Top Physics at the LHC is reviewed with special emphasis on cross section measurements, top properties studies and production mechanisms (single top production

Rolli, S

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Top Physics at the LHCSimona RolliTufts University9/11/06 Simona Rolli, PASCOS06 2The Top Quark in the Standard ModelThe LHC offers opportunity for further testing and precision measurementsDiscovered in 1995 at the TeVatron,flurry of measurementsWe still don’t know all about it- Mass  Precision <2%- Top width ~1.5 GeV   ?- Electric charge ⅔  -4/3 excluded @ 94% C.L. (preliminary)- Spin ½   Not really tested – spin correlations- BR(tWb) ~ 100%  At 20% level in 3 generations case  FCNC: probed at the 10% level- Production mechanisms    Single Top : not yet observedd bsu c ttop9/11/06 Simona Rolli, PASCOS06 3Talk Outlook• Strong pair production Standard top physics Early top physics• Top Properties Mass,Charge,W polarization,top polarization• Electroweak single top production Analysis strategies Vtb measurements• Using top for calibration purposes Jet energy corrections, b-jets, missing energy• A window to new physics• ConclusionsMost of the resultspresented are based onATLAS studies9/11/06 Simona Rolli, PASCOS06 4Production:  σtt(LHC) ~ 830 ± 100 pbStrong Pair  production at the LHCCross section LHC = 100  x TevatronBackground   LHC =   10  x TevatronttL+jets (l=e,µ) is the Golden channel 2.5 million events/year10%90%Decay9/11/06 Simona Rolli, PASCOS06 5LHC is a top factory             Seeing top is easyTop quark physics with b-tagSelection: High PT Lepton               Large Missing ET               4 high-PT jets (2 b-jets)                signal efficiency few %                very small SM backgroundS/B=O(100)Top signalW+jetsbackgroundTop mass (GeV)Number of Events• ‘Standard’ Top physics at the LHC:    - b-tag is important in selection    - Most measurements limited bysystematic uncertainties• ‘Early’ top physics at the LHC:    - Cross-section measurement (~ 20%)     - Decay properties9/11/06 Simona Rolli, PASCOS06 6Top quark physics without b-tag (early phase)A top peak can be seen without b-tag requirementMt (GeV/c2) MW (GeV/c2)events/5.1GeV/c2Mt (GeV/c2)full simulationW+jets andMC@NLO signalL=300pb-1|Mjj-MW|<10GeV/c2Selection semileptonic top: pT(lepton)>20GeV/c, missing ET>20GeV no b-tagging required hadronic top: Njet>4, pT(jet)>40GeV/c (0.4 cone algorithm) 3 jets with highest vector-sum pT identified as top of these, 2 leading jets in 3-jet rest frame identified as W9/11/06 Simona Rolli, PASCOS06 7Top Properties: MassLepton+jets• isolated lepton (e,µ): pT>20GeV/c, |η|<2.5• missing ET>20GeV• at least 4 jets: pT>20GeV/c (corrected), |η|<2.5 at least 2 light jets to reconstruct hadronic W 2 b-tagged jets to select the bjj system with highest PT• very effective in background rejection (S/B=10-430) mainly from bb, W/Z+jets and WbbMjj(GeV/c2)Mjj=79.6GeV/c2σjj=8.8GeV/c2Mt(GeV/c2)Mjjb=176GeV/c2σjjb=12GeV/c2full simulation, L=10fb-1δMt(stat)=0.05GeV/c2δMt(sys)=1.3GeV/c2Dileptons:• two opposite-signed leptons:pT(lepton)>20GeV/c, |η|<2.5• missing ET>40GeV• 2 b-jets: pT>25GeV/c (corrected), |η|<2.5• Final state reconstruction• 6 unknowns (neutrinos’ momenta), Mthypothesis conservation of transverse momentum mass-constrain each l–ν pair to MW mass-constrain each l–ν–b-jet system to Mt• weight assigned to each solution based on comparison w ith MC average w eight over w hole event sample• Mt from solution with highest mean weightfast simulationL=10fb-1δMt(stat)=0.04GeV/c2δMt(sys)=1.7GeV/c29/11/06 Simona Rolli, PASCOS06 8Top Properties: W PolarizationTop decays before hadronization spin information passed directly onto Wb SM predicts 70% longitudinal W and 30% left-handed W depending on Mt and MW only parametrize in terms of angle between direction of W in top rest frame direction of lepton in W rest frame Precision in measurements of the fractions F0(longitudinal) and FR Unfold selection and detector effectstb W ψlνATLAS (SL+DL)CMS (SL)ATLAS (SL+DL)CMS (SL)±0.012±0.016±0.003±0.004fast simulation L=10fb-1±0.053±0.022Sys±0.015±0.023StatFRF09/11/06 Simona Rolli, PASCOS06 9Top Properties: Charge• Aimed at confirming Qt=2/3 SM hypothesis non standard value Qt=–4/3 not yet excluded can arise from wrong W-b association• Two procedures for direct measurement Top e.m. coupling through photon radiation in tt events gg initial state dominance at LHC reduces ISR radiative tt production & (interfering) decay: x-section radiative tt decay: reduced by requiring high M(bjjγ) orMT(lνbγ) reconstruct charge of decay products(lepton/dilepton+jets) easy for W boson (Ql) challenging for b-jets l-b association: Mlb < Mt Systematics underwayQt=2/3Qt=–4/3fast simulationQ(b)=–0.109±0.007 Q(b)=0.112±0.0075σ b-b separation with 24pb–1fast simulation9/11/06 Simona Rolli, PASCOS06 10Single Top at LHC1 leptons + MET+ ≥ 2 jets+ 1(2) b-tags9/11/06 Simona Rolli, PASCOS06 11Why Single Top ?NANA< 4.4 pb< 5 pbD0 < 3.5 NA< 3.1pb<3.2 pbCDF62 +17-4 pb247 ± 25 pb10.6 ± 1.1 pbLHC σNLO0.1 pb1.98 ± 0.25 pb0.88 ± 0.11 pbTeVatron σNLOCombined (s+t)Associated tWt-channels-channelM(top) = 175 GeV/c2 Run II95% CLσt+s = 2.9 pb for m(top) = 175 GeV/c29/11/06 Simona Rolli, PASCOS06 12ATLAS analysis strategiesIn the late ‘90 several studies were conducted to produce a physics TDR. Currentstudies are meant to devise analysis strategies for early data taking and the fullstatistics, using the latest software tools.9/11/06 Simona Rolli, PASCOS06 13Wg channel9/11/06 Simona Rolli, PASCOS06 14s-channel9/11/06 Simona Rolli, PASCOS06 15Wt channel9/11/06 Simona Rolli, PASCOS06 16Vtb Measurement• Indirect measurement based on CKM unitarity constraint (3 generations)• Direct measurement based on electroweak single top production (σ∝|Vtb|2) measure yield of single top production combine with BR(tWb) and Mt (from tt channel) unbiased test of 3-generation structure of SM penalized by poor knowledge of W+jets, WQQ background no systematic effects taken into accounttheoreticalstat (30fb–1)5.5%4.7%ΔV tb/V tb11%0.54%2.3t-channel7.5%5.6%0.55s-channeluncertainties on σS/Bchannel9/11/06 Simona Rolli, PASCOS06 17Top quark pair production as calibration toolYou can use production of top quarkpairs to help calibrate LHC detectorsin complex event-topologiesYes            No            Cancel Calibrate light jet energy scale Calibrate missing ET Obtain enriched b-jet sample Leptons and trigger A candle for complex topologies:Note candles: 2 W-bosons                     2 top quarksleptonMissingenergyb-jetb-jetjetjet9/11/06 Simona Rolli, PASCOS06 18Calibrating jet energy scaleOne of the most relevant systematic effects on Mt jet energy: measurement of parton energy 1% uncertainty on absolute JES induces δMt~1GeV/c2 sizeable effects also from b-jet energy scale QCD radiation, underlying event, cone algorithm at start-up, 5÷10% uncertainty test-beam data in-situ correction with Z/γ+jet pT(jet) correction residual mass shift (2% on Mt) Mjj=MW additional constraint on JES clean Wjj sample needed 80% purity within ttlv+jets goal: 2÷3% uncertainty in 1 year (target 1%) Alternative method: PT balance in Z/γ + jet eventsL=10fb-1full simulationtt MC@NLO9/11/06 Simona Rolli, PASCOS06 19Calibrating b-jets• b-tagging techniques rely on impact parameter of decay tracks primary/secondary vertex separation soft leptons targeting b and c semileptonic decays• Typical performances efficiency ~60% on pT>40GeV/c jets light flavour rejection 1/εu~200• Jets from b-quarks need specific corrections semileptonic decays of heavy-flavoured quark neutrino induces a large shift on the jet energy effect enhanced if lepton is muon (MIP) jet direction affected as well as jet energy9/11/06 Simona Rolli, PASCOS06 20ttCalibrating the missing energyMissing ET (GeV)EventsPerfect detectorSUSY LSP ora mis-calibrated detector ?- Pµ(neutrino) constrained from kinematics: MW    known amount of missing energy per event- Calibration of missing energy vital for all(R-parity conserving) SUSY and most exotics!Range:  50 < PT < 200 GeVExample from SUSY analysis9/11/06 Simona Rolli, PASCOS06 21A window to new physics ?Cross section (a.u.)Mtt (GeV)400 GeV500 GeV600 GeVStructure in MttInterference from MSSM HiggsesH,A tt (can be up to 6-7% effect)4.3·10–41.8·10–56.5·10–5BR(L=100fb–1)1.4·10–36.6·10–53.4·10–4BR(L=10fb–1)tgqtγqtZqq=u,cFCNC decays: GIM suppressed in SM, canbe enhanced in SM extensions1.6 TeV resonanceMttFCNCMSSMHiggsResonancesSUSY?Like-sign tt ? Resonances in MttttXpp !!Z’, ZH, G(1), SUSY, ?9/11/06 Simona Rolli, PASCOS06 22Conclusions  1) Top quarks are produced by the millions at the LHC:        Almost no background:           to measure top quark properties will be easy  2) Top quarks are THE calibration signal for complex topologies:        Most complex SM candle at the LHC        Vital input for detector commissioning/calibration  3) Top quarks pair-like and singly produced……      as a window to new physics:       FCNC, SUSY, MSSM Higgs,             Resonances, anomalous couplings          Also important SUSY backgroundBackup Slides9/11/06 Simona Rolli, PASCOS06 24Top Mass Now9/11/06 Simona Rolli, PASCOS06 25Top Cross Section Now9/11/06 Simona Rolli, PASCOS06 26Top Properties Now9/11/06 Simona Rolli, PASCOS06 27W helicity Now

Top Physics at the LHC

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