With about 10 millions top-pair events per year at low luminosity, the LHC will be a top factory. Among all precision measurements that will be performed in the top quark sector, the determination of its mass is probably the most important and will help in constraining the Standard Model. A brief review of the direct and indirect measurements of the top quark mass will be given, with indications about the potential of ATLAS and CMS in different luminosity scenario

Chierici, Roberto

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Available on CMS information server CMS CR 2006/072CMS Conference Report10 Octobre 2006Top mass at the LHCR. ChiericiCERN, Geneva, SwitzerlandAbstractWith about 10 millions top-pairs event per year at low luminosity, the LHC will be a top-factory.Among all precision measurements that will be performed in the top quark sector, the determinationof its mass is probably the most important and will help in constraining the Standard Model (SM).A brief review of the direct and indirect measurements of the top quark mass will be given, withindications about the potential of ATLAS and CMS in different luminosity scenarios.Presented at ICHEP 2006, Moscow, Russia, 26 July-2 August, 20061 IntroductionTop-pair production at the LHC is important for several reasons: it will allow to precisely measure the top quarkquantum numbers, to perform tests of QCD and, thanks to its large mass and coupling to the Higgs boson, toinvestigate possible presence of new physics, both in production and in decay mechanisms. Moreover, top-pairevents are important as main background to many search channels and as an excellent environment for calibrationof the jet energy scales and the b-tagging algorithm. Among all precision measurement in the top sector, the topmass has a key role in the constraining of the SM. Indeed, the Higgs boson mass is related to the other parameters ofthe model via loop corrections; since m   is still the one with the largest experimental uncertainty, an improvementof its measurement will greatly reduce the allowed range for m  and eventually discriminate between the SM andthe presence of new physics.The studies presented in this report will give an indication about the realistic potential of CMS and ATLAS in thedetermination of the top quark mass and are largely based on full detector simulations and on up-to-date generationtools. They are documented in detail in the CMS physics TDR [1] and in several notes updating the ATLAS physicsTDR [2].2 Top mass determinationTop-pair production at the LHC happens mainly via gluon fusion. Final states result from the decay of two topquarks into Wb, with a branching ratio close to unity, and the subsequent decay of the W boson. The resultingchannels are therefore called fully hadronic (  46%), semileptonic (  44%) and fully leptonic (  10%), dependingon the type of W decay. Triggering top-pair events is relatively straightforward in the (semi)leptonic cases, wherethe inclusive triggers of single isolated lepton or di-leptons can be used, whereas it is more complex for the fullyhadronic final state because of the large component of QCD background. In this case the trigger selection is basedon the inclusive jet trigger, where the thresholds depend on the number of jets and are lowered with respect toreference [3] in order to increase the efficiency, and on a fast b-jet tagging algorithm.There are two main approaches to determine the top quark mass: the first one uses the direct reconstruction of theinvariant mass via the detection of the   final state, the second one infers the value of m   by measuring quantiteswhich are correlated to it. The use of different approaches in determining the top quark mass is essential to reduce,in combination, the contribution of the correlated systematic errors, dominating the error on the measurement.2.1 Direct measurementsDirect measurements are based on the reconstruction of the invariant mass from the detected decay products.The simplest final states are the (semi)leptonic ones, where the trigger and the selection are made simpler by thepresence of at least an isolated lepton. In semileptonic events the top mass is reconstructed from the hadronicdecay of the Wb system, after proper calibration of the light and b jet energies. Both ATLAS [4] and CMS [5] haveinvestigated the possibility of fitting the resulting spectrum by using kinematic constraints imposing the value ofthe W boson in the decay and additionally the equality of the two top masses in the event. This procedure resultsin a substantial improvement in the resolution and a reduced sensitivity to systematics due to radiation. Figure 1shows the two expected mass spectrum for CMS with only 1/fb of integrated luminosity.Direct reconstruction can be applied also for the fully leptonic channel. In this case it is necessary to use aconstrained fit imposing the W boson masses and the equality of the two decaying top quark masses to a test mass(6 constraints) to cope with the missing information from the undetected neutrinos (6 unknowns). Each fit mayhave a solution to which a weight can be associated. The mass test value which maximizes the weight will providethe value of m   which is the most compatible with the kinematics of the event. The dominating contribution to theerror in this channel comes from the b jet energy scale calibration [4, 6].The top mass can also be directly reconstructed in the fully hadronic channel, but the sensitivity is quite reducedbecause of the enormous QCD background and the combinatorial background coming from the wrong pairing ofjets to form the two top quarks. The measurement is expected to be dominated by systematic errors coming fromthe jet energy scale and the control of the background.Another interesting approach consists in using only high transverse momentum top-pair semileptonic candidates(with p  200 GeV/c), determining the mass by collecting all the energy in a cone around the flight direction ofthe candidate “hadronic” top quark. The advantage of this method is that it is much less sensitive to combinatorialbackgrounds and to jet calibration. However, it is very sensitive to the description of the underlying event, whose2 (GeV)fittopm50 100 150 200 250 300 350nr. of events020406080100120 -1L = 1 fb Signalother TtbarW+jetsFigure 1: Top quark mass spectrum for the semileptonic channel for 1/fb. Both signal and expected backgroundsare indicated.energy must be properly subtracted and whose description represents the main source of error.2.2 Indirect measurementsThere are ways to determine the top mass indirectly. The most promising consists in using exclusive semileptonicfinal states with the production of a J/   meson from the hadronization of the b quark. The meson is identifiedby its decay into two light leptons and the top mass is inferred from its correlation with the invariant mass ofthe three leptons in the event [7, 8], as shown in figure 2. The selection is affected by a very low backgroundcontribution, mostly coming from combinatorial in top-pair events. The advantage of this method is that the topmass is reconstructed without b-tagging and with a limited use of jet reconstruction, making it insensitive to themost relevant systematics sources in the direct reconstruction methods. On the other hand, the branching ratio forthe final state is so low that it will be necessary to use a few years of data taking before approaching a statisticalerror competitive with the systematic one.Another possibility is to exploit the large dependence of the top-pair cross-section to the value of the top quarkmass. A 5% uncertainty in the measurement of the cross-section would already give an error on the top quarkmass of 2 GeV/c  , with a systematic breakdown largely uncorrelated to direct measurements. In practice, however,the cross-section itself will suffer from large errors from the knowledge of the proton p.d.f.s and the error on theluminosity, which make this analysis extremely difficult.3 Error on the top massAt the LHC the measurement of the top quark mass will soon be dominated by systematic uncertainties, theprecise determination of which is currently under study. It is interesting, however, to already have an estimate ofthe ones expected to give major contributions. They can be divided into two broad categories: instrumental onesand theoretical ones. The former include the jet and lepton energy scale and resolution, the b-tagging efficiencyand fake rate and the luminosity measurement. Preliminary studies [9] show that the jet energy calibration usingexternal samples like Z(  )+jet or the W mass constraint in top-pair events can bring the uncertainty on the jetenergy scale to a level of a few percent and that the use of top events can also assure an internal calibration of theb-tagging at the level of 4-5%. Theoretical uncertainties, on the other hand, reflect our poor understanding of thereality of a p-p collision and the imperfect way in which it is implemented in our Monte Carlo (MC) simulation.This category include the description of the quarks and gluons p.d.f.s in the proton, the underlying event and theminimum bias, and the description of the radiation and fragmentation. It is difficult to assess today the magnitudeof the systematics effects due to MC modelling; nevertheless recipes already exist [10], though entirely basedon simulation, and are currently used by the analyses to quantify these errors in the absence of data. When the3)2 (GeV/ctm170 172 174 176 178 180)2 (GeV/c-lΨJ/Reconstructed m69707172737475a = 0.56 +/- 0.052b = (-25.3 +/-  8.3) GeV/c + bt = a m-lΨJ/mFigure 2: Correlation plot at full simulation between the top quark mass and the three-lepton invariant mass in theexclusive J/   decays method (CMS). Only final states with electrons or muons are used.data will be available many more constraints and MC tuning, especially with QCD data, will help in reduce quitesignificantly these uncertainties.Table 1 shows the approximate expected error contributions on the top quark mass in the different analyses thathave been briefly described and with the present knowledge of simulations and tools. The systematic error isMethod   m    m    m    m (stat.) (syst. instr.) (syst. theory)bbqq  0.2 1.0 0.6 1.1bbqq  (high p  ) 0.2 0.9 1.4 1.7bb  0.5 1.0 0.3 1.2bbqbbq 0.2 2.3 3.5 4.2J/   decays 0.5 0.5 1.4 1.5Via 	 0.1 0.7 4.0 4.1Table 1: Expected top mass error breakdown for different analyses. The second column presents the expectedstatistical error after one year of data taking at low luminosity, with exception of the J/   channel, where full LHCstatistics is considered. The third and fourth columns present the expected systematic error split into instrumentaland theory components, whereas the last column summarizes the experimental error. All numbers are expressed inGeV/c.decomposed into purely instrumental contribution and theory contribution; it is evident that the combination ofseveral analysis will greatly help in reducing the error, and that particular care will have to be taken for a com-plete understanding of the totally correlated systematic errors, especially when coming from MC modelling. Thepossibility of having a final LHC combined error of about 1 GeV/cseems very well in reach.4 Top mass determination as a commissioning toolStudying the top mass is not only interesting per se, but also an invaluable tool for commissioning detector andtools. In a study performed with only 100/pb of luminosity, corresponding to few days of nominal LHC operation,and with the additional hypothesis of non-availability of any b-tagging, the ATLAS collaboration [11] showedthat it is already possible to clearly separate a top signal in the three-jets invariant mass in semileptonic events, asshown in figure 3, by simply selecting those events where two of the three jets are close to the nominal W mass.This exercise can help having top enriched samples with which it will be possible to perform a first light jet energycalibration and an estimate of the b-tagging efficiency using the fourth jet in the event. This analysis could also4m(3jet) (GeV)100 150 200 250 300 350 400 450 500Events / ( 9.95714 )01020304050Events / ( 9.95714 )Figure 3: Three jets invariant mass after selection and 100/pb luminosity in ATLAS without use of b-tagging.lead to the first physics measurements at the LHC.5 ConclusionsThe sector of top quark physics in general and the top quark mass determination in particular are extremely impor-tant to understand both physics and detector at the LHC. Moreover, m   will be essential to constrain the validity ofthe SM or of new physics. Both ATLAS and CMS collaborations are very active in studying the realistic potentialof the experiments in this measurement, which can be performed with the use of very different methods and thatcan achieve a final combined error of 1 GeV/cor less.References[1] CMS Coll., CERN/LHCC/2006-021.[2] ATLAS Coll., CERN/LHCC/99-15.[3] CMS Coll., CERN/LHCC/2002-26.[4] ATLAS Coll., Eur. Phys. J. C 39 (2005) 63.[5] J.D’Hondt et al., CMS NOTE 2006/066.[6] M.Davids et al., CMS NOTE 2006/077.[7] P.Grenier, ATL-PHYS-2001-023.[8] A.Kharchilava, Phys. Lett. B476, 73 (2000)R.Chierici,A.Dierlamm, CMS NOTE 2006/058.[9] J.Heyninck, proceedings TOP 2006, Coimbra, Portugal, January 2006.[10] P.Bartalini et al., CMS NOTE 2005/013.[11] S.Bentvelsen, M.Cobal, ATL-PHYS-PUB 2005-024.I.Van Vulpen, Proceedings of Physics at the LHC 2006 (Krakow).5

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