Single Top Analysis Overview

1. Single Top Analysis Overview B.Andrieu (LPNHE, Paris) Outline: u Introduction u Preselection u Tagging and final selection u Background estimation u Summary

2. Introduction Two main diagrams for electroweak production of top (single top) at Tevatron • Signature: W (l n) +  2 jets ( 1 b jet) • Backgrounds: • QCD multijet with misidentified lepton (jet faking a lepton) and fake ETmiss • Top pair production: s(tt ) = 7 pb, 2 b jets in the final state • lepton + jets channel: Br = 0.34 , higher jet multiplicity • dilepton channel: Br = 0.02, same jet multiplicity as single top • W + jets: s(W+jets) ~ 1000 pb • s(W bb) ~ 10 pb  irreducible background • s(W W) ~ 10 pb, s(W Z) ~ 3 pb  negligible s -channel t -channel s(s) = 0.88 ± 0.14 pb s(t) = 1.98 ± 0.30 pb B.Andrieu Single Top Analysis Overview -

3. Preselection • First cuts • e + Etmiss + >=2 jets, veto on extra electron & muon  basic W + jets selection • Triangle cuts :cut in (Etmiss, Df(Etmiss, e )) or(Etmiss, Df(Etmiss, jet)) planes remove QCD background(difficult to model) • Use both “loose” and “tight” electron selection (likelihood cut at the end of electron selection based on shower shape and track matching) control QCD and W+jets background using “Matrix Method” NQCD= eQCD (eW Nl - Nt) / (eW – eQCD ) NW= eW (Nt – eQCD Nl) / (eW – eQCD ) eW known from Zee, eQCD known from this sample with inverted cut on ETmiss B.Andrieu Single Top Analysis Overview -

4. b tagging and final selection • Two main methods for b tagging of jets • Semi-leptonic decay of b SLT (Soft Lepton Tag) = find a lepton (m) in jet • b lifetime  JLIP (Jet Lifetime Probability), SVT (Secondary Vertex Tagger)= look for tracks with large impact parameter or for secondary vertex in jet • Ask for 1 (t channel) or 2 (s channel) b tagged jets • Final selection based on topology • Restrict number of jets (exactly 2)  reduce tt background • Find topological variables which separate single top from W +jets and tt , e.g. HT = pT (e) + Etmiss + pT (jet1) + pT (jet2) • Apply final cuts (e.g. HT > 160 geV) or build a likelihood distribution based on these variables L = PS (x1,…,xN ) / (PS (x1,…,xN ) + PB (x1,…,xN )) B.Andrieu Single Top Analysis Overview -

5. Estimation of QCD background • Preselection • NQCD known from data (Matrix Method) How well do we know eQCD? Does it evolve with event topology? • b tagging and final selection • Use a control sample which models the flavour content and kinematics of QCD background events in analysis sample, normalize this sample to NQCD , then apply b tagging and final cuts directly to this sampleHow to build a valid control sample?  Apply all preselection cuts except for one inverted cute -likelihood < 0.05  e contamination very small, topology similar to signal • Alternative method : use the Matrix Method after b tagging and final selection (statistics limited) • Does the QCD control sample behave the same as QCD background?Compare control sample distributions to result of Matrix Method B.Andrieu Single Top Analysis Overview -

6. Estimation of W + jets background • Preselection • NW known from data (Matrix Method) How well do we know eW? Does it evolve with event topology? • b tagging and final selection • SLT directly applied on simulation Is the Monte Carlo simulation of W + jets valid? How can we check it? • Lifetime taggers: tracking response not well simulated Use Tag Rate Functions (TRFs) to model the flavour content of W + jets background • Measure in data the probability for a jet to be b tagged, parametrized as a function of pT and h Is it really independent of event topology? • TRFs can be inclusive or flavour exclusive • Data used for TRFs measurement are QCD multijet samples Is the flavour content the same as in W + jets? • Inclusive TRFs can be applied on data, exclusive TRFs on Monte Carlo  Use data to model the kinematics (W + jets is dominant after preselection) and apply inclusive TRFs to data (normalized to NW known from Matrix Method) to predict N(W + jets) after tagging and final selection • Can we build a W + jets control sample?  Apply preselection cuts except for • one “stronger” cut to reduce QCD background: e-likelihood > 0.95  topology similar to all W + jets • one topological cut to reduce single top and tt: e.g. HT < 150 GeV  need to correct for kinematics • Cross check TRFs prediction on control sample (statistics limited) B.Andrieu Single Top Analysis Overview -

7. Summary: open questions • Preselection • Are eWand eQCD well known? • Are they really independent of further cuts? • Flavour content of W + jets • Control with W + 1 jet, Z + jets, gamma + jets, QCD? • Does it evolve with kinematics? How? • How can we do better than TRFs? • W + jets Monte Carlo simulation • Parton-jet matching: which method to use? Which Monte Carlo? • What errors should we assign to Monte Carlo predictions? • What cross checks can we do with data? • b tagging • How well do we simulate b and c fragmentation? • Is the mistag rate different between quark and gluon jets? B.Andrieu Single Top Analysis Overview -