Di-Lepton Channel in ATLAS S. Tokár, Comenius Univ. Bratislava 9/10/2015S. Tokár, Cz-Sk meeting, Košice1

Di-lepton Channel e  (97  1)%, ee (98  1)%,  (96  1)%  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 p T opposite signed leptons 9/10/2015S. Tokár, Cz-Sk meeting, Košice2 e e: Isolation (Calo  R=0.2) < 6 GeV   :  R(  -jet) > 0.2 L=100pb -1 Selection criteria: to maximize Significance: exactly two leptons, p T > 20 GeV at least two jets, p T > 20 GeV E T -mis > 35 GeV (ee,  ), >30 GeV (e  )

tt-bar cross section in DIL  Cross section determined as  Efficiency  = geom. acceptance  recon. efficiency 9/10/2015S. Tokár, Cz-Sk meeting, Košice3 From MC From exp. Expected x-section statistical error Inclusive template method ttWWZ   3 dominant sources of isol. leptons: tt, WW and Z   The proceses can be separated in the plane E T mis vs #jets ttWWZ  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. tt Xsec)

Inclusive template cross section results Total composition of the inclusive di-lepton selection versus number of jets 9/10/2015S. Tokár, Cz-Sk meeting, Košice4 Projected significance from pseudo- experiments for the inclusive template fit versus integrated luminosity including all channels and all systematic uncertainties.

Likelihood approach N sig N bkg The log-likelihood function used to extract N sig and N bkg given the N tot x i fixed total number of events N tot and the measurements (x i ): 9/10/2015S. Tokár, Cz-Sk meeting, Košice5 The Signal (S) and background (B) functions: by fitting MC  p T E t miss signal bkgdsignal/bkgd Distribution of  between the highest p T lepton and the E t miss momentum vector for (a) signal, (b) bkgd and (c) signal/bkgd composition according to the ratio obtained from the cut analysis. Expected statistical error evaluated on X-section measurement for the likelihood method for different luminosities.

Top quark mass in dilepton channel 9/10/2015S. Tokár, Cz-Sk meeting, Košice6  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)

DIL top mass: Matrix Element Method 9/10/2015S. Tokár, Cz-Sk meeting, Košice7 Tuning of the methods continues – Transfer function, etc

TOP MASS : Modified DALITZ-GOLDSTEIN Method 9/10/2015S. Tokár, Cz-Sk meeting, Košice8 Unknowns are four-momenta of the two ’s 8 equations of constraint M t t and tbar ellipses in three momentum Assuming top quark mass, M t, first 6 eq.’s constrain t and tbar three-momentum vectors to point somewhere on respective ellipses in three momentum (lepton, b-quark and assumed M t suffices to find such an ellipse for each combination) intersection points 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

PLAN FOR ATLAS STUDIES 9/10/2015S. Tokár, Cz-Sk meeting, Košice9 10 TeV LHC energy 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 jet energy uncertainty 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

9/10/2015S. Tokár, Cz-Sk meeting, Košice10 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 W-W- W+W+ b b tt l+l+ J/  →l + l - W→ , J/  →2  - first channel to use Main advantages of this channel:  Major systematic effects (from jet energy scale) are avoided.  Very clean signature

9/10/2015S. Tokár, Cz-Sk meeting, Košice11 Signal and Background in 20fb -1 Source Cross section (pb) Expected Events % of Total Signal ±2836% Wrong Pairing ±2836% W+Jets316086±95% Z+Jets243<120 (Zero found)<7% bbar 6140 (  b) <1<0.2% ttbar (  not from W) 1.85±5<0.5% Combinatorial~300±17~16% (?) Fake Muon<15<1%

9/10/2015S. Tokár, Cz-Sk meeting, Košice12 Unbinned Maximum Likelihood Method Top Mass: 175GeV (~ 20 fb-1) Top Mass (GeV) Result from the fit (GeV) Statistical Error for 20 fb -1 : 2.8GeV

Single top cross section in DIL 9/10/2015S. Tokár, Cz-Sk meeting, Košice13 studies: ATL-PHYS-INT , ATL-COM-PHYS (PUB draft) Signal: Wt-channel dilepton → Final state: 1 b-jet, 2 high-P T leptons, large missing E T → Small cross section, but clean signal → Privately produced 50k fast simulation events in release 12 ● Used premade AcerMC input files (acermc Wtlep) ● Ran through Pythia -> Atlfast -> TopView 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

Event Selection / results 9/10/2015S. Tokár, Cz-Sk meeting, Košice14 Selection cuts 2 OS leptons with P T > 30 GeV (no 3 rd one) ● DIL inv-mass out [81,101] GeV (ee/  only) ● Pt(jet1) > 30 GeV and Pt(jet2) < 20 GeV ● Missing E T > 20 GeV (e  ), 30 GeV (ee/  ) Vetoes Vetoes e e is in 1.37 < |  | < 1.52   is in 1.1 < |  | < 1.7  (mis-E T,  ) muons in 1.1 < |  | < 1.7 Using multivariate analysis (BDT) to improve discriminating power vs ttbar, Z events – different variable are used: MET, m(ll),  (MET,j 1 ),  (ll,j 1 ), HT,  R(l 1,l 2 ), pT(j 1 ), …

Conclusions 9/10/2015S. Tokár, Cz-Sk meeting, Košice15  DIL analyses are present at ATLAS, but are less advanced than lepton + jets one  Good perspectives are in the top mass determination, spin correlations,…