or composite, andwhether they couple to particles in the exactproportion predicted by the SM [i.e., for fer-mions (f ) proportional to mf

2 and for bosons(V) proportional to mV

4]. These properties arestudied via the new boson’s rate of decay intodifferent final states, the angular distributionsof the decay particles, and its rate of produc-tion in association with other particles such asW and Z bosons. The SM Higgs boson is pre-dicted to be an elementary particle with JP = 0+.Much progress is expected, as by the end of2012 the ATLAS and CMS detectors shouldbe able to triple the amount of data used forthe results presented here. The LHC will thenbe shut down in 2013 and 2014 to refurbish

parts of the accelerator so that it will be ableto reach its full design energy (14 TeV) andenable precise measurements of the propertiesof the new bosons and the full exploration ofthe physics of the TeV energy scale, especial-ly the search for physics beyond the SM.

It is known that quantum corrections makethe mass of a fundamental scalar particle floatup to the next highest physical mass scale cur-rently known, which, in the absence of ex-tensions to the SM, is as high as 1015 GeV. Afavored conjecture states that this is avoidedby a set of heavy particles not yet discovered.For each known SM particle there would be apartner with spin differing by half a unit; fer-mions would have boson partners and vice ver-sa, in a symmetry called supersymmetry. Thishappens because in quantum mechanics, cor-rections involving fermions and bosons haveopposite signs for their amplitudes and hencecancel each other. In the minimal supersym-metry model, five types of Higgs bosons arepredicted to exist. Furthermore, the lighteststable neutral particle of this new family ofsupersymmetric particles could be the particleconstituting dark matter. If, as conjectured, suchparticles are light enough, they ought to revealthemselves at the LHC.

The discovery of the new boson suggeststhat we could well have discovered a funda-mental scalar field that pervades our universe.Astronomical and astrophysical measurementspoint to the following composition of energy-matter in the universe: ~4% normal matter that“shines,” ~23% dark matter, and the rest form-ing “dark energy.” Dark matter is weakly andgravitationally interacting matter with no elec-tromagnetic or strong interactions. These arethe properties carried by the lightest supersym-metic particle. Hence the question: Is dark mat-ter supersymmetric in nature? Fundamentalscalar fields could well have played a criticalrole in the conjectured inflation of our uni-verse immediately after the Big Bang and inthe recently observed accelerating expansionof the universe that, among other measure-ments, signals the presence of dark energy inour universe.

The discovery of the new boson is widelyexpected to be a portal to physics beyond theSM. Physicists at the LHC are eagerly look-ing forward to establishing the true nature ofthe new boson and to the higher-energy run-ning of the LHC, to find clues or answers tosome of the other fundamental open questionsin particle physics and cosmology. Such a pro-gram of work at the LHC is likely to take sev-eral decades.

References and Notes1. S. L. Glashow, Nucl. Phys. 22, 579 (1961).2. S. Weinberg, Phys. Rev. Lett. 19, 1264 (1967).3. A. Salam, in Elementary Particle Physics: Relativistic

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4. F. Englert, R. Brout, Phys. Rev. Lett. 13, 321(1964).

5. P. W. Higgs, Phys. Lett. 12, 132 (1964).6. P. W. Higgs, Phys. Rev. Lett. 13, 508 (1964).7. G. S. Guralnik, C. R. Hagen, T. W. B. Kibble, Phys. Rev.

Lett. 13, 585 (1964).8. P. W. Higgs, Phys. Rev. 145, 1156 (1966).9. T. W. B. Kibble, Phys. Rev. 155, 1554 (1967).

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Hadron Collider Workshop, G. Jarlskog, D. Rein,Eds. Aachen, Germany, 1990, p. 474, CERN90-10-V-2/ECFA 90-133-V-2, http://cdsweb.cern.ch/record/215298.

22. M. Della Negra et al., in Proceedings of the LargeHadron Collider Workshop, G. Jarlskog, D. Rein,Eds. Aachen, Germany, 1990, p. 509, CERN90-10-V-2/ECFA 90-133-V-2, http://cdsweb.cern.ch/record/215298.

23. LHC Computing Grid, Technical Design ReportCERN-LHCC-2005-024 (2005).

24. ATLAS Collaboration, ATLAS: Letter of Intent fora General-Purpose pp Experiment at the LargeHadron Collider at CERN, CERN-LHCC-92-004(1992).

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Acknowledgments: The construction, and now theoperation and exploitation, of the large and complexATLAS and CMS experiments have required the talents,the resources, and the dedication of thousands ofscientists, engineers, and technicians worldwide. Manyhave already spent a substantial fraction of their workinglives on these experiments. This paper is dedicated toall our colleagues who have worked on these experiments.None of these results could have been obtained withoutthe wise planning, superb construction, and efficientoperation of the LHC accelerator and the WLCGcomputing.

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Hadron ColliderJourney in the Search for the Higgs Boson: The ATLAS and CMS Experiments at the Large

M. Della Negra, P. Jenni and T. S. Virdee

DOI: 10.1126/science.1230827 (6114), 1560-1568.338Science 

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