The ATLAS experiment sensitivity, with an expected luminosity of 1 fb^-1, to the study of the top quark properties are presented. The measurements of the top charge, the spin and spin correlations, the SM decay (t -> bW) and rare top decays associated to flavour changing neutral currents (t -> qX, X=gamma,Z,g) are discussed

Veloso, F

CERN Document Server

1top quark properties:prospects at ATLASfilipe velosofilipe.veloso@lipc.fis.uc.ptTOP2008 23.05.2008contents 2• top properties: prospects for 1 fb−1 at ATLAS- mass → see Jorgen Sjolin and Roger Wolf’s talk- tt¯ resonances → see Florent Chevallier’s talk- charge- tt¯ spin correlations- W polarisation in top decays- anomalous Wtb couplings- rare top decays and FCNC• summarymonte carlo and common selection 3• monte carlo:- common full simulation samples were used(signals: MC@NLO, AcerMC and TopReX)(fast simulation were also used for specific purposes)• common event selection (charge, tt¯ spin correlations, Wpolarisation, anomalous Wtb couplings and tt¯ resonances):- semileptonic topology - dileptonic topologyPSfrag replacementsℓbbνjjPSfrag replacementsℓℓbbννj- FCNC analyses have different selectionscharge 4W (+1)t (+2/3)νℓ+ (+1)b (−1/3)W (−1)t (−4/3)ν¯ℓ− (−1)b (−1/3)SM: exotic:CDF: 4/3 excluded at 87% CL [CDF note 8967]DØ: 4/3 excluded at 92% CL [PRL 98, 041801 (2007)]need to determine:1. lepton charge2. lepton/b-jet pair3. b quark charge→ charge weighting→ semileptonic b decayEntries  195307Mean    74.33± 9.562e+04 m(lepton,bjet) [GeV]0 50 100 150 200 250 300 350 400 450 500310×-2Events / 2 GeVc010002000300040005000 Entries  195057Mean    183.1± 1.346e+05 ATLAS preliminary155→ wrong pairright pair ←mℓbϵ = 31% P = 86%charge 5charge weighting:Qbjet =∑ q|~j · ~p|κ∑ |~j · ~p|κκ = 0.5Entries  279629Mean   0.0007±-0.0984 b jet charge-1 -0.5 0 0.5 1Events / 0.0201000200030004000500060007000Entries  279790Mean   0.0006± 0.1068 ATLAS preliminary-1L = 1 fbEntries  5545Mean   0.0045± -0.0804 b jet charge-1 -0.5 0 0.5 1Events / 0.02020406080100120140160Entries  5530Mean   0.0045± 0.0949 ATLAS preliminary-1L = 1 fbestimate for the W+jets background: S/B = 7 : 1Entries  12647Mean   0.0030± -0.0772 b-jet charge-1.5 -1 -0.5 0 0.5 1 1.5Events / 0.02050100150200250300350Entries  1572Mean   0.0088± -0.0035 S + BBATLAS preliminary-1L = 1 fbEntries  12647Mean   0.0126± 0.6750 top charge-4 -3 -2 -1 0 1 2 3 4 5 6Events / 0.1050100150200250300350400450S + BBATLAS preliminary-1L = 1 fbQcomb = −0.077± 0.003(stt.)± 0.003(bck.)Qt = Qℓ +Qb−jet× Cb = 0.67± 0.04(stt.) (Cb = Qb/Qcomb = 4.33± 0.28)charge 6semileptonic b decay:b→ q−ν¯, b¯→ q¯+ν, (q = , c)Charge-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-1Events / 1 fb020406080100120140160180200220-1Events / 1 fbB-mesonD-mesonOtherFakeATLASCharge-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-1Events / 1 fb020406080100120140160180200220240-1Events / 1 fbB-mesonD-mesonOtherFakeATLASQ¯(+)nons = −0.32± 0.05 Q¯(−)nons = 0.30± 0.05Q¯nons =N(+)− N(ℓ−)N(ℓ+) + N(ℓ−)Q¯(comb)nons = −0.31± 0.04PSfrag replacementsℓ (−1)ν¯b (−1/3)c(+2/3)W (−1)charge 7systematic uncertainties on Qcomb:source weighting b-decayjet scale 0.7% 0.3%b-jet scale 1.9% 6%Δmt 1.3% 7%PDF 0.6% —ISR 2.8% 15%FSR 7.8% 8%MC modelling 18% 16%pile-up 11% 1.8%background 3% —total 23% 25%ATLAS will be able to distinguish bothcharge hypotheses with a significanceabove 5σ for 1 fb−1 of datatt¯ spin correlations 8t quarks decay before hadronisation→ spin information is conservedA =σ(t↑ t¯↑) + σ(t↓ t¯↓)− σ(t↑t¯↓)− σ(t↓t¯↑)σ(t↑ t¯↑) + σ(t↓ t¯↓) + σ(t↑t¯↓) + σ(t↓t¯↑)1Nδ2Nδ cosθ1δ cosθ2=14(1− A|α1α2| cosθ1 cosθ2)1NdNd cos=12(1− AD |α1α2| cos)PSfrag replacementsθℓ+θbθνθℓ−θb¯ θν¯ℓ+bνℓ−b¯ ν¯tt¯CMtCMt¯CMα → spin analysing powerPSfrag replacementsℓ+ℓ−tCMt¯CMASM = 0.422ASMD = −0.290(mtt¯ < 550 GeV)cosθ → angle between the t (in tt¯rest frame) and the t decay product(in t rest frame) → angle between the two spin analy-sers (in the corresponding t rest frame)tt¯ spin correlation 9A = 0.67± 0.34 (stat+sys) AD = −0.40± 0.14 (stat+sys)W polarisation 10W polarisation: [Phys. Rev. D 45 (1992) 124]1NdNd cosΨ=32F0sinΨp22+ FL1− cosΨ22+ FR1+ cosΨ22W helicity states:FSM0 = 0.695 FSML = 0.304 FSMR = 0.001 F0 + FL + FR = 1Ψ→ the angle between the ℓ (in W rest frame) and the W (in t rest frame)FL = 0.29± 0.02± 0.03 F0 = 0.70± 0.04± 0.02 FR = 0.01± 0.02± 0.02tt¯ spin correlations and W polarisation 11systematic uncertainties:FL F0 FR A ADwith fast simulation:factorisation 0.000 0.001 0.001 0.029 0.006structure function 0.003 0.003 0.004 0.033 0.012ISR 0.001 0.002 0.001 0.002 0.001FSR 0.009 0.007 0.002 0.023 0.016b fragmentation 0.001 0.002 0.001 0.031 0.018hadronization scheme 0.010 0.016 0.006 0.006 0.008pile-up (2.3 events) 0.005 0.002 0.006 0.001 0.005input top mass (2 GeV) 0.015 0.011 0.004 0.028 0.013with full simulation:b tagging efficiency (5%) 0.007 0.002 0.005 0.027 0.07b jet energy scale (5%) 0.02 0.002 0.02 0.07 0.015light jet energy scale (5%) — — — 0.11 0.017S/B scale (20%) 0.004 0.002 0.001 0.000 0.004total 0.03 0.02 0.02 0.15 0.08anomalous Wtb couplings 12L = − gp2b¯γμ (VLPL + VRPR) tW−μ −gp2b¯σμνqνMW(gLPL + gRPR) tW−μ + h.c.VRgLgR[Phys. Rev. D67 (2003) 014009; Eur. Phys. J. C50 (2007) 519]anomalous couplings                                                                                                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                   PSfrag replacementsWt bθ`bθ∗`= pi − θ`bν`fit to cosθ∗ℓ :ρR,L = FR,L/F0ρSMR= 5.1× 10−4ρSML = 0.423angular asymmetries:ASMFB= −0.2225ASM+ = +0.5482ASM− = −0.839700.10.20.30.40.50.60.7-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1FiVL=1 and gR=gL=VR=0cosθ*lFL=ΓL/ΓtotßA+FR=ΓR/Γtot(rescaled)ßßA-F0=Γ0/ΓtotßßAFBSManomalous Wtb couplings 13kinematics reconstruction:minimize χ2 by looping on jets and scanning on pνz :χ2 =(mℓνj −mt)2σ2t+(mjbjcjd −mt)2σ2t+(mℓν −mW)2σ2W+(mjcjd −mW)2σ2Wmt = 175 GeV, σt = 14 GeVmW = 80.42 GeV, σW = 10 GeVprobabilistic analysis:LS = N=1PsgnLB = N=1Pbck.LR = log(LS/LB)LR > 0.1PSfrag replacementsnP =nntot)B/LS (L10 = logRL-3 -2 -1 0 1 2 3events/bin05001000150020002500 ATLASPreliminary-1L = 1 fb (no b-tag)µe+)B/LS (L10 = logRL-3 -2 -1 0 1 2 3events/bin02004006008001000 ATLASPreliminary-1L = 1 fb (b-tag)µ   e+→ acceptreject ← → acceptreject ←expected signal and background events:no b-tag with b-tagsignal 12627± 103 6586± 74background 4011± 203 851± 94anomalous Wtb couplings 14l*θcos -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 10100200300400500ATLAS-1L = 1 fb (b-tag)µ   e+l*θcos -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1correction factor01234567ATLAS-1L = 1 fb (b-tag)µ   e+expected values with statistical errors:no b-tag with b-tagρL 0.402 ± 0.050 0.453 ± 0.048ρR -0.008 ± 0.008 -0.004 ± 0.007AFB -0.220 ± 0.025 -0.229 ± 0.026A+ 0.560 ± 0.024 0.542 ± 0.028A− -0.845 ± 0.012 -0.830 ± 0.014sources of systematic uncertainties (with b-tag):source ρL ρR AFB A+ A−jet energy scale 0.038 0.001 0.010 0.004 0.002luminosity 0.007 0.000 0.006 0.005 0.001top mass 0.026 0.003 0.013 0.008 0.006background 0.005 0.000 0.003 0.002 0.004ISR+FSR 0.052 0.006 0.024 0.028 0.015MC generator 0.012 0.008 0.009 0.011 0.000pile-up 0.146 0.006 0.012 0.041 0.022total 0.163 0.012 0.033 0.052 0.027anomalous Wtb couplings 15constraints on the anomalous couplingsρLρRA+A−VRgLgRrare top decays and FCNC 16Wtq¯′, ν¯q, `bZt`−`+u, ctγu, ctgu, cBR(t→ FCNC) in several models:SM QS 2HDM FC 2HDM MSSM /R SUSYt→ qγ ∼ 10−14 ∼ 10−9 ∼ 10−6 ∼ 10−9 ∼ 10−6 ∼ 10−6t→ qZ ∼ 10−14 ∼ 10−4 ∼ 10−7 ∼ 10−10 ∼ 10−6 ∼ 10−5t→ qg ∼ 10−12 ∼ 10−7 ∼ 10−4 ∼ 10−5 ∼ 10−5 ∼ 10−4[Acta Phys. Polon. B 35 (2004) 2695]rare top decays and FCNC 17different selection cuts applied to the FCNC analyses:PSfrag replacementsqγ ℓℓ+ℓ−bνgZPSfrag replacementsqγℓℓ+ ℓ−bνgZPSfrag replacementsqγℓℓ+ℓ−bνgZkinematics reconstruction (no b-tag used):minimize χ2 by looping on jets (and leptons) and scanning on pνz :χ2 =mFCNCt −mt2σ2t+mℓνj −mt2σ2t+ mℓν −mW2σ2W+mℓbℓc −mZ2σ2Zmt = 175 GeVσt = 14 GeVmW = 80.42 GeVσW = 10 GeVmZ = 91.19 GeVσZ = 3 GeVrare top decays and FCNC 18after final selection:tt¯→ bWqγ:total bkg. 650± 66signal % 7.6± 0.2tt¯→ bWqZ:total bkg. 125± 56signal % 7.6± 0.2tt¯→ bWqg:total bkg. 19253± 359signal % 2.9± 0.1probabilistic analysis:LS = N=1PsgnLB = N=1Pbck.LR = ln(LS/LB)no cut on LRPSfrag replacementsnP =nntotnormalised discriminant variables:RL-6 -4 -2 0 2 4 600.050.10.150.20.250.30.350.4 ATLASPreliminary-1L = 1 fbµe+RL-8 -6 -4 -2 0 2 4 6 800.050.10.150.20.250.30.350.40.45ATLASPreliminary-1L = 1 fb3lRL-6 -4 -2 0 2 4 600.050.10.150.20.25ATLASPreliminary-1L = 1 fbµe+tt¯→ bWqγ tt¯→ bWqZ tt¯→ bWqg− Signal ATLFAST + Signal FullSim Background ATLFAST +◦ Background FullSimrare top decays and FCNC 19expected 95% CL limits:−1σ expected +1σtt¯→ bWqγ:3.8× 10−4 6.8× 10−4 1.0× 10−3tt¯→ bWqZ:1.9× 10−3 2.8× 10−3 4.2× 10−3tt¯→ bWqg:7.2× 10−3 1.2× 10−2 1.8× 10−2systematic uncertainties:source t→ qγ t → qZ t → qgjet energy calibration 2% 5% 4%luminosity 10% 6% 10%top mass 6% 12% 5%backgrounds σ 7% 12% 15%ISR/FSR 17% 7% 9%pile-up 22% 0% 13%generator 4% 14% 4%χ2 4% 7% 9%total 32% 25% 27%10-510-410-310-210-1110 -5 10 -4 10 -3 10 -2 10 -1 1BR(t→qγ)BR(t→qZ)LEPZEUS(q=u only)ZEUS(q=u only)(630 pb-1)CDFCDF(2 fb-1)ATLAS (1 fb-1)95% C.L.EXCLUDEDREGIONSsummary 20expectedprecisionsfor 1 fb−1top charge (2/3 versus -4/3) ≥ 5σspin correlations:A 50%AD 34%W polarisation:F0 5%FL 12%FR 0.03angular asymmetries:AFB 18%A+ 11%A− 4%anomalous couplings:VR 0.15gL 0.07gR 0.15top FCNC decays (95% C.L.):Br(t→ qγ) 10−3Br(t→ qZ) 10−3Br(t→ qg) 10−2

top quark properties: prospects at ATLAS

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