Camacho Toro, Reina Coromoto

HAL Clermont Université

Searches for ttbar resonances in ATLASReina Coromoto Camacho ToroTo cite this version:Reina Coromoto Camacho Toro. Searches for ttbar resonances in ATLAS. Beyond The Stan-dard Model of Particle Physics, Jul 2012, Qui Nhon, Vietnam. 2012. <in2p3-00760281>HAL Id: in2p3-00760281http://hal.in2p3.fr/in2p3-00760281Submitted on 3 Dec 2012HAL 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.ATL-PHYS-PROC-2012-28530/11/2012SEARCHES FOR tt¯ RESONANCES IN ATLASREINA CAMACHO TOROLaboratoire de Physique Corpusculaire (LPC), Campus des Ce´zeaux. 24, Avenue des Landais BP80026. 63171 Aubie`re Cedex, France.E-mail: reina.coromoto.camacho.toro@cern.chOn behalf of the ATLAS collaborationA search for tt¯ resonances has been performed using 2.05 fb−1 of pp collisions at√s = 7 TeVrecorded by ATLAS in 2011. The dilepton, resolved lepton+jets and a boosted lepton+jetstopology have been considered. No evidence of a resonance was found. Limits were set onthe production cross-section times branching ratio to tt¯ for narrow and wide resonances. Thesearch excludes at 95% confidence level narrow leptophobic topcolor Z′ bosons with massesbetween 500 and 1150 GeV and wider Kaluza-Klein gluons with masses between 500 and1500 GeV.1 IntroductionThe top quark is the heaviest elementary particle known, with a mass close to the scale ofelectroweak symmetry breaking. The large top quark mass hints at an intimate connectionbetween the top quark and beyond the Standard Model (BSM) physics. So far, searches formanifestations for such physics have remained unsuccessful, and such searches therefore continueto explore a wide spectrum of possible signatures. A promising approach are BSM theoriespredicting the existence of new heavy particles decaying into top quark pairs.In the SM, the top quark decays approximately 99% of the time into a W boson and ab-quark. The top quark decay final states are determined by the decay of the W , which de-cays approximately 33% into a charged lepton-neutrino (leptonic decay) pair and 67% into aquark-antiquark pair (hadronic decay). If the lepton is a τ , it will decay subsequently into anelectron/muon-neutrino pair or hadronically. This results in three decay topologies: hadronic,lepton plus jets and dilepton.This report summarizes recent results of the search for new heavy particles decaying to tt¯pairs using the ATLAS detector 1 at the CERN Large Hadron Collider (LHC) in three differentfinal state topologies: the dilepton channel 2, the resolved lepton plus jets channel 2,3 and theboosted lepton plus jets channel4. The latter is increasingly important in searches for resonanceswith masses above 1 TeV where the hadronically decaying top quark daughters merge into oneor two jets. Only final state topologies containing at least one electron or muon expected tocome from the decay of one of the W bosons produced in the top quark decays are considered.All analyses use 2.05 fb−1 of data collected by ATLAS in the first half of 2011.The benchmark model used to quantify the experimental sensitivity to narrow resonancesis a leptophobic Z ′ boson arising in models of strong electroweak symmetry breaking throughtop quark condensation (specifically the leptophobic scenario known as Model IV 5). The modelused for wide resonances is a Kaluza-Klein (KK) gluon gKK, which appears in Randall-Sundrum(RS) models with one warped extra dimension 6,7,8. This model is taken as a proxy for colouredresonances. Previous searches for tt¯ resonances recently carried out did not show evidence fornew resonances and limits were set on the mass of a Z ′ boson excluding mZ′ > 900 GeV9 aswell as placing upper limits on the coupling strengths of a heavy colour-octet vector particle.2 Event selectionEvents are selected using a single lepton trigger. Each event is required to have at least oneoﬄine-reconstructed primary vertex with at least five tracks, and it is discarded if any jet withpT > 20 GeV is identified as out-of-time activity or calorimeter noise.2.1 Dilepton channelThe event must contain exactly two isolated leptons of opposite charge. Electron candidatesare required to have pT > 25 GeV and |η| < 2.47 a, while muon candidates are required to havepT > 20 GeV and |η| < 2.5. Two or more jets with pT > 25 GeV and |η| < 2.5 are also required.Jets are reconstructed using the anti-kt algorithm (R = 0.4)10 and calibrated using a correctionfactor obtained from simulation 11. To suppress the Z plus jets background, events are requiredto have EmissT> 40 GeV and an invariant dilepton mass not compatible with the Z boson massrange (|mZ −mll| > 10 GeV) for ee and µµ events, where EmissT is the module of the missingtransverse momentum in the event. In the eµ channel the non-tt¯ background is suppressed byrequiring the scalar sum of the transverse momenta of the leptons and jets, HT, to be largerthan 130 GeV.2.2 Resolved lepton plus jets channelThe event must contain exactly one isolated lepton. Electrons must have pT > 25 GeV and|η| < 2.47, while muons must have pT > 20 GeV and |η| < 2.5. Events where an electronshares an inner detector track with a non-isolated muon or with a second lepton with pT >15 GeV are rejected. The total event fraction of tt¯ is increased by applying the following eventcuts: first, in the electron channel, EmissT> 35 GeV and MT(lepton,EmissT) > 25 GeV, whereMT(lepton,EmissT) is the lepton-EmissTtransverse mass. In the muon channel, EmissT> 20 GeV andEmissT+MT(lepton,EmissT) > 60 GeV is required. Finally, the leading jet must have pT > 60 GeVand the event must contain at least four jets with pT > 25 GeV and |η| < 2.5, where at leastone of the jets must be tagged as a b-quark jet using the MV1 algorithm 12. The jet multiplicityrequirement is relaxed to at least three jets if one of the jets has jet mass mj > 60 GeV, sincefor boosted top quarks two of the quark daughters from the hadronic top quark decay can bemerged into a single jet. Jets are reconstructed using the anti-kt algorithm (R = 0.4).2.3 Boosted lepton plus jets channelThe lepton selection and multijet background rejection is similar to the one used in the leptonplus jets resolved channel. The jet selection is modified for the expected boosted topology. Thelepton and the jet from the leptonic top quark decay are expected to be collimated, therefore a jetwith pT > 30 GeV and with an η−φ separation ∆R from the lepton candidate of 0.4 < ∆R < 1.5of the selected lepton is required. The decay products from the hadronic top quark decay areaATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in thecentre of the detector and the z−axis along the beam pipe. The x−axis points from the IP to the centre ofthe LHC ring, and the y−axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φbeing the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ asη = −lntan(θ/2).expected to be merged into a single fat jet back-to-back to the leptonic top quark decay. Fatjet candidates are reconstructed using the anti-kt algorithm with a distance parameter R = 1.0.Fat jets that potentially overlap with the jet associated with the leptonic top quark decay arenot considered (∆R(j, j) > 1.5).An analysis of the substructure of the fat jet can be used to increase the tt¯ signal. To thisend two observables are considered in this analysis. The first one is the jet mass, mj , obtainedby adding the four vectors of all jet constituents (topological clusters). The second one is thelast splitting scale,√d1213. Jet constituents are reclustered with the kt algorithm and√d12represent the scale at which the fat jet would split into two, i.e the last step of the clusteringsequence. For fat jets containing the hadronic decay of a boosted top quark is typically aroundthe W−mass. Therefore, at least one fat jet must satisfy √d12 > 40 GeV, pT > 250 GeV andmj > 100 GeV. The leading fat jet is taken as the hadronic top quark candidate.3 Backgrounds and signalsThe irreducible SM tt¯ and electroweak single top quark backgrounds are simulated using theMC@NLO generator 14,15 interfaced to the Herwig 16 parton showering and hadronizationmodel and the Jimmy17 underlying event model. In the dilepton channel, the small contributionof events with at least one fake lepton is estimated from data using the Matrix Method 18 whichaccounts for small backgrounds with both one (W plus jets) and two fake leptons (multijetbackground). In the lepton plus jets channel, the W plus jets samples are simulated with theAlpgen generator and they are normalized using data-driven techniques. In the lepton plus jetsresolved channel, the multijet background is derived from a jet-triggered sample where exactlyone jet is required with a high electromagnetic fraction. The full EmissTdistribution of thissample and the simulated tt¯, single top, W plus jets and Z plus jets background samples arefitted to the data to calculate its normalization. In the lepton plus jets boosted channel theMatrix Method is used to estimate the multijet background contribution. The Z plus jets eventsare simulated using the Alpgen 19 Monte Carlo. In the dilepton channel, this background isnormalized in a data control sample orthogonal to the signal sample. Diboson WW , ZZ andWZ samples are produced using the Herwig generator and Jimmy model.Samples for topcolor Z ′ bosons were generated with the Pythia 20 Monte Carlo. Samplesfor KK gluons were generated with Madgraph21 showered with the Pythia Monte Carlo. Anyinterferences with SM processes were assumed to be negligible.4 tt¯ mass reconstruction4.1 Dilepton channelIn the dilepton channel the tt¯ mass reconstruction present ambiguities due to the presenceof two neutrinos. The effective mass, defined as the scalar sum of HT + EmissT, is used asthe discriminating variable. Figure 1(c) shows the HT + EmissTdistribution for data and SMexpectation.4.2 Resolved lepton plus jets channelIn this channel, the event EmissTcorresponds to the neutrino’s transverse momentum pT andthe neutrino’s longitudinal momentum (pz) is determined by imposing the W boson mass con-straint. The leptonically decaying W boson candidate is reconstructed from the lepton and thereconstructed neutrino.The tt¯ pair is reconstructed differently depending on the highest mass jet in the event. Forevents where all jets have masses below 60 GeV, the dRmin algorithm 2 is used to identify astt¯ final state daughters those jets whose η − φ separation, ∆R, with the lepton or closest jetsatisfies: ∆R > 2.5 − 0.015 × mj . If a jet was discarded and more than three jets remain,the procedure is iterated. The tt¯ mass is reconstructed from the lepton, the neutrino and theleading four jets or three jets (if only three jets remain). For events where there is a jet witha mass greater than 60 GeV, the hadronic top quark candidate is built combining this jet withclosest jet in ∆R and the leptonic top quark candidate is built combining the reconstructedleptonically decaying W boson candidate with the closest jet, among the remaining jets. The tt¯mass spectrum is presented in Figure 1(a).4.3 Boosted lepton plus jets channelThe tt¯ system is reconstructed adding the four-momenta of the hadronically decaying top quarkcandidate, with the semileptonically decaying top quark candidate reconstructed from the lep-tonically decaying W boson candidate and the closest jet. The reconstructed tt¯ mass spectrumis presented in Figure 1(b).(a) (b) (c)Figure 1: HT + EmissT distribution in the dilepton channel (a) and reconstructed tt¯ mass in the lepton plus jetschannel resolved (b) and boosted (c) after all the selection requirements. The hatched area shows the backgroundnormalization uncertainties 3,4.5 ResultsFigure 1 shows good agreement between data distributions and background expectations forthe different discriminant variables used without visible evidence of a resonance structure abovethe expected backgrounds. Good agreement is seen in the shapes of kinematic distributions aswell. The tt¯ mass and HT + EmissTdistributions are compared with the background-only andsignal-plus-background hypothesis to verify that no statistically significant excess is observed.This was done using the BumpHunter 22 tool for the lepton plus jets channels and a likelihoodratio test statistic for the dilepton channel. No significant excess was found. Given the absenceof a new physics signal, limits were set using a Bayesian approach 23 (taking into account all thenormalization and shape systematic uncertainties) on the cross-section times branching ratio asa function of mass. The predicted resonance cross-sections versus mass are used to convert thecross-section upper limits into mass limits.For the dilepton channel, the observed (expected) limit for a Kaluza-Klein gluon in Randall-Sundrum models, gKK, ranges from 18.5 (11.3) pb at mgKK = 600 GeV to 2.3 (2.7) pb atmgKK = 1300 GeV, excluding mgKK between 500 (500) GeV and 1080 (1070) GeV at 95% CL.No mass limit is set on a narrow leptophobic topcolour Z ′ boson mass, mZ′ , since this channelhas insufficient sensitivity to it. Limits on the gKK production cross section time branchingfraction are shown in Figure 2.Figure 2: Observed (black points) and expected (dashed line) upper limits on σ × BR(Z′ → tt¯) (a) and σ ×BR(gKK → tt¯) for the dilepton channel. The dark and light green bands show the range in which the limit isexpected to lie in 68% and 95% of experiments, respectively 2.For the lepton plus jets resolved channel, the observed (expected) limit for σ×BR(Z ′ → tt¯)ranges between 4.8 (6.0) pb at mZ′ = 600 GeV and 0.95 (0.62) pb atmZ′ = 1300 GeV. The massrange 500 < mZ′ < 880 GeV (500 < mZ′ < 1010 GeV) is excluded at 95% C.L. The observed(expected) limit for σ × BR(gKK → tt¯) ranges between 5.0 (6.0) pb at mgKK = 500 GeV and1.6 (0.9) pb at mgKK = 1300 GeV. The mass range 500 < mgKK < 1130 GeV (500 < mZ′ <1360 GeV) is excluded at 95% C.L., as shown in Figure 3.(a) (b)Figure 3: Observed (black points) and expected (dashed line) upper limits on σ × BR(Z′ → tt¯) (a) and σ ×BR(gKK → tt¯) for the lepton plus jets resolved channel. The dark and light green bands show the range in whichthe limit is expected to lie in 68% and 95% of experiments, respectively 2.For the lepton plus jets boosted channel, the observed (expected) limit for σ×BR(Z ′ → tt¯)ranges between 7.7 (10.4) pb at mZ′ = 600 GeV to 0.56 (0.39) pb at mZ′ = 1300 GeV. Themass range between 600 < mZ′ < 1200 GeV is excluded at 95% C.L. The observed (expected)limit for σ ×BR(gKK → tt¯) ranges between 2.8 (2.9) pb at mgKK = 700 GeV to 0.8 (0.6) pb atmgKK = 1300 GeV. The mass range between 700 < mgKK < 1500 GeV is excluded at 95% C.L.6 ConclusionResults of a search for tt¯ resonances in the resolved lepton plus jets, boosted lepton plus jets anddilepton topologies have been presented. The different analyses use 2.05 fb−1 of pp collisionsat√s = 7 TeV collected with the ATLAS detector in 2011. The data was compared to SMbackground prediction and found to be compatible within uncertainties. The tt¯ mass (HT +(a) (b)Figure 4: Observed (black points) and expected (dashed line) upper limits on σ × BR(Z′ → tt¯) (a) and σ ×BR(gKK → tt¯) for the lepton plus jets boosted channel. The dark and light green bands show the range in whichthe limit is expected to lie in 68% and 95% of experiments, respectively 4.EmissT) distribution in the lepton plus jets (dilepton) channels was used to set upper limits at95% C.L. on the cross-section times branching ratio to tt¯ of a narrow leptophobic topcolor Z ′boson and a wide Kaluza-Klein gluon excitation gKK in the Randall-Sundrum model. Theyexclude Z ′ bosons with masses between 500 and 1150 GeV and KK gluons with masses between500 and 1500 GeV.References1. ATLAS Collaboration, JINST 3, S08003 (2008).2. ATLAS Collaboration, Eur. Phys. J. C 72, 2083 (2012), arXiv:1205.5371.3. ATLAS Collaboration, Tech. Rep. ATLAS-CONF-2012-029.4. ATLAS Collaboration, JHEP 041, 1209 (2012), arXiv:1207.2409.5. R. M. Harris, C. T. Hill and S. J. Parke, Phys. Lett. B 345, 483-489 (1995).6. K. Agashe et al., Phys. Rev. D 77, 015003 (2008).7. B. Lillie, J. Shu and T. M. P. Tait, Phys. Rev. D 76, 115016 (2007).8. B. Lillie, L. Randall and L.-T. Wang, JHEP 09, 074 (2007), arXiv:hep-ph/0701166.9. T. Aaltonen et al. (CDF Collaboration), Phys. Rev. D 84, 072004 (2011).10. M. Cacciari, G. P. Salam and G. Soyez, JHEP 04, 063 (2008), hep-ph/0802.1189.11. The ATLAS Collaboration, Accepted by Eur. Phys. J.(2011), arXiv:1112.6426.12. ATLAS Collaboration, Tech. Rep. ATLAS-CONF-2011-102.13. ATLAS Collaboration, JHEP 1205, 128 (2012), arXiv:1203.4606.14. S. Frixione and B. R. Webber, JHEP 06, 029 (2002).15. S. Frixione, P. Nason and B. R. Webber, JHEP 08, 007 (2003).16. G. Corcella et al., JHEP 0101, 010 (2001).17. J. Butterworth, J. R. Forshaw and M. Seymour, Z. Phys. C 72, 637 (1996).18. ATLAS Collaboration, Phys. Lett. B 707, 459 (2012).19. M. L. Mangano et al., JHEP 0307, 001 (2003).20. T. Sjostrand, S. Mrenna and P. Z. Skands, JHEP 0605, 026 (2006).21. J. Alwall et al., JHEP 0709, 028 (2007).22. G. Choudalakis, arXiv:1101.0390 [physics.data-an].23. I. Bertram et al. (DØ Collaboration), FERMILAB-TM-2104, Fermilab, 2000.

Searches for ttbar resonances in ATLAS

Abstract

Similar works

Full text

Available Versions

HAL Clermont Université

HAL-IN2P3