The Large Hadron Collider (LHC) will provide a huge amount of top-antitop events, making the LHC a top quark factory, producing 1 tt pair per second at a luminosity of 10^33cm-2s-1. A large top quark sample will be available from the start of LHC and will play an important role in commissioning the CMS and ATLAS detectors. An overview of the top quark measurements during the first data-taking period is given

Vander Donckt, M.

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Available on CMS information server CMS CR-2008/017The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report20 May 2008Top Physics at LHCM. Vander DoncktAbstractThe Large Hadron Collider (LHC) will provide a huge amount of top-antitop events, making the LHCa top quark factory, producing 1 tt pair per second at a luminosity of 1033cm−2s−1. A large top quarksample will be available from the start of LHC and will play an important role in commissioning theCMS and ATLAS detectors. An overview of the top quark measurements during the first data-takingperiod is given.Presented at Moriond QCD and High Energy Interactions,March 2008,La Thuile,ItalyTop quark physics at LHCM. Vander Donckt,on behalf of CMS and ATLAS CollaborationsCERN, 1211 Gene`ve 23, SwitzerlandThe Large Hadron Collider (LHC) will provide a huge amount of top-antitop events, makingthe LHC a top quark factory, producing 1 tt¯ pair per second at a luminosity of 1033cm−2s−1. Alarge top quark sample will be available from the start of LHC and will play an important rolein commissioning the CMS and ATLAS detectors. An overview of the top quark measurementsduring the first data-taking period is given.1 IntroductionThe Large Hadron Collider (LHC) will start to collide protons in July 2008. The large amount oftt¯ pairs produced will provide the opportunity to measure many top properties with a precisionnever reached before. The LHC can be considered as a real top quark factory as eight (three)millions of tt¯ pairs (single tops) will be produced per year during its first phase, correspondingto an initial luminosity of 1033cm−2s−1. Top quark pair production at the LHC happens mainlyvia gluon fusion. Final states result from the decay of two top quarks into Wb, with a branchingfraction close to unity, and the subsequent decay of the W boson. The resulting channels aretherefore called fully hadronic (46%), semi-leptonic (44%) and fully leptonic (10%), dependingon the W decays.Both ATLAS 1 and CMS 2 have conducted extensively studies in the top quark sector, trying tosearch for new physics by probing the Standard Model. Moreover, tt¯ events provide an excellentenvironment for calibration of the jet energy scale and b-tagging algorithms.2 Early top signalThe top quark pair production cross section will be one of the first relevant measurements tobe performed. The main challenge is to select a pure top quark sample without the applicationof b-tagging identification algorithms.ATLAS expects to isolate tt¯ events without requestingb-tagging within the first 100pb−1 of data. The selection of tt¯ pairs in the semi-leptonic eventsis performed by requesting a lepton with pt > 20GeV/c, 3 jets with ET > 40GeV , 1 jet withET > 20GeV and missing ET > 20GeV/c. The expected signal significance as a function of theintegrated luminosity is shown in figure 1.0 20 40 60 80 100 120 140 160 180 20002468101214161820)-1Integrated luminosity (pbExpected significanceBackground (nominal)Background (x2)Figure 1: ATLAS Top signal significance as a function of the accumulated statistics, for W+jets background andtwice this background to account for QCD background.3 Commissioning with top quarksTop quark events at the LHC are very complex and their reconstruction is based on almost eachcrucial aspect of the detector. Top quark are therefore perfect for the commissioning of thedetectors and measuring the physics performance of the experiments at start-up.The measurement of the jet energy scale calibration factors 4 can be extracted from the eventscollected in the first weeks of data taking: the W boson mass constraint can be used to reachan uncertainty of better than 1% with an integrated luminosity of about 300pb−1. The mainchallenge is the control of the pile-up collisions, for which vertex counting methods can beenvisaged.A tag and probe method is used by CMS3 to measure the b-tagging efficiency with semi-leptonictt¯ pairs. The b-tagged jet associated to the hadronic W is used as a tag while the jet on theleptonic side is probed. A 6% (10%)uncertainty can be reached in the barrel (endcaps) for anintegrated luminosity of 1fb−1.4 Cross-section and mass measurementsSeveral methods have been explored for different final state topologies and the studies haveshown that the expected uncertainty, for an integrated luminosity of 10 fb−1, on the mass mea-surement could reach this goal if detectors and theoretical systematic uncertainty are well undercontrol 5,6,7. A similar program for the cross section also exists, but the systematic uncertaintyis at present limited to 10% because of the currently relatively large uncertainties on b-taggingalgorithms and the luminosity.Among the large list of systematic contributions that enter the mass and cross-section measure-ments, the major effects (depending on the decay channel) are the light and b-jet energy scale,the initial/final state radiation modeling, the minimum-bias and underlying events estimate, theparton distribution functions, the uncertainty on b-tag performances.Figure 2: CMS barrel uncertainty of the b-tagging efficiency as a function of the b-jet ET .A 1.5% uncertainy is needed on the b-jet energy scale calibration to reach the precision of 1GeV/c2 on the top quark mass using the semi-leptonic tt¯ events. Most of the systematic effectshave to be extracted from independent data samples to have a robust and reliable evaluation.Both ATLAS and CMS have a detailed program during the initial period of data taking, tomeasure these effects directly from data.5 Spin correlationsUnlike light quarks, the top quark decays before hadronization or depolarization can take place.The study of the angular correlation between the decay products (b-jet, lepton or light quarkfrom W) in the top (anti-top) restframe and the top (anti-top) direction in the tt¯ restframegives the unique opportunity to study the top quark spin. A CMS analysis8 computes that thecorrelation coefficient can be measured with a total relative uncertainty of 17% with 10fb−1of semi-leptonic tt¯ data. The main sources of systematic uncertainties for this analysis are theb-tagging efficiency and the jet energy scale and multiplicity. ATLAS9 combines di-leptonic andsemi-leptonic analyses for 10fb−1 to reach a 4% precision for Standard Model spin correlations.6 Single top productionProduction of top quark events at LHC is not limited to tt¯ pairs. Indeed, single top productionvia the s and t channel, as well as the associated production of a top quark and a W boson,account for a third of the overall production cross-section of top quarks at the LHC. The NLOtotal cross section for each process amounts to 10 pb, 250 pb and 70 pb respectively 10. Becauseof the different final states and topologies, different selection strategies have to be developedand different analysis will be performed both in ATLAS11 and CMS12. The general systematicconsiderations accounted for the tt¯ mass and cross-section measurements apply also to measure-ments with single top quark events. A detailed overview of the CMS analysis for cross sectionmeasurement in single top quark production, reported in 12, shows that the t-channel analysisappears as the most favourable case due to its higher production cross section and better sig-nal over background ratio. The precision that can be achieved with an integrated luminosityof 10 fb1 is: ∆σσ= 2.7%(stat) ± 8.1%(syst) ± 3%(luminosity) The measurement of the crosssections are directly related to the |Vtb| matrix element in the CKM matrix. If the SM holds,stringent limits on couplings of new interactions can be established using the measurement ofthe t-channel cross section.7 Search for new physicsFlavour changing neutral currents (FCNC) are strongly suppressed in the Standard Model.Inthe top quark sector of the SM, these contributions limit the FCNC decay branching ratios tothe gauge bosons, BR(t → qX) (X = Z;γ,g), to below 10−10. Both ATLAS13 and CMS14 haveanalyses ready to search for FCNC in top quark decays as possible hints for physics beyond theStandard Model. The expected sensitivity reach with 10fb−1 (shown in figure 3) extends by 2orders of magnitudes that of the Tevatron ?.Figure 3: CMS branching ratios of FCNC top decays including (solid line) and excluding (dashed line) thecontribution from systematic uncertainties as a function of integrated luminosity, for t → qZ (left) and t → qγ(right), assuming a 5-sigma discovery level for an integrated luminosity of L=10fb−1.8 ConclusionsThe LHC will open a new era of precision measurements in the top quark physics that will leadto a thorough determination of the top quark properties. CMS and ATLAS are ready to exploitthe first top events to commission the detectors. This will be a first step towards stringent testsof the SM which are powerful probes in the search for new physics.References1. ATLAS Collaboration, CERN-LHCC-99-14/15,ATLAS-TDR-14/15 (1999).2. CMS Collaboration, CERN/LHCC 2006-001/021 (2006) CMS-TDR-8.1/8.23. S.Lowette et al., CERN-CMS-NOTE-2006-013.4. J.D’Hondt et al., CERN-CMS-NOTE-2006-025.5. I. Borjanovic et al.,Eur. Phys. J. C 39S2, 2005 (63) [arXiv:hep-ex/0403021].6. J. Heyninck et al.,CERN-CMS-NOTE-2006-06.7. M. Davids et al.,CERN-CMS-NOTE-2006-077.8. M. Baarmand et al., CERN-CMS-NOTE-2006-111.9. F.Hubaut et al., Eur.Phys.J. C 44, 2005 (13-33)10. Q. H. Cao and C. P. Yuan, Phys. Rev. D 71, 2005 (054022) [arXiv:hep-ph/0408180].11. A. Lucotte et al., ATLAS-PHYS-PUB-2007-00512. V. Abramov et al., CERN-CMS-NOTE-2006-08413. J.Carvalho et al., Eur.Phys.J.C 52, 2007 (999-1019)14. C. Karafasoulis et al;, CERN-CMS-NOTE-2006-09315. F. Abe et al., Phys. Rev. Lett. 80, 1998 (2525).

Top quark physics at the LHC

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