Di-Lepton Channel in ATLAS

1. Di-Lepton Channel in ATLAS S. Tokár, Comenius Univ. Bratislava S. Tokár, Cz-Sk meeting, Košice

2. Di-lepton Channel • A combination of single lepton and di-lepton triggers – the overall trigger efficiency is: e (971)%, ee (981)%, (961)% • Preselection: two high pT opposite signed leptons L=100pb-1 e: Isolation (CaloR=0.2) < 6 GeV : R(-jet) > 0.2 Selection criteria: to maximize Significance: • exactly two leptons, pT> 20 GeV • at least two jets, pT> 20 GeV • ET-mis > 35 GeV (ee,), >30 GeV (e) S. Tokár, Cz-Sk meeting, Košice

3. tt-bar cross section in DIL From MC From exp. • Cross section determined as Efficiency  = geom. acceptance  recon. efficiency Expected x-section statistical error • Inclusive template method • 3 dominant sources of isol. leptons: tt, WW and Z • The proceses can be separated in the planeETmisvs#jets Normalised 2D templates are created for each channel and relative contribution of tt, WW and Z determined (binned likelihood fit to pseudoexp -10 params –inc. ttXsec) S. Tokár, Cz-Sk meeting, Košice

4. Inclusive template cross section results Total composition of the inclusive di-lepton selection versus number of jets Projected significance from pseudo-experiments for the inclusive template fit versus integrated luminosity including all channels and all systematic uncertainties. S. Tokár, Cz-Sk meeting, Košice

5. Likelihood approach The log-likelihood function used to extract Nsigand Nbkggiven the fixed total number of events Ntot and the measurements (xi): The Signal (S) and background (B) functions: by fitting MC Expected statistical error evaluated on X-section measurement for the likelihood method for different luminosities. Distribution of  between the highest pT lepton and the Etmissmomentum vector for (a) signal, (b) bkgd and (c) signal/bkgd composition according to the ratio obtained from the cut analysis. S. Tokár, Cz-Sk meeting, Košice

6. Top quark mass in dilepton channel • No top quark mass determination in DiL channel is in the ATLAS book • Presented in in the top quark mass group: • Matrix element method (P. Cavalleri, LPNHE Paris) • Modified Dalitz-Goldstein method (K.Sliwa et al., Tufts Univ./CDF) • Top mass using J/ - lepton correlations (S. Behar and Y. Rozen, Technion) S. Tokár, Cz-Sk meeting, Košice

7. DIL top mass: Matrix Element Method Tuning of the methods continues – Transfer function, etc S. Tokár, Cz-Sk meeting, Košice

8. TOP MASS : Modified DALITZ-GOLDSTEIN Method • Unknowns are four-momenta of the two ’s • • 8 equations of constraint Assuming top quark mass, Mt, first 6 eq.’s constrain t and tbarthree-momentum vectors to point somewhere on respective ellipses in three momentum (lepton, b-quark and assumed Mt suffices to find such an ellipse for each combination) The last 2 eq.’s - projection of an ellipse from 3-dim momentum space onto a transverse momentum plane is also an ellipse, the t and tbar transverse momenta are equal and opposite - ellipses should then cross each other, intersection points are the solutions for the t and t-bar momenta satisfying all the constraints S. Tokár, Cz-Sk meeting, Košice

9. PLAN FOR ATLAS STUDIES • Monte Carlo studies with events with 10 TeV LHC energy • • evaluate error on top mass achievable within 1 year, 2 years etc. of low luminosity running • • dominant error will be systematic (statistical scales with √N), most likely the most important factor will still be jet energy uncertainty • • backgrounds could be different than in CDF, on the other hand much larger statistics should allow MUCH more stringent selection cuts and most likely in background reduction to less than 10% level, or better • • use support vector machine (Ben Whitehouse) and tagging information to enrich the sample of top-like events with just the kinematics cuts to bring S/B to 10/1 or better S. Tokár, Cz-Sk meeting, Košice

10.  W+ W- t l+ b b t Top Mass via J/ – lepton correlation • The invariant mass of the J/-lepton (from the W) is highly correlated with the top mass. • Create the correlation graph • Measure the InvMass in data • Read the Top mass from the correlation graph J/→l+l- • Main advantages of this channel: • Major systematic effects • (from jet energy scale) are avoided. • Very clean signature W→, J/→2 - first channel to use S. Tokár, Cz-Sk meeting, Košice

11. Signal and Background in 20fb-1 S. Tokár, Cz-Sk meeting, Košice

12. Unbinned Maximum Likelihood Method Top Mass: 175GeV (~ 20 fb-1) Statistical Error for 20 fb-1: 2.8GeV S. Tokár, Cz-Sk meeting, Košice

13. Single top cross section in DIL studies: ATL-PHYS-INT-2009-013, ATL-COM-PHYS-2009-058 (PUB draft) Signal: Wt-channel dilepton → Final state: 1 b-jet, 2 high-PTleptons, large missing ET → Small cross section, but clean signal → Privately produced 50k fast simulation events in release 12 ● Used premade AcerMC input files (acermc.006220.Wtlep) ● Ran through Pythia -> Atlfast -> TopView121403 Backgrounds: → Top pair (Dilepton mode + l+jets mode with fake leptons) → Diboson production (WW, ZZ, WZ) → Z+jets and W+jets with fake leptons ● Cross section is huge, so might be an issue... → Other single top channels with fake leptons S. Tokár, Cz-Sk meeting, Košice

14. Event Selection / results • Selection cuts • 2 OS leptons with PT > 30 GeV (no 3rd one) • ● DIL inv-mass out [81,101] GeV (ee/ only) • ● Pt(jet1) > 30 GeV and Pt(jet2) < 20 GeV • ● Missing ET > 20 GeV (e), 30 GeV (ee/) • Vetoes • e is in 1.37 < || < 1.52 •  is in 1.1 < || < 1.7 • (mis-ET,) <0.5 or >2.6 • 2 muons in 1.1 < || < 1.7 Using multivariate analysis (BDT) to improve discriminating power vsttbar, Z events – different variable are used: MET, m(ll), (MET,j1), (ll,j1), HT, R(l1,l2), pT(j1), … S. Tokár, Cz-Sk meeting, Košice

15. Conclusions • DIL analyses are presentat ATLAS, but are lessadvancedthanlepton + jets one • Good perspectives are in the top mass determination, spin correlations,… S. Tokár, Cz-Sk meeting, Košice