Some models, including the so-called Randall-Sundrum model, predict the existence of heavy Kaluza-Klein resonances decaying mainly into top quarks. The prospects to discover those resonances at the LHC, using the ATLAS experiment, are discussed. Especial emphasis is given in the techniques used to identify and reconstruct top quarks at high pT

Isobe, T

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ATL-PHYS-PROC-2009-12404October2009Search for resonances decaying into top quarkpairs with ATLASTadaaki Isobe on behalf of the ATLAS collaborationInternational Center for Elementary Particle Physics (ICEPP),the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, JapanAbstract. Some models, including the so-called Randall-Sundrum model, predict the existence ofheavy Kaluza-Klein resonances decaying mainly into top quarks. The prospects to discover thoseresonances in the ATLAS experiment at the LHC are discussed. Special emphasis is given to thetechniques used to identify and reconstruct top quarks at high-pT.Keywords: Extra dimension, top quarkPACS: 12.10.Dm, 14.65.HaINTRODUCTIONThe top quark could be one of the most important handles in searches for new physics atthe LHC. The top quark may couple to new particles (Z’, g*, H±, and tˆ) in many exten-sions of the standard model [1, 2, 3]. In the Randall-Sundrum warped extra dimensionmodel, Kaluza-Klein resonances are expected to exist. The Kaluza Klein excitation ofthe gluon is predicted to couple strongly to heavy quarks (Br(g*→t¯t) = 92.3 % in [4]).The reconstruction of the decay of a heavy resonance is an experimental challenge.The decay products from the highly boosted top quark are collimated in a narrow cone.Standard jet reconstruction algorithms will fail to resolve the partons from hadronicallydecaying tops. Merging of jets will thus limit the performance of the traditional approachtowards top reconstruction and a new paradigm is required to explore resonances withvery high mass. In this contribution the ATLAS discovery potential for heavy t¯t reso-nances is discussed, as well as the first attempts to establish a new technique for theidentification of high-pT top quarks.At Tevatron experiments, a search for a narrow-width heavy resonance decaying intotop quark pairs X→t¯t in pp¯ collisions at √s = 1.96 TeV has been performed [5]. D0reports the search for such resonances in the lepton plus jets channel, and there is noevidence for a narrow resonance of Z’ decaying to t¯t up to 700 GeV/c2 [6].HIGH MASS GAUGE BOSON SEARCH IN TOP QUARK PAIREVENTSATLAS estimated the discovery potential of the ATLAS detector for a generic narrow Z’boson in a model independent way [7]. For this estimation, the production cross-section,couplings with top, and mass of the Z’ are free parameters, while its intrinsic widthis required to be small compared to its reconstructed width. To obtain a very pure t¯t [GeV]tt M0 200 400 600 800 1000 1200 1400 1600 1800 2000Evts/20GeV01020304050607080Reconstructed EvtsCombinatorial BackgroundZ’ : 700  GeVATLASMass [GeV]700 800 900 1000 1100 1200 1300 1400 1500 Br) [pb]× σ(-210-110110210-1Luminosity = 1 fb-1Luminosity = 10 fb-1Luminosity = 30 fb-1Luminosity = 100 fb-1Luminosity = 300 fbATLASFIGURE 1. Reconstructed t¯t invariant mass spectra for a 700 GeV/c2 Z’→t¯t process (left). Discoverypotential of a generic narrow t¯t resonance for different integrated luminosity scenario (right).sample, top quark pairs in the “lepton+jets” channel are selected, where lepton is eithera muon or an electron. In order to reproduce trigger cuts, and to reduce the backgrounds,such as multi-jets, W+jets, Z+jets, dibosons and single-top production, the followingcriteria are required for event selection. Standard offline selection cuts are applied tothe events satisfying a high-pT isolated lepton trigger. The standard offline selection cutsconsist of exactly one isolated lepton (plT > 40 GeV/c, |ηl | < 2.5), high missing transverseenergy (6ET > 20 GeV), at least four jets (p jetT > 40 GeV/c, |ηjet| < 2.5) on which exactlytwo b-tagged jets. Then, the kinematics of the event is reconstructed as follows: the“hadronic top quark” is reconstructed combining the nearest b-tagged jet to the hadronicW, which is reconstructed from the two light (non b-tagged) jets which are closest inthe η ×φ (smallest ∆R). Two leptonic W boson candidates are formed because of thetwo-fold ambiguity in the reconstructed longitudinal neutrino momentum (pνz ). The oneproviding the leptonic top mass closest to the mean value of the hadronic top mass ischosen. In the leftmost panel of Fig. 1 the reconstructed t¯t mass distribution is shownfor a 700 GeV/c2 Z’→t¯t process. The reconstructed Z’ mass resolution is 40 GeV/c2for 700 GeV/c2 Z’. The level of SM backgrounds can be reduced by applying cutson the reconstructed hadronic W and both top masses. The level of instrumental andphysical SM backgrounds is estimated to be small: (SM t¯t)/(SM background)∼ 70. Thereconstructed top quark pairs includes the combinatorial background, which is due tothe t¯t events with a wrong b-jet, light jet or pνz assignment and is also important for thebackground estimation.The discovery potential is determined from the t¯t reconstructed mt¯t spectrum.The counting method is used with a sliding window technique to detect any excessof event in a mass window. The window width is optimized for narrow resonancesearches (ΓZ′ < 50 GeV/c2). Right panel of Fig. 1 shows the discovery potential of ageneric narrow t¯t resonance for different integrated luminosity scenario. The result-ing sensitivity shows that a 700 GeV Z’ resonance produced with a cross-section ofσ (pp→Z’)·Br(Z’→t¯t) > 11 pb should produce an excess of selected events S/√Bgreater than 5 σ (statistics and systematics combined) with 1 fb−1 of√s = 14 TeV data.The sensitivity in the high mass region is limited by the efficiency for the reconstruc-tion of high-pT top quarks. A different approach is necessary for the reconstruction ofhigh-pT top quarks of pT > 500 GeV/c.RECONSTRUCTION OF BOOSTED TOP QUARKThe identification of highly boosted tops is an experimental challenge. For hadronicdecay of the W (nearly 70 % of cases) from a top quark, three jets are produced in anarrow cone. For large top quark pT, the separation between the three jets becomes sosmall that the individual decay products can no longer be distinguished by the standardjet reconstruction algorithms. At higher-pT, top quark are thus increasingly reconstructedas a single jet, which is a so-called “mono-jet”.One of the approaches for the reconstruction of such boosted hadronically decayingtop quarks is the reconstruction of sub-jets within high-pT jets [8]. The idea for thereconstruction of underlying sub-jets is to use the kT clustering jet algorithm. It fulfilsthe theoretical requirements on jet algorithms, and tends to reconstruct the sub-jets ina jet, due to fragmentation and radiation effects. The characteristic scale of the sub-jets splitting is logarithmically below the jet scale in the case of QCD jets, since in theregion around the jet strongly-ordered DGLAP-like QCD evolution dominates. ATLASexamined the discrimination of signal hadronic top quarks and jets originating from asingle hard parton using sub-jets technique with requiring a further advanced selectionprocedure [9]. At first, high-pT kT jets are considered (pjetT > 300 GeV/c). Then, amultivariate sequential selection is used, with several discriminant variables: the mass ofthe jet and the energy scale values at which the mono-jet splits into two, three and fourjets. The mass of a kT jet is defined as the invariant mass of all the jet’s constituents,which are the calorimeter cells. The left panel of Fig. 2 shows the selection efficiencyfor the signal and the QCD background. The selection efficiency increases with ptopT . Ahigh efficiency is achieved for top mono-jets while keeping the light jet efficiency below25 %.In addition to the jet substructure, the long lifetime of the B-hadron embedded in thetop mono-jet is expected to be useful signature to identify top-jets. Tagging of b-jets hasproven to be a very powerful tool. The ATLAS collaboration has developed a suite ofb-tagging algorithms based on the precise reconstruction of charged tracks in the innerdetector. In reference [10] the IP3D+SV1 algorithm, that combines impact parameterand secondary vertex information to construct a b-tag likelihood, was applied to topmono-jets. In the rightmost panel of Fig. 2 the signed impact parameter significancedistribution for light jets (continuous line), b-jets (dashed line) and top mono-jets (dottedline). The tail towards positive values in the significance distribution for b-jets is dueto the large lifetime of B-hadrons. The distribution for top mono-jets is much moresymmetric than that for b-jets due to the large uncertainty on the B-hadron flight path.While the tagging performance is degraded with respect to b-jets of the same transversemomentum, the lifetime signature remains a powerful tool to distinguish top mono-jetsfrom jets originating in light quarks or gluons. For jets with a transverse energy of 1 TeV,the light jet rejection power is a factor 6 for a top mono-jet tagging efficiency of 60 %. [GeV]TJet p200 400 600 800 1000 1200 1400 1600 1800Efficiency00.20.40.60.81 > 0.6LyZ' signalQCD backgroundATLAS Preliminarysigned transverse IP significance-25 -20 -15 -10 -5 0 5 10 15 20 25-410-310-210-110FIGURE 2. Left figure shows the selection efficiency for jets coming from the hadronic top quark(circle) and coming from hard light partons (triangle). yL stands for the likelihood variable defined in[9]. Right figure shows the signed impact parameter significance of all tracks inside light jets (continuousline), b-jets (dashed line) and hadronic top mono-jets (dotted line).SUMMARYATLAS estimated the discovery potential for the Z’ narrow resonance decaying intotop quark pairs in the lepton+jets channel. In order to reconstruct the boosted topquarks from heavy t¯t resonances with a high efficiency, ATLAS developed kinematicvariables and selection criteria. Especially, jet mass information, sub-structure of jetsand b-tagging technique are useful for the high-pT top mono-jets reconstruction.ACKNOWLEDGMENTSI would like to thank the ATLAS collaboration members of the Top and Exotics for theircomments and helpful discussions. Among them, I am grateful to P. Savard, M. Vos,G. Brooijmans, K. Terashi, S. Asai, L. Pribyl, C. Petridou, and D. Pallin.REFERENCES1. K. Agashe el al., Phys. Rev. D 77, 015003, 2008.2. B. Lille el al., JHEP 0804, 087, 2008.3. S. Matsumoto el al., Phys. Rev. D 75, 055006, 2007.4. B. Lille, L. Randall, and L. Wang, JHEP 0709, 074, 2007.5. T. Altonen el al., Phys. Rev. Lett. 102, 222003, 2009.6. V. Abazov el al., Phys. Lett. B 668, 98, 2008.7. G. Aad el al., CERN-OPEN-2008-020, 2008.8. J. M. Butterworth, B. E. Cox and J. R. Forshaw, Phys. Rev. D 65, 096014, 2002.9. The ATLAS collaboration, ATL-PHYS-PUB-2009-081, 2009.10. M. Vos, ATL-PHYS-CONF-2008-016, 2008.

Search for resonances decaying into top quark pairs with ATLAS

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