To appear in the proceedings of 41st Rencontres de Moriond on QCD and Hadronic Interactions, La Thuile, Italy, 18-25 Mar 2006.The new CERN proton-proton collider, the LHC, is about to start in 2007 its data taking. Millions of top quarks will be available out of these data, allowing to perform a wide range of precision measurements and searches for new physics. An overview of the planned top physics program accessible with ttbar events is given for the ATLAS and CMS experiments. A particular emphasis is put on the precision measurements of the top mass, top polarization and searches for new physics in top production and decay

Hubaut, F.

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Top physics at LHC with ttbar eventsF. HubautTo cite this version:F. Hubaut. Top physics at LHC with ttbar events. 41st Rencontres de Moriond on QCD andHadronic Interactions, Mar 2006, La Thuile, Italy. 2006. <in2p3-00068033>HAL Id: in2p3-00068033http://hal.in2p3.fr/in2p3-00068033Submitted on 10 May 2006HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.in2p3-00068033, version 1 - 10 May 2006TOP PHYSICS AT LHC WITH tt¯ EVENTSF. HUBAUTOn behalf of the ATLAS and CMS collaborationsCPPM, CNRS/IN2P3, Univ. Me´diterrane´e,163 Av. de Luminy, Case 902, 13288 Marseille Cedex 9, FranceThe new CERN proton-proton collider, the LHC, is about to start in 2007 its data taking.Millions of top quarks will be available out of these data, allowing to perform a wide range ofprecision measurements and searches for new physics. An overview of the planned top physicsprogram accessible with tt¯ events is given for the ATLAS and CMS experiments. A particularemphasis is put on the precision measurements of the top mass, top polarization and searchesfor new physics in top production and decay.1 IntroductionThe top quark plays a central role in the physics programs of present and future high energyphysics collider experiments 1,2. It is a privileged tool for precise studies of the Standard Model(SM) of particle physics, being by far the heaviest fundamental particle and the only quarkdecaying before hadronization takes place. As a consequence, a precise measurement of itsmass constitutes a crucial test of the electro-weak sector and puts constraints onto the Higgsboson mass via radiative corrections, while its very short lifetime offers a unique window onbare quarks. Moreover, its mass, close to the electro-weak symmetry breaking (EWSB) scale,suggests that the top quark could play a special role in the EWSB mechanism and that newphysics might be preferentially coupled to it. It is therefore a sensitive probe to new physics.Top quark events are also a major source of background for many search channels, and preciseunderstanding of top signal is crucial to claim new physics. Last but not least, top quark eventsprovide an essential tool for understanding and calibrate the detector: triggering, b-tagging, jetenergy scale can all benefit from top signal.Ten years after the top quark discovery at the Tevatron Fermilab collider, we still knowquite little about its production and decay mechanisms, because of limited statistics. Thisleaves plenty of room for new physics in the top quark sector. Even if Tevatron run II startsto incisively probe it 3, a new opportunity for top quark physics will be opened by the CERNproton-proton facility LHC (Large Hadron Collider), that will collide beams with a center ofmass energy of 14 TeV at a design luminosity of 1034 cm−2s−1 and will start to take data in 2007.The main reason is that the LHC will be a top factory, with cross-sections for producing tt¯ pairsand single tops roughly 100 times larger than at Tevatron: 8 (3) millions of tt¯ pairs (single tops)will be produced per year during its first phase, corresponding to 10 fb−1, the initial luminositybeing about 10 times lower than the design one. Single top, which has not been yet discovered,is described in more details elsewhere in these proceedings 4. We will here focus on tt¯ channels,m(3jet) (GeV)100 150 200 250 300 350 400 450 500Events / ( 4.182 )0102030405060Events / ( 4.182 )σ ∼ 15 GeVFigure 1: Reconstructed top mass with semileptonic tt¯ events using 100 pb−1 of simulated data assuming nob-tagging (left), and 10 fb−1 with nominal b-tagging (right). The dashed (shaded) histogram on the left (right)represents the background (combinatorics and W+4 jets).whose SM final states are completely determined by the W -boson decay: as all top quark decaythrough t → W+b, semileptonic (44%), dileptonic (10%) and all hadronic (46%) tt¯ final statesare possible.2 Top mass measurementThe semileptonic channel provides a clean and abundant top quark sample, 2.5 million of eventswith electron or muon being expected with 10 fb−1 of LHC data. With one isolated high pTlepton to trigger on, four high pT jets, and only one neutrino, it is the most promising channel forprecise measurements. Even without b-tagging, a pessimistic scenario, and simply forming the 3jets combination giving the highest pT , a clear top signal should be observed quickly above thebackground, dominated by the combinatorics andW+4 jets events. This is illustrated in Figure 1(left) obtained with 100 pb−1 of fully simulated data, i.e. one day of running at 1033 cm−2s−1.This simple top sample extraction should enable initial measurements of cross-section and mass,and give important feedbacks on detector performance (such as jet energy scale and b-tagging)and on the reliability of Monte Carlo description. The improvement of detector performancewith data and time will increase the purity of the top sample. This is illustrated in Figure 1(right), which assumes a realistic b-tagging with a b-jet efficiency of 60% coupled with a light(c) jet rejection above 100 (10). The full event topology is reconstructed after estimating theunknown neutrino momentum and correctly associating the two b-jets to the b and b¯ quarks.The background is well under control, with a signal over non-tt¯ background ratio above 50 and adominant contribution from tt¯ events themselves (combinatorics and tt¯→ τ +X events). With10 fb−1 of data, 70 000 events should be selected with a purity around 65%.As a consequence of such a large statistics, measurements will be dominated by systematicuncertainties. The mass measurement is dominated by uncertainties on the b-jet energy scaleand final state radiations. The latter can be reduced with a kinematic fit to the entire tt¯ eventby reconstructing both leptonic and hadronic sides. A total error on the top mass below 2 GeVshould be feasible, possibly reaching an ultimate precision around 1 GeV. Exploiting fully thehigh statistics, different sub-samples and various methods can be used to perform top massmeasurements 5, leading to different source of systematic errors and allowing therefore reliablecross-checks. This is also the case with dileptonic and hadronic channels: the measurement ismore difficult for various reasons, but a total accuracy around 2-3 GeV seems achievable. Forall present analyses, a major source of systematics comes from the b-jet energy scale. This led to00.20.40.60.8-1 -0.5 0 0.5 1cos Y1/N dN/dcos Y F0=0.699±0.005FL=0.299±0.003FR=0.002±0.003Ycos -1 -0.5 0 0.5 1Y1/N dN/dcos 00.20.40.60.8 0.04–=0.710F0.02–=0.29LF0.02–=0.00RFFigure 2: Reconstructed angular distribution for W polarization measurement with semileptonic tt¯ events, in-cluding background. Events were processed through fast (left, 10 fb−1) and detailed (right, 0.5 fb−1) simulationof the detector. Full lines are the results of a fit allowing to extract W helicity state probabilities F0, FL and FR.the development of an alternative method, based on the identification of J/Ψ originating froma b-quark decay and using the linear correlation between the J/Ψ-lepton invariant mass and thetop mass 5,6. Due to a small branching ratio, this approach requires however a high luminosity(100 fb−1) to achieve a precision around 1 GeV.3 Top propertiesThe high statistics available at LHC will allow to study many properties of the top quark, besidesits mass. Using the same fully reconstructed tt¯ events, it will be possible to search indirectlyfor new physics in top production and decay mechanisms. As an example, the top quark spinproperties, through W polarization and tt¯ spin correlation measurements, can lead to a deepinsight of the nature of the top quark couplings to fermions and to the mechanisms (SM ornot) responsible for its production. The measurement of the SM predicted, but never measured,top spin correlationa, with asymmetries between spin-like and spin-unlike pairs 7, constitutes atest of the top production. The W boson polarization measurement complements this studyby testing only the top decay. Both measurements are directly accessible through angulardecay distributions in top and W rest frames. Because of its high mass, the top quark decaysbefore hadronization or spin flip, thus leaving an imprint of its spin on its daughter angulardistributions. This is illustrated in Figure 2, showing the reconstructed cosΨ distribution,where Ψ is the angle between the charged lepton direction in the W rest frame and the Wdirection in the top quark rest frame. The longitudinal (F0), left (FL) and right (FR)W helicitystate probabilities are extracted from a two parameter fit on this distribution 8. Results fromfast (left) and detailed (right) simulated data samples are in good agreement. Similar angulardistributions are used for spin correlation measurement 7. A precision of 1-2% is obtained forF0 and FR and 4% for the top spin correlation asymmetries after 10 fb−1, as shown in Table 1 9.These results are dominated by the same systematics as for the top mass measurement. Adeviation from SM values could point to anomalous gtt¯ or tWb couplings, a top spin differentfrom 1/2, or an anomalous decay such as t→ H+b.aAt LHC, t and t¯ spins are not polarized in tt¯ pairs, but rather correlated.Table 1: ATLAS sensitivity to W polarization components (F0, FL, FR) and top spin correlation asymmetries (Aand AD) after 10 fb−1 and combination of dileptonic and semileptonic channels.Parameters F0 FL FR A ADSM values 0.703 0.297 0.000 0.422 -0.290Statistical errors 0.004 0.003 0.003 0.014 0.008Systematic errors 0.015 0.024 0.012 0.023 0.0104 Direct searches for new physicsDirect searches for new physics in top production and decay mechanisms have also been inves-tigated. As an example, a heavy resonance decaying to tt¯ might enhance the cross-section andproduce a peak in the tt¯ invariant mass spectrum, as predicted by various models beyond theSM. A 1 TeV “generic” narrow resonance 2 may be discovered at LHC with 30 fb−1 of datafor a cross-section down to 800 fb. Many models beyond the SM include a complicated EWSBsector, with implications for top decays. Examples include the possible existence of chargedHiggs bosons, or possibly large flavor changing neutral currents (FCNC) in top decays. The topdecay to a light charged Higgs boson t→ H+b can be searched for through an excess of tt¯ eventswith τ -jets or a deficit of dilepton events. H+ with mass up to 160 GeV may be observed and170 GeV may be excluded with 30 fb−1 of data 10. In the SM, FCNC top decays are highly sup-pressed (BR < 10−13-10−10). However, several extensions of the SM can lead to very significantenhancements of these BRs (up to 10−3), which could be detected directly in tt¯ events. ATLASand CMS studies show that the current limits on the branching ratios of these processes can beimproved by a factor 102 to 103 already with 10 fb−1 of data 2, such a sensitivity allowing tostart to probe models beyond the SM.5 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, as its mass, couplings and polarization.Besides stringent tests of the SM, those measurements also constitute powerful probes in thesearch for new physics and could lead to new discoveries in the first years of the LHC lowluminosity data taking period.References1. D. Chakraborty et al., Ann. Rev. Nucl. Part. Sci. 53, 301 (2003) [hep-ph/0303092].2. M. Beneke et al., CERN-TH-2000-100 (2000) [hep-ph/0003033].3. L. Shabalina, D. Whiteson, J. Leveque and A. Gresele, these proceedings.4. M. Mohammadi Najafabadi, these proceedings.5. I. Borjanovic et al., Eur. Phys. Jour. C 39 S2, 63 (2005) [hep-ex/0403021].6. A. Kharchilava, Phys. Lett. B 476, 73 (2000) [hep-ph/9912320].7. W. Bernreuther et al., Nucl. Phys. B 690, 81 (2004) [hep-ph/0403035].8. G.L. Kane et al., Phys. Rev. D 45 124 (1992).9. F. Hubaut et al., Eur. Phys. Jour. C 44 S2, 13 (2005) [hep-ex/0508061].10. S. Banerjee and M. Maity, J. Phys. G 28, 2443 (2002).

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