This paper describes the top quark physics measurements that can be performed
with the first LHC data in the ATLAS and CMS experiments.Comment: 6 pages, 2 figures. Talk given at `V Workshop Italiano sulla Fisica
  pp a LHC', Perugia, Italy, 30 January - 2 February 200

Acharya, Bobby S.

Cavallari, Francesca

Corcella, Gennaro

Di Sipio, Riccardo

Petrucciani, Giovanni

English

arXiv

This paper describes the top quark physics measurements that can be performed with the first LHC data in the ATLAS and CMS experiments.This paper describes the top quark physics measurements that can be performed with the first LHC data in the ATLAS and CMS experiments

Acharya, Bobby Samir

CERN Document Server

Available on CMS information server CMS CR-2008/021The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report6 May 2008Re-discovery of the top quark at the LHC and firstmeasurementsB.S. Acharya, F. Cavallari, G. Corcella, R. Di Sipio, G. PetruccianiAbstractThis paper describes the top quark physics measurements that can be performed with the first LHC inthe Atlas and CMS experiments.Presented at V Workshop italiano sulla fisica p-p ad LHC,30/01/2008-02/02/2008,Perugia,ItalyIL NUOVO CIMENTO Vol. ?, N. ? ?Re-discovery of the top quark at the LHC and first measurementsB. S. Acharya(1), F. Cavallari(2), G. Corcella(3)(4), R. Di Sipio(5)and G. Petrucciani(4)(1) Abdus Salam International Center for Theoretical Physics and INFN Sezione di Trieste,Italy(2) INFN, Sezione di Roma, Italy(3) Museo Storico della Fisica e Centro Studi e Ricerche E. Fermi, Italy(4) Scuola Normale Superiore and INFN, Sezione di Pisa, Italy(5) Universita` di Bologna and INFN Sezione di Bologna, ItalySummary. — This paper describes the top quark physics measurements that canbe performed with the first LHC data in the ATLAS and CMS experiments.PACS 14.65.Ha – Top quarks.1. – Top quark production at the LHCThe Large Hadron Collider (LHC) is the new CERN proton-proton collider thatwill start taking data in the course of 2008. Initially the collisions will take place at√s = 10 TeV, moving on to the planned√s = 14 TeV in early 2009.Top quarks at the LHC are produced mainly in tt¯ pairs via gluon fusion or qq¯scattering. The gluon-fusion mechanism accounts for about 90% of the cross-sectionand the onset of this subprocess at high energy is responsible for the much larger cross-section at the LHC compared to the Tevatron at Fermilab. The total pair-productioncross-section is estimated [1] to be σ(tt¯) =(833+52−39)pb. Single top production occursvia electroweak interactions, with a total cross-section of about 300 pb.Because |Vtb| ' 1, the top quark decays almost always as t → Wb. tt¯ events canbe then classified depending on how the W decays as: hadronic if both W ’s decayhadronically (branching ratio BR ' (2/3)2 ' 44%), semi-leptonic if one W decaysleptonically and one hadronically (BR ' 2 × 1/9 × 2/3 ' 15% for each lepton typee, µ or τ) or di-leptonic if both W ’s decay leptonically (BR ' (1/3)2 ' 11.1%).tt¯ events can be used to investigate many relevant observables and properties of topquarks: production cross-section, top-quark mass, charge, polarization, rare decays, suchas the ones involving flavour changing neutral currents (FCNC). The top quark is theheaviest and less studied quark: the study of its properties may open a window tonew physics. tt¯ events are also going to be important as a ‘validation tool’ of manyc© Societa` Italiana di Fisica 12 B. S. ACHARYA, F. CAVALLARI, G. CORCELLA, R. DI SIPIO and G. PETRUCCIANIphysics variables: in fact, tt¯ events contain leptons, jets, b-jets and missing transverseenergy. They can therefore be used to study the b-tagging performance and calibratethe jet energy scale. The quick observation of top-quark pair events will give theneeded credibility to possible signals of new physics. Top events also constitute animportant background for many searches beyond the Standard Model: as a first step, thedistributions of all important kinematic variables of these events must be well understood.Most of the top properties studies are performed using the semi-leptonic or di-leptonicchannels, since the presence of isolated leptons and missing energy ensure a higher puritythan the all-hadronic channel.In this paper we describe the tt¯ event topology, selection and possible measurements,with particular emphasis on the early data. The LHC luminosity will gradually increasein the first months; here we review the experiments potential in the top physics sectorfor a typical integrated luminosity between 100 pb−1 and 1 fb−1.A typical selection for 100 pb−1 has to take into account that the detector may not beperfectly calibrated and aligned. As a consequence, physics analysis tools like b-taggingor missing energy calculation will not be yet fully commissioned. Trigger selections andoﬄine selection cuts must then rely on simple and robust criteria.2. – Event selection2.1. Semi-leptonic events . – Semi-leptonic events with one electron or muon in the finalstate are characterized by one isolated lepton, some missing transverse energy, two heavyflavour jets and two light jets. The main backgrounds are single W boson productionwith associated jets, including the heavy-flavour channel (Wbb¯+jets), and QCD eventswith heavy flavour jets which produce leptons from B decays or fake leptons.In the early data-taking period, the most reliable triggers to study these events willlikely be the single-lepton triggers, which typically have a threshold at pT ' 20 GeV.Additional cuts will then be performed oﬄine to improve the lepton identification andthe isolation. At least four jets are required in the event within the tracker acceptance.b-tagging can be applied to these if a particular analysis requires it. Often a loose cuton the missing ET is applied, as well as additional kinematic cuts on the reconstructedW and top masses, or on the scalar sum of the transverse energies of leptons and jets.A typical efficiency in this channel is around 5% and the signal over background ratioS/B '3.2.2. Di-leptonic events . – Di-leptonic events with electrons or muons in the finalstate are characterized by two isolated leptons, missing transverse energy and two heavyflavour jets. The main backgrounds in this channel are single Z → l+l− or W → lνproduction with associated jets, including heavy-flavoured jets. In the W boson eventsthe second lepton can arise from the decay of heavy flavour jets or from a fake. Bosonpair production is also an important background.The presence of two isolated leptons allows one to select these events with high purity,especially the e−µ channel. These events often fire the single- and double-lepton triggers,following which additional cuts can be applied oﬄine to improve the lepton identificationand isolation. Two jets are required in the event within the tracker acceptance in orderto perform the b-tagging, if needed. A missing-ET cut and cuts on the reconstructeddilepton mass further suppress the QCD and Z backgrounds. A typical efficiency in thischannel is around 3-4%, with a signal over background ratio S/B ' 5− 10.RE-DISCOVERY OF THE TOP QUARK AT THE LHC AND FIRST MEASUREMENTS 32.3. Startup scenario. – A CMS study [2, 3] has shown that already with 10 pb−1it is possible to select some top-pair events and reconstruct a mass peak, as shown inFigure 1. This Monte Carlo study simulates mis-calibrated calorimeters and a mis-aligned tracker in realistic startup conditions. Relaxed cuts are applied in the eventselection and redundancy in the reconstruction process is used. For example, jets arereconstructed both with calorimeters and by using tracks, and lepton-based b-tagging isemployed, instead of methods based on vertex reconstruction.3. – Analysis for L between 100 pb−1 and 1 fb−1Both CMS and ATLAS have performed detailed Monte Carlo studies of the topphysics reach with an integrated luminosity between 100 pb−1 and 1 fb−1 [4, 5]. Atypical selection for 100 pb−1 has to take into account reduced detector calibration andalignement, and the possibility to evaluate efficiencies directly from the data.In the CMS study [4] the calorimeter calibration and tracker alignement are mis-calibrated at a level that is expected to be reachable with 100 pb−1 integrated luminosity.Jets are reconstructed using the iterative cone algorithm; the missing transverse energy iscalculated using calorimetric towers and taking into account the muon momentum. Bothoptions to use or not the b-tagging are considered. In the first case, b-jets are identifiedusing the track-counting algorithm and a loose b-tagging is generally employed. Thissimple algorithm considers the transverse impact parameter of the N -th track in a jet,where the tracks are sorted by impact parameter. In the di-leptonic events with taus(eτ and µτ), the τ -jet tagging algorithm is used to select a narrow, isolated calorimetricjet, well matched with one or three tracks and possibly with some indication of largeimpact parameter. Techniques to evaluate the efficiency and misidentification rate forthe b-tagging and lepton-selection directly from the data have been studied.In ATLAS, the cross-section is determined both in the semi-leptonic and dileptonchannel. For the semi-leptonic channel the measurement is performed with and with-out relying on b-tagging. A strategy is studied to evaluate the W+jets backgroundnormalization using the inclusive W , Z and Z+jets cross-sections via the followingrelation: σ(Wincl)/σ(W + nj) = σ(Zincl)/σ(Z + nj), which is valid to a few percentlevel as suggested by [6].A crucial point in these analyses is the control of the QCD background, characterizedby a very large cross-section and small efficiency. Thus, extremely large Monte Carlosamples would be needed to correctly estimate the contamination by these processes.To calculate the efficiency, two techniques were used by the CDF and D0 experimentsat the Tevatron: the cut factorization and the ‘matrix’ method. The cut-factorizationtechnique consists in finding two sets of uncorrelated cuts, measuring the two efficienciesseparately on Monte Carlo events and assuming that the total efficiency is the productof the two. The matrix method estimates the QCD background directly from the databy extracting the QCD efficiency in a signal-free control region, by loosening one of theanalysis cuts.4. – Top Physics Reach at Start-upBoth ATLAS and CMS are putting great effort into studies with very low integratedluminosity. The first measurement is the rediscovery of the top quark: event selection inthe di-lepton and semi-leptonic channels and mass peak reconstruction. This is withinreach already with about 10 pb−1.4 B. S. ACHARYA, F. CAVALLARI, G. CORCELLA, R. DI SIPIO and G. PETRUCCIANIWith about 100 pb−1, the tt¯ cross-section can be measured with an error of the orderof 15%, and a first mass measurement can be given with a few hundred pb−1. However,a precise mass measurement (below 1 GeV precision) requires detailed understandingof many systematic effects, due to both the detector and the techniques of analysis:calibrations, energy scales, missing-energy evaluation, possible biases in kinematic recon-structions, etc.4.1. tt¯ Invariant Mass Spectrum. – Starting from L = 1 fb−1 ATLAS [7] and CMS willgive a measurement of the tt¯ invariant-mass spectrum. This spectrum will be sensitive tott¯ resonances and other new physics effects. Figure 2 shows the expected ATLAS reachfor a generic narrow tt¯ mass resonance.4.2. Single Top Events . – Besides tt¯-pair production, single top quarks will also beproduced via electroweak processes at the LHC. There are three main production chan-nels: the t-channel has the largest cross-section (∼ 240 pb), followed by tW associatedproduction (∼ 60 pb) and finally the s-channel (∼ 10 pb). The single-top productioncross-section can give a measurement of the CKM matrix element Vtb. The mostimportant backgrounds come from tt¯ and W+jets events, which are difficult to reduce.An analysis by ATLAS [8] has shown statistical significance already for O(100 pb−1). Astudy by CMS in [9] shows that the t-channel will be easily observable with 10 fb−1 [9].Furthermore, rescaling the statistical and systematic uncertainties obtained with the10 fb−1 analysis, it is found that single-top production in the t-channel can also bevisible at 5σ confidence level even with a luminosity of 1 fb−1.4.3. Top Charge. – A preliminary study by ATLAS [7] has concluded that it is possibileto measure the top charge by pairing the lepton and the b-jet via their invariant mass.The jet-charge computation is made from the charges of the jet tracks weighted by theirproximity to the jet axis. A discrimination between charges +2/3 and −4/3 is possiblewith 1 fb−1.4.4. FCNC in Top Decays . – Searches for flavour changing neutral currents in top-quark decays will benefit from the high luminosity and large production cross-section fortt¯ pairs. According to a study by CMS [9], with 10 fb−1 t → qZ and t → qγ decays willbe observable with 5σ significance down to a BR ∼ 10−3, comparable to the estimatedsignals in some Beyond-SM models, such as R-parity violating SUSY. Preliminary results[7] show that even with an integrated luminosity of 1 fb−1 ATLAS is sensitive to FCNCdecays with BR(t → qZ) ∼ 10−3 and BR(t → qγ) ∼ 10−4.5. – ConclusionsThe LHC will give high statistics samples of tt¯ pairs that will allow detailed studiesof the top quark properties. Rediscovery of the top quark is possible already at verysmall integrated luminosity (10 pb−1). Such a quick confirmation of top-quark pairevents will give credibility to possible signals of new physics in the ATLAS and CMSexperiments. With more luminosity, ATLAS and CMS will measure precisely manyimportant properties of top quarks and will be sensitive to various sources of new physics,such as tt¯ resonances and flavour changing neutral currents. The importance of top quarkevents at the LHC cannot be emphasised enough.RE-DISCOVERY OF THE TOP QUARK AT THE LHC AND FIRST MEASUREMENTS 5REFERENCES[1] NLL resummation of the heavy-quark hadroproduction cross section, Roberto Bonciani,Stefano Catani, Michelangelo Mangano, Nucl Phys B, 529 (1998) 424-450[2] Expectations for observation of top quark pair production in the dilepton final state with thefirst 10 pb−1 of CMS data, The CMS Collaboration, CMS PAS TOP-08-0012008[3] Observability of Top Quark Pair Production in the Semileptonic Muon Channel with thefirst 10 pb−1 of CMS Data The CMS Collaboration, CMS PAS TOP-08-0052008[4] Measurements of the tt¯ cross section in the dilepton channel with L = 100 pb−1 using theCMS Detector, The CMS Collaboration, CMS PAS TOP-08-0022008[5] Determination of the Top quark production cross-section in ATLAS The ATLASCollaboration, ATLAS Note in preparation.[6] F.A.Berends et al., Phys. Lett. B, 224 (1989) 237[7] Top Quark Properties, The ATLAS Collaboration, ATLAS Note in preparation.[8] Prospects for Single top measurements in ATLAS The ATLAS Collaboration, ATLASNote, in preparation.[9] CMS Physics TDR: Volume II (PTDR2), Physics Performance: CERN-LHCC-2006-021, 25June 2006.Fig. 1. – CMS Monte Carlo study of the semileptonic muon tt¯ channel with the first 10 pb−1data: invariant mass of the three jets with the highest vectorially-summed ET .6 B. S. ACHARYA, F. CAVALLARI, G. CORCELLA, R. DI SIPIO and G. PETRUCCIANI]2Mass [GeV/c500 1000 1500 2000 2500 3000]2Resolution [GeV/c0204060801001201401601802001.81711x-607.785  !f(x) = 1.62261Fit function : ATLAS PreliminaryFigure 15: Z′ mass resolution as a function of theZ′ mass.]2Mass [GeV/c500 1000 1500 2000 2500 3000 Br) [pb]! "(-210-110110210-1Luminosity = 1 fb-1Luminosity = 10 fb-1Luminosity = 30 fb-1Luminosity = 100 fb-1Luminosity = 300 fbATLAS PreliminaryFigure 16: Discovery potential of a generic nar-row tt resonance as a function of the integratedluminosity.6.3 Discovery potential6.3.1 Method and resultsThe method used to extract the discovery sensitivity consists in counting the number of Standard Modelttevents in a sliding mass window over the invariant mass spectrum. The width of the window is twicethe detector resolution for a given resonance mass. Then, the lowest cross-section times branching ratio(" ×Br) is computed for the discovery of a resonance at a given mass.The produced resonances are expected to have a width smaller than the resolution, leading to agaussian shape for the reconstructed invariant mass. The discovery potential is thus estimated here onlyfor narrow tt resonances.This method, explained in detail in [75], requires as input the Z′ mass resolution (Figure 15), thereconstruction efÞciency of both Standard Modeltt events and resonance events, and the purity of Þnalsamples.The resulting sensitivity is shown on Figure 16. For example, a 700 GeV Z′ resonance produced witha (" ×Br) of 11 pb should be discovered with a 5" signiÞcance after 1 fb−1 of data taking. The tt massspectrum associated with such a case is shown in Figure 17.6.3.2 Measurement uncertaintiesUncertainties on the sensitivity arise from:• the reconstruction efÞciency for the Z′ signal and tt background. The main contribution arisesfrom the expected error on the b-tag efÞciency, which is set to ±5 %.• the background contribution (Standard Modeltt). The main contribution comes from the tt cross-section uncertainty +6.2−4.7 %.• the tt mass resolution (Figure 15). The effect on the discovery potential increases with the reso-nance mass.• the luminosity of collected data.The discovery potential is given as a function of the tt resonance mass. The position of the resonancemass peak depends on the accuracy of the jet energy scale. This will be constrained by the W and topevents themselves.29Fig. 2. – Reach for a generic narrow resonance decaying into tt¯ pairs in ATLAS.

Re-discovery of the top quark at the LHC and first measurements

Acharya, Bobby

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Re-discovery of the top quark at the LHC and first measurements.

Re-discovery of the top quark at the LHC and first measurements

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