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

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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

Gregory Vermisoglou, NCSR "Demokritos"

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 BR99.9%

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

Gregory Vermisoglou, NCSR "Demokritos"

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:

Gregory Vermisoglou, NCSR "Demokritos"

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. tWb and the Wlνl (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.

Gregory Vermisoglou, NCSR "Demokritos"

Gregory Vermisoglou, NCSR "Demokritos"

- Various cuts:
- (1e+1γ) || (1μ +1γ) from L1 and HLT trigger
- 1 isolatede(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)

Gregory Vermisoglou, NCSR "Demokritos"

After all cuts: Backgrounds Normalized to L = 10fb-1

Gregory Vermisoglou, NCSR "Demokritos"

So, the efficiency of all cuts is 2.6% and the background that survives is the ttbar background with a rejection factor0.8x10-6. We expect 6.4background events at L = 10fb-1

Gregory Vermisoglou, NCSR "Demokritos"

The expected event number can be calculated assuming 3σ signal significance for discoveryat 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, BRWlν =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)

Gregory Vermisoglou, NCSR "Demokritos"

Gregory Vermisoglou, NCSR "Demokritos"

- Various cuts:
- Double Muon || Double Electron L1 and HLT selection
- 2 isolated STOSe 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)

Gregory Vermisoglou, NCSR "Demokritos"

After all cuts: Backgrounds Normalized to L = 10fb-1

Gregory Vermisoglou, NCSR "Demokritos"

So, the efficiency of all cuts is 3.77% and the main background that survives is the leptonic ttbar background with a rejection factor4.6x10-6. We expect 3.9background events at L = 10fb-1

Gregory Vermisoglou, NCSR "Demokritos"

The expected event number can be calculated assuming 3σ signal significance for discoveryat 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, BRWlν =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)

Gregory Vermisoglou, NCSR "Demokritos"

- 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

Gregory Vermisoglou, NCSR "Demokritos"