Status of FCNC top decays(t->γ+q ,t->Z +q )

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Presentation transcript:

Status of FCNC top decays(t->γ+q ,t->Z +q ) Grigoris Vermisoglou, Costas Karafasoulis , Aristotelis Kyriakis, Institute of Nuclear Physics NCSR “Demokritos” Leonardo Benucci, Andrea Giammanco, Fabrizio Palla INFN, Pisa OUTLINE Why FCNC of top quark? Signal Topology Relevant Background Analysis - Plots Future plans Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

Gregory Vermisoglou, NCSR "Demokritos" 1.WHY FCNC OF TOP QUARK? Top is the only quark that has no time to form strong bound states because it decays very fast( τdecay <τhadronization where : τdecay =1/(Γ~1.5 GeV) , τhadronization ~1/(ΛQCD ~0.2 GeV)) makes it a very clean source of providing fundamental information. Not only the heaviest of the quarks but also up to now a point like particle  possible deviations from SM are more likely to be observed in the top quark sector In SM the dominant decay of the top quark is t  Wb with a BR99.9% The Flavor Changing Neutral Current (FCNC) decays of the top quark t  qV (V= g, γ, Z) that occur at one loop are GIM suppressed in SM by a factor mb2/MW2 in the amplitude In SM their expected BR is very small (of the order of 10-11 to 10-13) So any observation at the LHC would be a signal of new Physics Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

WHY FCNC OF TOP QUARK?(Cont) Significant increase in their BR ( by several orders of magnitude) are predicted in SUSY extensions of the SM In the MSSM which we study R-parity is violated. This means that LSP of the model is unstable and either the leptonic or the baryon number is violated. In this model it is prefered the second choice which has as a consequence the proton to be unstable and the the BR of FCNC of top decays to be increased by 8 or 9 orders of magnitude. The BR are summarized in the following Table: FCNC decay BR in SM BR in SUSY Exper. limits (95% CL) tqγ 5x10-13 2x10-5 < 0.003 (HERA) tqZ 1.3x10-13 8x10-4 < 0.08 (LEP-2) tqg 5x10-11 10-3 <0.29 (CDF+TH) Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

2. SIGNAL PRODUCTION: tγ q,tZ0 q 1. The goal is to test the discovery potential of the FCNC of the top quark in the CMS/LHC environment The FCNC decays: t  qγ ,t  Z0 q (q = u, c) were studied, with Z0 l+l- 2. We used TOPREX event generator in order to produce ttbar pairs. One of the tops was forced to decay in the above FCNC decays while the other according to the SM i.e. tWb and the Wlνl (l = e, μ). 3. Initial and final QED and QCD radiative corrections have been taken into account. We produced 10000 fully simulated signal events for each of the above FCNC top decays in our home institute. Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

3.ANALYSIS OF THE CHANNEL:tq+γ 3.1RELEVANT BACKGROUND(SO FAR) Background for FCNC tqγ Process σ x BR Events (L= 10fb-1) Events Analyzed 1 830pb(NLO) 8.3M 1.3M 2 10069pb(LO) 100Μ 735K 3 4307pb(LO) 43Μ 545Κ Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

ANALYSIS-CUT DEFINITION Various cuts: (1e+1γ) || (1μ +1γ) from L1 and HLT trigger 1 isolated e (Pt > 25GeV/c) or μ (Pt > 15GeV/c) that forms with Missing Energy (PtEmis > 25GeV/c) a Transverse Mass in the range(0, 120)GeV/c2 (compatible with the W transverse mass). If 2 isolated leptons exist the event is rejected Only ONE b-jet with (Pt > 40GeV/c) Transverse W + b-jet mass in the range (80GeV/c2 , 200GeV/c2) An isolated photon with (Pt >50GeV/c) an a light jet with (Pt >50GeV/c) Cosine of azimuthal angle between (W+b-jet) && (γ + light-jet) vectors < -0.95 (back to back in the XY plane) (γ + light-jet) invariant mass in the range (150GeV/c2 , 200GeV/c2) Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

CHANNEL:tq+γ(Cont.) FCNC top Mass After all cuts: Backgrounds Normalized to L = 10fb-1 Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

SIGNAL EFFICIENCY – BACKGROUND REJECTION Cut Signal efficiency(%) Background Survived(%) t-tbar->inclusive Wjets_50_85 Wjets_85_150 1(L1+HLT) 81.54 26.584 6.707 13.76 2(W->lν) 39.88 5.068 1.45 2.566 3(b-jet) 14.45 2.1 0.0546 0.212 4(SM-top) 11.39 1.37 0.034 0.096 5(ljet+γ) 5.15 0.0048 0.0 6(cosxy) 3.67 0.0022 7(FCNC-top) 2.6 0.0000775 So, the efficiency of all cuts is 2.6% and the background that survives is the ttbar background with a rejection factor 0.8x10-6. We expect 6.4 background events at L = 10fb-1 Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

Gregory Vermisoglou, NCSR "Demokritos" BR LIMITof tq+γ The expected event number can be calculated assuming 3σ signal significance for discovery at 99% CL from the formula : sqrt(S + B) – sqrt(B) > 3/2. For B = 6.4 we have S = 10 events The upper limit for the BR can be calculated from the formula: S=10.0, L=10fb-1, σ = 830pb, BRWlν =0.22, ε = 0.026 The calculation gives a BR upper limit : 1.1x10-4 which is one order of magnitude better that the current experimental limit from HERA and approaches the SUSY estimation( ~ 2x10-5) Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

4.ANALYSIS OF THE CHANNEL:tq+Z0 RELEVANT BACKGROUND(SO FAR) Background for FCNC: tZ0 q Process σ x BR Events (L= 10fb-1) Events Analyzed 1 85pb(NLO) 850K 871K 2 70.3pb(LO) 703K 393K 3 26.7pb(LO) 267K 197K Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

ANALYSIS-CUT DEFINITION Various cuts: Double Muon || Double Electron L1 and HLT selection 2 isolated STOS e with (Pt> 20GeV/c) or μ with (Pt > 15GeV/c) and invariant mass around the nominal Z0 mass (±10GeV/c2) . If 2 Z0s found reject the event An extra isolated e (Pt > 20GeV/c) or μ ( Pt > 15GeV/c) that forms with Missing Energy (PtEmis > 20GeV/c) a Transverse Mass < 120GeV/c2 (compatible with the W transverse mass) MT2 (l+ν)=((pT(l)+Emiss)2-(px(l)+px(ν))2-(py(l) +py(ν))2)1/2 Only ONE b-jet with (Pt > 40GeV/c) Transverse W + b-jet mass in the range (60GeV/c2 , 200GeV/c2) ΜT2(W+b)=((Mw2 +(PT(W))2)1/2 +PT(B))2-(PT(W)+PT(B))2 Only ONE extra light jet with (Pt >30GeV/c) Cosine of azimuthal angle between (W+ b-jet) && (Z0 + light-jet) vectors < -0.5 (back to back in the XY plane) Events with (Z0 + light-jet) invarint mass in the range (150GeV/c2 , 200GeV/c2) Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

CHANNEL: tq+Z0 (Cont.) FCNC top Mass After all cuts: Backgrounds Normalized to L = 10fb-1 Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

4.5 SIGNAL EFFICIENCY- BACKGROUND REJECTION Cut Signal efficiency(%) Background Survived(%) t-tbar->leptonic WWjets->inclusive ZWjets->inclusive 1(L1+HLT) 89.8 47.39 19.34 20.30 2(Z0->l+ l- ) 42.08 1.5 0.086 2.042 3(W->lν) 22.88 0.019 0.0 0.235 4(b-jet) 7.96 0.00666 5(SM-top) 7.19 0.005 6(ligth-jet) 5.83 0.0015 7(cosxy) 5.38 0.0013 8(FCNC-top) 3.77 0.00046 So, the efficiency of all cuts is 3.77% and the main background that survives is the leptonic ttbar background with a rejection factor 4.6x10-6. We expect 3.9 background events at L = 10fb-1 Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

BR UPPER LIMIT for tq+Z0 The expected event number can be calculated assuming 3σ signal significance for discovery at 99% CL from the formula : sqrt(S + B) – sqrt(B) > 3/2. For B = 3.9 we have S = 8.2 events The upper limit for the BR can be calculated from the formula: S=8.2, L=10fb-1, σ = 830pb, BRWlν =0.22, BRZ→ll=0.067 , ε = 0.0377 The calculation gives a BR upper limit : 8.8x10-4 which is two orders of magnitude better that the current experimental limit from LEP-2 and approaches the SUSY estimation( ~ 8x10-4) Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"

Gregory Vermisoglou, NCSR "Demokritos" 5. FUTURE PLANS Study other types of background (Wγ, Zjets, ZZ inclusive, γ+jets,EW single top, Zbb, Wbb, QCD) Perform the same studies in high luminosity case Ioannina, 16 April 2006 Gregory Vermisoglou, NCSR "Demokritos"