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ATLAS

ATLAS. Study of ATLAS Sensitivity to FCNC Top Quark Decays J.Carvalho, N.Castro, L.Chikovani, T.Djobava, J.Dodd, S.McGrath, A.Onofre, J.Parsons, F.Valoso Workshop, Tbilisi, October 18/2005. Introduction. We study the ATLAS experiment sensitivity to FCNC top quark decays

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ATLAS

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  1. ATLAS Study of ATLAS Sensitivity to FCNC Top Quark DecaysJ.Carvalho, N.Castro, L.Chikovani, T.Djobava, J.Dodd, S.McGrath, A.Onofre, J.Parsons, F.ValosoWorkshop, Tbilisi, October 18/2005

  2. Introduction • We study the ATLAS experiment sensitivity to FCNC top quark decays ( t→Zc/u, t→γc/u, t→gc/u ) at the LHC • In SM, FCNC top quark rare decays are strongly suppressed (loop suppression, heaviness of gauge bosons )Br < 10-13 - 10-10 • Supersymmetric extensions to the SM lead to significantly larger predictions for FCNC branching ratios on the level 10-8 - 10-6 still too small to be observable at the LHC • Observation of significantly larger Br’s will be a clear signal of new physics (new dynamical interactions of top quark, multi-Higgs doublets, exotic fermions or other possibilities ) •The Supersymmetric models (without R-parity) have much higher value : Br ~ 10-4 • Although FCNC processes associated with the decay of top quarks ( t→γc/u, t→Zc/u ) have been studied at CDF, the amount of data collected up to now is not comparable with the statistics expectedat LHC Workshop,Tbilisi,18/2005

  3. t g g g t q t q t Introduction LHC will be a” top factory” • NLO prediction that (t-tbar) = 833 pb ( which is  100X larger than at FNAL) in pp collisions at √ s = 14 TeV implies that more than 8 thousands t-tbar pairsper day will be produced in low luminosity (10-33cm -2 s-1) running of LHC • Production processes at LHC: Top Quark Pair Production through gluon fusion and quark-antiquark annihilation 90%gg + 10% q-qbar Workshop,Tbilisi,18/2005

  4. Introduction The Analysis presented here focus on two final state topologies: • Leptonic mode t → Xq ( X = Z, γ, g ; q = u,c ) t → Wb →lνb • Hadronic mode t → Xq ( X = Z, γ, g ; q = u,c ) Different types of analysis ( sequential & probabilistic ) are applied to each FCNC decay mode and their results are compared t → Wb →jjb Workshop,Tbilisi,18/2005

  5. Introduction The results obtained by the different analyses is presented within two approaches : • Branching ratio sensitivities ( assuming a 5σ signal significance for discovery ) • 95% confidence level limits ( in thehypothesis of signal absence) Workshop,Tbilisi,18/2005

  6. Monte Carlo Event Generation • The MC generation of the background processes : b-bbar, Z/γ**+jets, WW, ZZ, ZW }PYTHIA . W+jets} HERWIG Single top quark production, SM t-tbar & Signal } ONETOP & TopReX Initial and final state QED and QCD radiation (ISR+FSR), multiple interactions, fragmentations and decays of unstable particles were enabled. • The generated background and signal events were passed through the ATLAS fast simulation ATLFAST 2.0, 2.14 or 2.53 and ATLFAST-B packages. • ATLFAST was used with its default parameters for low and high luminosity running. • The b-jet tagging efficiency was set to 60% for low luminosity ( L= 10 fb-1 ) and 50% for high luminosity ( L = 100 fb-1) • The rejection factor of c - jets is 10 and of light quarks jets 100. • The CTEQ2L and CTEQ5L Parton Distribution Functions were used. Workshop,Tbilisi,18/2005

  7. t → Zq ( q = u, c ) • CDF Br < 33% @ 95%CL • The enormous QCD backgrounds at hadron colliders make the search for the signal via hadronic decays t-tbar → WbZq → 6-jets very difficult. • Z → l+ l- ( l = e,m )significantly reduces the backgroundsprovides a relatively clean experimental signature. The final state is determined by the decay of W: * t-tbar → Zq,Wb → llj, l νb leptonic mode *t-tbar → Zq,Wb →llj, jjb hadronic mode The hadronic W decay mode has a much larger Branching ratio, but suffers from larger backgrounds. Workshop,Tbilisi,18/2005

  8. t-tbar → Zq,Wb → llj, l νbSequential Analysis ( leptonic mode ) • Experimental Signature: ● three isolated charged leptons ( with m_ll≈ m_z ) ● missing transverse momentum pT ● two jets : light _jet + b_jet • Backgrounds : *Z+jets dominated by ● qg→ Zq ; ● qqbar→Zq thresholds were imposed at the generator level on the invariant mass, m^=√s=130 GeV and transverse momentum ( pT > 50 GeV )hard parton interaction. σ = 3186pb2.1X 107events * ZW ( 26.58pb ) ; * Wjj ( 18000pb ) * Wbb ( 300pb ) * Single top: W-gluon Fusion (244pb), Wt (60pb), s-channel (10 pb) * t-tbarSM ( 833pb ) : dileptonic decay, semileptonic decay, hadronic decay Workshop,Tbilisi,18/2005

  9. t-tbar → Zq,Wb → llj, l νbSequential Analysis ( leptonic mode ) • The final topologies of most dangerous backgrounds: ● WZ→ l+ l-l±ν38 000 events ● t-tbar→W+ bW-b→l+ νbl-νb 3.9x106 events • selection cuts: ● three isolated carged leptons withpTl > 30 GeV (e) ,|η| < 2.5; Z+jets, t-tbar,Wjj,Wb-bbar,Wt ● at least two jets with pTjet > 50 GeV, |η| <2.5, ΔRjj > 0.4, ΔRlj > 0.4; ZW ●pTmiss > 30 GeV Z+jets ● A like sign, same-flavor pair of isolated leptons was required to reconstruct to the Z mass within a window of m_ll=mz ± 6 GeV , Zmass resolution ~ 2.9 GeV. t-tbar ● A peak at the top quark mass in the Zj invariant mass distribution within mass window m-llj =mt± 12 GeV, m-llj =mt ± 24 GeV ( The top quark mass resolution σ(m-llj ) = 14 GeV ) after all cuts, have (for L=100 fb-1) • signal efficiency = 6.1 % • number of surviving background events : 2 WZ , 4 t-tbar Workshop,Tbilisi,18/2005

  10. a)Invarint mass distribution of lepton pairs m-ll b)Invarint mass distribution of t→Zq→llj for leptonic mode Workshop,Tbilisi,18/2005

  11. t-tbar → Zq,Wb → llj, jjbSequential Analysis ( hadronic mode ) • Experimental Signature: ● twoisolated charged leptons ( with m_ll≈ m_z ) ● fourjets : threelight _jet + b_jet • Backgrounds : *Z+jets dominated by ● qg→ Zq ; ● qqbar→Zq thresholds were imposed at the generator level on the invariant mass, m^=√s=130GeV and transverse momentum ( pT > 50 GeV ) hard parton interaction. σ(Z+jets) = 3186pb2.1X 107events * ZW ( 26.58pb ) ; * Wjj ( 18000pb ) * Wbb ( 300pb ) * Single top: W-gluon Fusion (244pb), Wt (60pb), s-channel (10 pb) * t-tbarSM ( 833pb ) : dileptonic decay, semileptonic decay, hadronic decay Workshop,Tbilisi,18/2005

  12. t-tbar → Zq,Wb → llj, jjbSequential Analysis ( hadronic mode ) • The final topologies of most dangerous backgrounds: ● Z +jets→ l+ l- + jets 2.1x106events ● t-tbar→W+ bW-b→l+ νbl-νb + l± νbjj 28.x106 events • selection cuts: ● two isolated charged leptons withpTl > 30 GeV (e) ,|η| < 2.5; t-tbar (semileptonic decay mode) ● at least four jets with pTjet > 50 GeV, |η| <2.5, ΔRjj > 0.4, ΔRlj > 0.4; WZ ● A like sign, same-flavor pair of isolated leptons was required to reconstruct to the Z mass within a window of mz ± 6 GeV , Z mass resolution ~ 2.9 GeV. ● invariant mass mjj = mw ± 16GeV , W mass resolution ~ 8 GeV. Z+jets ● one jet tagged as b-jet Z+jets ,WZ ● invariant mass mjjb = mt ± 8GeV , mjjb mass resolution ~ 18.5 GeV. WZ &Z+jets ● A peak at the top quark mass in the Zj invariant mass distribution within mass window m-llj =mt± 12 GeV, m-llj =mt ± 24 GeV, Z mass resolution σ(m-llj ) =14 GeV after all cuts, have (for L=100 fb-1) • signal efficiency = 0.4 % • Number of surviving background events : two Z+jets, four t-tbar Workshop,Tbilisi,18/2005

  13. a)Invarint mass distribution of jet pairs, m-jj b)Invarint mass (m-jjb) distribution of t→Wb→jjb ( hadronic mode) Workshop,Tbilisi,18/2005

  14. a)Invarint mass distribution of lepton pairs m-ll b)Invarint mass(m-llj) distribution of t→Zq→llj ( hadronic mode ) Workshop,Tbilisi,18/2005

  15. t-tbar → Zq,Wb → llj, l νbProbabilistic Analysis ( leptonic mode ) • selection cuts: ● three isolated charged leptons withpTl > 25GeV (e) ,|η| < 2.5; Z+jets, t-tbar,Wjj,Wb-bbar,Wt ● at least two jets with pTjet > 20 GeV, |η| <2.5, ZW ●pTmiss > 20 GeV Z+jets ● A like sign, same-flavor pair of isolated leptons was required to reconstruct to the Z mass by calculating m-ll invariant mass. t-tbar ● the top quark mass (m-llj ) reconstruction was done by associating the non-b- jet with lepton pair. Last cut : ●LR > 5.62 Workshop,Tbilisi,18/2005

  16. t-tbar → Zq,Wb → llj, l νbProbabilistic Analysis ( leptonic mode ) • Following the final selection, a probabilistic type of analysis was applied. Signal ( Ρi signal ) and background-like( Ρi back. ) probabilitieswere computed using Probability Density Functions ( p.d.f. ) constructed from relevant physical variables. The signal Ls=Πi=1n Ρi signaland background LΒ= Πi=1n Ρi back likelihoods (n is the number of p.d.f.) were used to built the discriminant variable, defined as LR= Ls / LΒ . For the t→Zq channel the p.d.f. were based on the following physical distributions : ■ minimum invariant mass m-ll of the three possible combinations of two leptons (only the three leading leptons were considered ); ■ transverse momentum of the third lepton pTl3 ■ the jll invariant mass and ■ transverse momentum of the most energetic non-b jet pTjet Workshop,Tbilisi,18/2005

  17. Invarint mass (m-llj) distribution of t→Zq→llj Probabilistic Analysis ( leptonic mode ) Workshop,Tbilisi,18/2005

  18. Distribution of variables based on wich the p.d.f were built (t→Zq)SM background and signal discriminant variable distributions Workshop,Tbilisi,18/2005

  19. t-tbar →γq,Wb →γj, l νbSequential Analysis ( leptonic mode) ■ CDF Br < 3.2% @ 95%CL ■ The presence of a high pT photon in t-tbar →γq,Wbevents is not sufficient to reduce the QCD multy-jet background to a manageable level. Therefore, the analysis considered the final state with a the leptonic decay of W boson. • Experimental Signature: ●one isolated highpT photon ●one isolated highpT chargedlepton ● missing transverse momentum pT ● two jets : light _jet + b_jet • Backgroundi s dominated by the events with a real W→lν and either a real or fake photon.: * Wjj +Wbb HERWIG * Single top: W-gluon Fusion ,Wt (60pb), s-channel (10 pb) ONETOP * t-tbarSM ( 833pb ) : dileptonic decay, semileptonic decay, hadronic decay PYTHIA Workshop,Tbilisi,18/2005

  20. t-tbar →γq,Wb →γj, l νbSequential Analysis ( leptonic mode) • selection cuts: ● one isolated photons withpTγ> 40 GeV (e) ,|η| < 2.5; ● one isolated charged leptons withpTl > 20 GeV (e) ,|η| < 2.5; ● # jets = 2 with pTjet > 20 GeV, |η| <2.5, (# b-jets = 1, pTjet > 30 GeV) ●pTmiss > 20 GeV ● invariant mass m(lνb) = mt ± 20 GeV ● A peak at the top quark mass in the m-γj invariant mass distribution within mass window m-γj =mt± 12 GeV, The top quark mass resolution σ(m-γj ) = 7.7 GeV after all cuts, have (for L=100 fb-1) • Number of surviving 155 background events dominated byt-tbar, Wjj,+Wb-bbar,Wt (60) Workshop,Tbilisi,18/2005

  21. t-tbar →γq,Wb →γj, l νbProbabilistic Analysis ( leptonic mode) • selection cuts: ● one isolated photon withpTγ> 75 GeV ,|η| < 2.5; ● a maximum of two isolated charged leptons withpTl > 20 GeV (e) ,|η| < 2.5; ● # jets = 2 with pTjet > 20 GeV, |η| <2.5, (# b-jets = 1, pTjet > 30 GeV) ●pTmiss > 20 GeV ● A peak at the top quark mass in the m-γj invariant mass distribution Just like t→Zu(c) channel, a probabilistic type of analysis was used. The p.d.f. were built based on the following variables : ■ transverse momentum of the leading photon pTγ ■ the γj invariant mass ■ the number of jets Last cut : ●LR > 3.13 Workshop,Tbilisi,18/2005

  22. The Distribution of variables based on which the p.d.f were builtt →γu(c)Probabilistic Analysis ( leptonic mode) Workshop,Tbilisi,18/2005

  23. SM background and signal discriminant variables distribution for t →γu/cProbabilistic Analysis ( leptonic mode) Workshop,Tbilisi,18/2005

  24. t-tbar → gq,Wb → jj, l νbProbabilistic Analysis ( leptonic mode ) • Experimental Signature: ●one isolated highpT chargedlepton ● missing transverse momentum (pT ) from the undetected neutrino ● at least three isolated jets : twolight _jets+ b_jet Fully hadronic final state • Backgrounds : *Z+jets * W + jets * Single top: W-gluon Fusion ,Wt * t-tbarSM : dileptonic decay, semileptonic decay • A search for a FCNC t-gluon-q coupling (with q=u,c) through the decay t→gq would be overwhelmed by background from QCD multi-jet events ■More restrictive event selection is necessary Workshop,Tbilisi,18/2005

  25. t-tbar → gq,Wb → jj, l νbProbabilistic Analysis ( leptonic mode ) • selection cuts: ● one isolated charged lepton withpTl > 20 GeV ,|η| < 2.5; ●no photons withpTγ> 5 GeV (e) ,|η| < 2.5; to reject events assigned to t→γq ●at least three jets with pTjet > 20 GeV, |η| <2.5, (# b-jets = 1, pTjet >40GeV) ●pTmiss > 20 GeV • The events were classified as ‘3 jets’ or ‘4 jets’ • The ‘3 jets’ sample ■transverse momentum of the leading jet pTg > 75 GeV ■ the gj invariant mass 125 <m_t < 200 GeV As for the other channels , a probabilistic type of analysis was adopted, using the following variables to built the p.d.f. : ■the gq invariant mass ■ the blν invariant mass ■transverse momentum of the b-jet ■transverse momentum of the second non-b jet ; Last cut : ■angle between the lepton and the gluon αlg●LR > 1.13 Workshop,Tbilisi,18/2005

  26. The Distribution of variables based on which the p.d.f were builtt →gu(c)Probabilistic Analysis ( leptonic mode) ‘3 jets’ Workshop,Tbilisi,18/2005

  27. SM background and signal discriminant variables distribution for t →gu/cProbabilistic Analysis ( leptonic mode) ‘3 jets’ Workshop,Tbilisi,18/2005

  28. t-tbar → gq,Wb → jj, l νbProbabilistic Analysis ( leptonic mode ) • The ‘4 jets’ sample ■transverse momentum of the leading jet pTg > 100 GeV ■ the gj invariant mass 150 <m_t < 190 GeV The p.d.f. were built based on the following physical distributions : ■ the blν invariant mass, ■ transverse momentum of qg, ■transverse momentum of blν , ■angle between the lepton and the gluon ( αgl ), ■angle between the lepton and b-jet ( αlb ), ■angle between the gluon and the second non-b jet ( αgq ). Last cut: ●LR > 1.13 Workshop,Tbilisi,18/2005

  29. The Distribution of variables based on which the p.d.f were builtt →gu(c)Probabilistic Analysis ( leptonic mode) ‘4 jets’ Workshop,Tbilisi,18/2005

  30. SM background and signal discriminant variables distribution for t →gu/cProbabilistic Analysis ( leptonic mode) ‘4 jets’ Workshop,Tbilisi,18/2005

  31. Branching Ratio Sensitivity( 5σ significance discovery hypothesis ) • Expected top quark FCNC decay branching ratios sensitivities of the ATLAS experiment were estimated for both the sequential and probabilistic analysis under two different hypothesis: • 95 % confidence level limits ( hypothesis of absence of signal ) • 5σ significance discovery hypothesis Assuming a signal discovery with a 5σ significance, the branching ratio for each channel studied is estimated by: 5√B Min. BR = ---------------------------------------------- 2 x L x σ(t-tbar)x BR x ε Workshop,Tbilisi,18/2005

  32. Branching Ratio Sensitivity for a luminosity L =10 fb-1, L =100 fb-1 ( 5σ significance discovery hypothesis ) t→Zu (c) Workshop,Tbilisi,18/2005

  33. Branching Ratio Sensitivity for a luminosity L =10 fb-1, L =100 fb-1 ( 5σ significance discovery hypothesis ) Workshop,Tbilisi,18/2005

  34. 95 % confidence level limits ( hypothesis of absence of signal ) • Sequential Analysis For the sequential analysis of tZq decays at integrated luminosity 100fb-1 was evaluatedthe 95% CL upper limits. Assuming the Poisson processes with backgrounds 95% CL upper limits on the number of signal events for both, leptonic and hadronic decay modes were derived. Using the NLO Calculation for σ(t-tbar), these limits were then converted into limits on the Branching Ratios for both decay modes. • Probabilistic Analysis The modified frequent likelihood method was used to evaluate the 95% CL upper limits. the Full information of the discriminant variables were used to derive 95% CL upper limits on the number of signal events for each channel. No cuts on the discriminant variables were used. Using the NLO calculation for σ(t-tbar), these limits were then converted into limits on the Branching Ratios for each of studied FCNC top decay channels. Workshop,Tbilisi,18/2005

  35. The expected 95% confidence level limits on the FCNC top decays branching ratio in the absence of signal hypothesis t→Zu (c) ( L =10 fb-1, L =100fb-1 ) Workshop,Tbilisi,18/2005

  36. The expected 95% confidence level limits on the FCNC top decays branching ratio in the absence of signal hypothesis Workshop,Tbilisi,18/2005

  37. Electroweak Single Top Quark Production • 3 separate production processes: W-gluon fusion (250 pb) W*/s-channel (10 pb) Wt (60 pb) Workshop,Tbilisi,18/2005

  38. Top Quark Rare Decays (cont’d) t  Hq • Tblisi group has been studying various approaches to t  Hq • Earlier results for t tbar  Hq Wb  (b-bbar)j (lb) for mH = 115 GeV • Sensitive to Br(t  Hq) = 4.5 X 10-3 (100 fb-1) • New results for t tbar  Hq Wb  WW*q Wb  (l lj) (lb) • ≥ 3 isolated leptons with pT(lep) > 30 GeV • pTmiss > 45 GeV • ≥ 2 jets with pT(j) > 30 GeV, incl. ≥ 1 jet with b-tag • kinematic cuts to take advantage of angular and other correlations • Sensitive to Br(t  Hq) = 2.4 X 10-3 for mH = 160 GeV (100 fb-1) Workshop,Tbilisi,18/2005

  39. Conclusions • The Sensitivity of the ATLAS experiment to the FCNC t → X q ( X = Z, γ, g and q=u,c ) decays of the top quark was performed. • Different types of analysis ( sequential and probabilistic ) were used to obtain the FCNC branching ratio sensitivities ( assuming a 5σ signalsignificance for discovery ) or the 95% CL limits on the FCNC branching ratios ( in absence of signal ). • In the Sequential analysis the kinematic cuts were applied in sequence for the signal and backgrounds in order to evaluate the expected branching ratio sensitivities. In probabilistic analysis signal and background-like probabilities were computed using Probability Density Functions constructed from relevant physical variables. The results obtained from both types of analysis coincide and are compatible. • The expected branching ratio sensitivities obtained by the different analysis are in good agreement, being in the range from 10-3 to 10-5 ( for L =100 fb-1 ) • The impact of Systematic errors on the final results have been studied • Studies of the top quark FCNC decays provide a sensitive window onto possible new physics beyond the SM Workshop,Tbilisi,18/2005

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