Marina Cobal

Università di Udine

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Physics at Hadron Colliders Part II

The structure of an event

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One incoming parton from each of the protons enters the hard process, where then a number of outgoing particles are produced. It is the nature

of this process that determines the main characteristics of the event.

Hard subprocess: described by matrix elements

An event: resonances

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The hard process may produce a set of short-lived resonances, like the Z0/W± gauge bosons.

Resonances

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•In this range the momentum scale is known at the permill level. • it is a cross-check of the detector performance in particular for the lepton energy measurements

The structure of an event: ISR

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One shower initiator parton from each beam may start off a sequence of branchings, such as q → qg,

which build up an initial-state shower.

Initial state radiation: spacelike parton shower

The structure of an event: FSR

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The outgoing partons may branch, just like the incoming did, to build up final-state showers.

Final state radiation: timelike parton showers

An event: Underlying events

•  Proton remnants ( in most cases coloured! ) interact: Underlying event,consist of low pT objects.

• There are events without a hard collision ( dependent on pT cutoff)

An event: Underlying events

Underlying event: •  Multi-parton interaction •  Beam-beam remnants •  Initial/final state radiation

Underlying Event

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•  Studying underlying event is crucial for understanding high pT SM events at LHC.

•  ingredient for many analyses. In fact they

affect: the jet reconstructions and lepton isolation, jet tagging etc..

•  One can look at charged track multiplicities Nch in transverse regions which are little affected by the high pT objects. •  Reasonably described by models

TransverseRegion

TransverseRegion

Toward Region

Away Region

3/ = q6

3/2 = q6

3/- = q6

3/-2 = q6

Leading Charged-Particle Jet = 0q

> [G

eV]

Tp<

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8R=0.2 Transverse region

ATLAS

[GeV]jetTp

10 20 30 40 50 60 70 80 90 100

MC

/DAT

A

0.8

1

1.2

The structure of an event: Pile up

In addition to the hard process considered above, further semi-hard interactions may occur between the partons of two other incoming hadrons.

‘Pile-up’ is distinct from ‘underlying events’ in that it describes events coming from additional proton-proton interactions, rather than additional

interactions originating from the same proton collision. ���

Pile up

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2012 ATLAS event; Z in µµ with 25 primary vertices

Z in µµ event with 25 vertices

•  Multiple interactions between partons in other protons in the same bunch crossing –  Consequence of high

rate (luminosity) and high proton-proton total cross-section (~75 mb)

•  Statistically independent of hard scattering –  Similar models used

for soft physics as in underlying event

Et ~ 58 GeV

Et ~ 81 GeV without pile-up

Prog.Part.Nucl.Phys.60:484-551,2008

Pile up

Et ~ 58 GeV

Et ~ 81 GeV with design luminosity pile-up

Prog.Part.Nucl.Phys.60:484-551,2008

Pile up •  Multiple interactions

between partons in other protons in the same bunch crossing –  Consequence of high

rate (luminosity) and high proton-proton total cross-section (~75 mb)

•  Statistically independent of hard scattering –  Similar models used

for soft physics as in underlying event

Challenge Pile up: example ETmiss

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•  Requirements on track vertexing

•  Number of reconstructed vertices proportional to the pile-up

•  Measure pile-up density event by event: Use it to subtract from the jets energy a pile-up term. do the same with isolation cones.

without PU suppression

with PU suppression

Important for quantities, affected by soft hadrons, for example; ET

miss = -| Σ pT |

Use data!

σtot=σ

EL+σ

SD+σ

DD+σ

ND

•  Inelastic hadron-hadron events selected with an experiment’s “minimum bias trigger”.

•  Usually associated with inelastic non-single-diffractive events (e.g. UA5, E735, CDF … ATLAS?)

Minimum bias events

¡  Need minimum bias data if want to: 1)  Study general characteristics

of proton-proton interactions 2)  Investigate multi-parton

interactions and the structure of the proton etc.

3)  Understand the underlying event: impact on physics analyses?

¡  In parton-parton scattering, the UE is usually defined to be everything except the two outgoing hard scattered jets: Beam-beam remnants.

1)  Additional parton-parton interactions.

2)  ISR + FSR ¡  Can we use “minimum bias” data

to model the “underlying event”? Ø  At least for the beam-beam

remnant and multiple interactions?

The underlying event

¡  The “soft part” associated with hard scatters

Minimum bias

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•  Non head-on collisions, with only low pT objects. Those are the majority of the events in which there is a small momentum transfer

Δp ~ h/Δx

•  Distributed uniformly in η: dN/dη = 6 •  On average the charged particles in the final

state have a pT~500 MeV

Not well described by models! Shape is sort of OK Normalisation is off

Minimum bias

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•  It is interesting by its own to study such events. Also an ingredient for many analyses you will see.

•  A necessary first step for precision measurements (such as top-quark mass)

•  A key ingredient to modelling pile-up •  As can be seen most of the events

do have quite low pT

•  Anyhow those events constitute a noise of few GeV per bunch crossing

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Monte Carlo Simulations •  Attempt to simulate all

physics and experimental aspects as well as possible in MC

•  Examples shown here: –  Pile-up –  Jet response –  Electron acceptance on

detector level –  Corrections from quark

to jets

•  Use data ('data-driven' techniques) to verify that MC is correct w.r.t all relevant aspects

•  Apply corrections (a.k.a. scale factors) to MC where necessary

[GeV]T

p20 30 40 210 210×2

No

n-p

ert

urb

ativ

e c

orr

ect

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ATLASPreliminary Simulation

Pythia 6 AMBT2B CTEQ6L1

Pythia 6 AUET2B LO**

Pythia 6 Perugia 2010

Pythia 8 4C

Herwig++ 2.5.1 UE7000-2

Pythia 6 AUET2B CTEQ6L1

Uncertainty

= 2.76 TeVs

R=0.4tanti-k

|y|<0.3

(a) R= 0.4

[GeV]T

p20 30 40 210 210×2

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n-p

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ATLASPreliminary Simulation

Pythia 6 AMBT2B CTEQ6L1

Pythia 6 AUET2B LO**

Pythia 6 Perugia 2010

Pythia 8 4C

Herwig++ 2.5.1 UE7000-2

Pythia 6 AUET2B CTEQ6L1

Uncertainty

= 2.76 TeVs

R=0.6tanti-k

|y|<0.3

(b) R= 0.6

Figure 2: Non-perturbative correction factors for the inclusive jets cross section for anti-kt jets with R=0.4 (a) and R= 0.6 (b) in the jet rapidity |y|< 0.3 as a function of the jet pT for Monte Carlo simulations

with various tunes. The correction factor derived from PYTHIA 6 with the AUET2B CTEQ6L1 tune (full-

square) is used as the correction factors for the NLO prediction in this measurement, with the uncertainty

indicated by the shaded area.

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[GeV]jetTp

20 30 40 210 210×2 310

MC

/ R

espo

nse

Dat

aR

espo

nse

0.90.920.940.960.98

11.021.041.061.08

1.1 = 0.4, LCW+JESR tanti-kData 2012

ATLAS Preliminary| < 0.8d = 8 TeV, |s

+jeta+jetZ

Multijet

Total in situ uncertainty

Statistical component

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Monte Carlo Simulations •  MC contains two aspects

–  description of detector response → efficiency, resolutions –  description of shapes (physics model) → acceptance

•  This allows to translate the cross section measurement into a determination

of a correction:

N.B. assuming good description of efficiency and acceptance by MC – uncertainty ?

Monte Carlo for Processes with jets

Parton shower

MC simulation of LHC event

Fig. 1 Pictorial representation of a tt̄h event as produced by an event generator. The hard interaction (bigred blob) is followed by the decay of both top quarks and the Higgs boson (small red blobs). Additionalhard QCD radiation is produced (red) and a secondary interaction takes place (purple blob) beforethe final-state partons hadronise (light green blobs) and hadrons decay (dark green blobs). Photonradiation occurs at any stage (yellow).

on the understanding of LHC physics. The construction, maintenance, validation and extension of eventgenerators is therefore one of the principal tasks of particle-physics phenomenology today.

The inner working of event generators

Fig. 1 pictorially represents a hadron-collider event, where a tt̄h final state is produced and evolves byincluding effects of QCD bremsstrahlung in the initial and final state, the underlying event, hadronisationand, finally, the decays of unstable hadrons into stable ones. Event generators usually rely on the fac-torisation of such events into different well-defined phases, corresponding to different kinematic regimes.In the description of each of these phases different approximations are employed. In general the centralpiece of the event simulation is provided by the hard process (the dark red blob in the figure), whichcan be calculated in fixed order perturbation theory in the coupling constants owing to the correspond-ingly high scales. This part of the simulation is handled by computations based on matrix elements,which are either hard-coded or provided by special programs called parton-level or matrix-element (ME)generators. The QCD evolution described by parton showers then connects the hard scale of colouredparton creation with the hadronisation scale where the transition to the colourless hadrons occurs. Theparton showers model multiple QCD bremsstrahlung in an approximation to exact perturbation theory,which is accurate to leading logarithmic order. At the hadronisation scale, which is of the order of afew ΛQCD, QCD partons are transformed into primary hadrons (light green blobs) by applying purelyphenomenological fragmentation models having typically around ten parameters to be fitted to data.The primary hadrons finally are decayed into particles that can be observed in detectors. In most caseseffective theories or simple symmetry arguments are invoked to describe these decays. Another impor-tant feature associated with the decays is QED bremsstrahlung, which is simulated by techniques thatare accurate at leading logarithmic order and, eventually, supplemented with exact first-order results. Aparticularly difficult scenario arises in hadronic collisions, where remnants of the incoming hadrons mayexperience secondary hard or semi-hard interactions. This underlying event is pictorially represented bythe purple blob in Fig. 1. Such effects are beyond QCD factorisation theorems and therefore no completefirst-principles theory is available. Instead, phenomenological models are employed again, with moreparameters to be adjusted by using comparisons with data.

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H"t" t"

p"p"

Hard partonic scattering Incoming parton distributions

QCD and QED radiation

Hadronisation Particles

Additional partonic scatters

Detector simulation

A Monte Carlo Event

Initial and Final State parton showers resum the large QCD logs.

Hard Perturbative scattering:

Usually calculated at leading order in QCD, electroweak theory or some BSM model.

Perturbative Decays calculated in QCD, EW or some BSM theory.

Multiple perturbative scattering.

Non-perturbative modelling of the hadronization process.

Modelling of the soft underlying event

Finally the unstable hadrons are decayed.

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Uncertainties

•  Statistical uncertainties, due to finite number of events •  Systematic uncertainties, due to errors and biases in the analysis

•  Simplest, most-often-used approach: assume that systematic errors are mutually independent, i.e. uncorrelated –  make list of all sources of systematic uncertainties –  remove those that are correlated with others –  repeat analysis for variation of each uncertainty separately –  add variations up in quadrature

•  More complex treatment of systematics not addressed today •  Most analysis work goes into dedicated studies aiming to minimize the systematic

uncertainty

Table of uncertainties 6 6 Results

section from the different sources.

Table 1: Summary of the individual contributions to the systematic uncertainty on the stt mea-surement. The uncertainties are given in pb. The statistical uncertainty on the result is givenfor comparison.

Source e+e� µ+µ� e±µ⌥

Trigger efficiencies 4.1 3.0 3.6Lepton efficiencies 5.8 5.6 4.0Lepton energy scale 0.6 0.3 0.2Jet energy scale 10.3 10.8 5.2Jet energy resolution 3.2 4.0 3.0b-jet tagging 1.9 1.9 1.7Pileup 1.7 1.5 2.0Scale (µF and µR) 5.7 5.5 5.6Matching partons to showers 3.9 3.8 3.8Single top quark 2.6 2.4 2.3VV 0.7 0.7 0.5Drell–Yan 10.8 10.3 1.5Non-W/Z leptons 0.9 3.2 1.9Total systematic 18.6 18.6 11.4Integrated luminosity 6.4 6.1 6.2Statistical 5.2 4.5 2.6

6 Results

The tt production cross section is measured by counting events after applying the selectioncriteria described in section 3. Table 2 shows the total number of events observed in data andthe number of signal and background events expected from simulation or estimates from data.Table 3 lists the mean acceptance (which contains contributions from W ! tnt, with leptonict decays) multiplied by the selection efficiency and the branching fraction in the dilepton finalstate, and the measured cross section for each of the three final states, e+e�, µ+µ�, and e±µ⌥,which give compatible results. The e+e� and µ+µ� channels have two additional sources ofuncertainty, arising from the DY background estimation and from the propagation of the JESto the ET/ estimation, which limit the precision of the measurement of stt in those final states.

A combination of the three final states using the BLUE method [37] yields a measured crosssection of stt = 239.0± 2.1 (stat.)± 11.3 (syst.)± 6.2 (lum.) pb for a top-quark mass of 172.5 GeV.In the combination, the systematic uncertainties are 100% correlated across channels, exceptthose associated to the lepton efficiencies, which have a correlation coefficient of 0.64 for e+e�with e±µ⌥ and 0.55 for µ+µ� with e±µ⌥. Finally, the uncertainties associated with the data-based estimates and the statistical uncertainties are taken as uncorrelated.

In this analysis the dependence of the acceptance on the top-quark mass is found to be quadraticwithin the present uncertainty of the top-quark mass [38]. The cross-section dependence in therange 160–185 GeV can be parametrized as

stt/stt (mt = 172.5) = 1.00 � 0.009 ⇥ (mt � 172.5)� 0.000168 ⇥ (mt � 172.5)2 (1)

where mt is given in GeV. Assuming a top-quark mass value of 173.2 GeV [38], a cross sectionvalue stt = 237.5 ± 13.1 pb is obtained.

Example: CMS top pair production in di-lepton channel

Experimental aspects

Theory uncertainties

backgrounds

8 7 Summary

corresponding to 5.3 fb�1. The result obtained by combining the three final states is stt =239± 2 (stat.)± 11 (syst.)± 6 (lum.) pb, in agreement with the prediction of the standard modelfor a top-quark mass of 172.5 GeV.

Table 2: Number of dilepton events after applying the event selection and requiring at least oneb jet. The results are given for the individual sources of background, tt signal with a top-quarkmass of 172.5 GeV and stt = 252.9 pb, and data. The uncertainties correspond to the statisticaland systematic components added in quadrature.

Number of eventsSource e+e� µ+µ� e±µ⌥

Drell–Yan 386±116 492±148 194±58Non-W/Z leptons 25±10 114±46 185±72Single top quark 127±28 157±34 413±88VV 30±8 39±10 94±21Total background 569±120 802±159 886±130tt dilepton signal 2728±182 3630±250 9624±504Data 3204 4180 9982

Table 3: The total efficiencies etotal, i.e. the products of event acceptance, selection efficiency andbranching fraction for the respective tt final states, as estimated from simulation for a top-quarkmass of 172.5 GeV, and the measured tt production cross sections, where the uncertainties arefrom statistical, systematic and integrated luminosity components, respectively.

e+e� µ+µ� e±µ⌥

etotal (%) 0.203 ± 0.012 0.270 ± 0.017 0.717 ± 0.033stt (pb) 244.3 ± 5.2 ± 18.6 ± 6.4 235.3 ± 4.5 ± 18.6 ± 6.1 239.0 ± 2.6 ± 11.4 ± 6.2

Acknowledgments

We congratulate our colleagues in the CERN accelerator departments for the excellent perfor-mance of the LHC and thank the technical and administrative staffs at CERN and at other CMSinstitutes for their contributions to the success of the CMS effort. In addition, we gratefully ac-knowledge the computing centres and personnel of the Worldwide LHC Computing Grid fordelivering so effectively the computing infrastructure essential to our analyses. Finally, we ac-knowledge the enduring support for the construction and operation of the LHC and the CMSdetector provided by the following funding agencies: BMWF and FWF (Austria); FNRS andFWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS,MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES and CSF (Croatia); RPF (Cyprus);MoER, SF0690030s09 and ERDF (Estonia); Academy of Finland, MEC, and HIP (Finland); CEAand CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH(Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU(Republic of Korea); LAS (Lithuania); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mex-ico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR(Dubna); MON, RosAtom, RAS and RFBR (Russia); MESTD (Serbia); SEIDI and CPAN (Spain);Swiss Funding Agencies (Switzerland); NSC (Taipei); ThEPCenter, IPST, STAR and NSTDA(Thailand); TUBITAK and TAEK (Turkey); NASU (Ukraine); STFC (United Kingdom); DOEand NSF (USA).

Individuals have received support from the Marie-Curie programme and the European Re-search Council and EPLANET (European Union); the Leventis Foundation; the A. P. Sloan

SM processes

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•  No hope to observe light objects ( W,Z,H) in the fully hadronic final state! •  We need to rely on the presence of an isolated lepton!

•  Fully hadronic final states can be extracted from the backgrounds only with hardO(100 GeV) pT cuts-> works for heavy objects!

QCD Sector

Snapshot of QCD

QCD vertices

Colour factors

QCD Potential

Jets from quarks and gluons •  Quarks and gluons cannot exist as free particles -> hadronization •  Collimated stream of charged and neutral hadrons -> QCD jets

Where do Jets come from at LHC?

1.8 TeVs =

14 TeVs =

pT

(TeV)

inclusive jet cross-section dσ 2

dηdpT η=0

nb

TeV

•  Fragmentation of gluons and (light) quarks in QCD scattering

•  Most often observed interaction at LHC

Multi-jet events at LHC

Jet multiplicity

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•  Another possible test of QCD is obtained by checking the jet multiplicity •  Tests also the modelling of the radiation

Where do Jets come from at LHC? •  Decay of heavy Standard

Model (SM) particles Prominent example:

t→ bW → jjjt→ bW → lν j

qq q q WW Hjjʹ′ ʹ′→ →% %

top mass reconstruction

Where do Jets come from at LHC?   Associated with particle production in Vector Boson Fusion (VBF)

E.g., Higgs

Where do Jets come from at LHC?

•  Decay of Beyond Standard

Model (BSM) particles –  E.g., SUSY

electrons or muons jets

missing transverse

energy

,jets

,leptons

Te f Tjf T ppM p= + +∑ ∑ l

What is a jet?

How to identify jets? Jet algorithm should collect all particles in the same way for: •  Leading order partons •  Partons+gluon emission •  Parton shower (soft) •  Hadrons-> detector

Jets •  Definition (experimental point of view):

bunch of particles generated by hadronisation of a common confined source –  Quark, gluon fragmentation

•  Signature –  Energy deposit in EM and HAD

calorimeters –  Several tracks in the inner detector

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•  Calorimeter energy measurement

- Gets more precise with increasing particle energy

- Gives good energy measure for all particles except µ’s and ν’s

-  Does not work well for low energies

-  Particles have to reach calorimeter, noise in readout

jet algorithms

Jet Reconstruction Task

Jet Reconstruction •  How to reconstruct the jet?

–  Group together the particles from hadronization

–  2 main types •  Cone •  kT

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Jet reconstruction algorithms: cone

Jet reconstruction algorithms: Kt

Di-jet quark flavours

arXiv:1210.0441v3

Jet physics: jet energy scale Before looking at jet physics be aware of few issues, first of all when we have steeply falling cross sections-> we have a sensitivity of its measurement from the energy scale -Jet energy determined from calorimeter (+tracking information) -Sophisticated calibration procedure

Different contributions to JES error. (jets reconstructed with the Anti-kT alogrithm cone 0.6 that is used in ATLAS)

Jet physics: JES calibration from data

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Different physics processes can be used to calibrate the JES. - recoil against Z and photons -reconstruction of W’s in ttbar events Such methods are useful for different energy ranges and can be used at different ECM

Jet production

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•  NLO QCD works over ~9 orders of magnitude! •  excellent exp. progress: jet energy scale

uncertainties at the 1-2% level •  for central rapidities: similar exp. and theo.

uncertainties, 5 - 10% •  inclusive jet data : starts to be important tool for

constraining PDFs, eg.also by using ratios at different c.o.m. energies