Prospects on Lonely Top Quarks searches in ATLAS Benoit Clément LPSC – CNRS/IN2P3 – UJF Grenoble – Grenoble INP Physics at LHC 2008 – Split, Croatia.

THIS IS OUR BEST GUESS AT THE MOMENT Analysis strategy - Goal Determine what will be the sensitivity of the ATLAS experiment to single top production for a given luminosity, taking into account : - Expected event statistics (S/B, S/√B …) - Expected systematics. Identify the critical points of the analysis and develop method to optimize the analysis strategy. Inputs Our current knowledge of what ATLAS data will look like - theoretical cross sections - detector full simulation - trigger and reconstruction algorithm efficiencies THIS IS OUR BEST GUESS AT THE MOMENT

Top quark electroweak production Top quark production in pp Strong interaction pp -> gluon  top pairs (833 pb) dominant mode. Electroweak interaction Wtb vertex (single top) 3 diagrams : s-channel, t-channel And associated production Wt. Cross section ~ |Vtb|² Sensitivity to new physics : H+, W’, 4th generation quarks. Generic signature: 1 central, high-pT b-jet (top decay) 1 leptonic W (high-pT lepton + MET) 1 or 2 extra jets (including b-jets) t-channel Wt s-channel σNLO = 246 ± 12 pb σNLO = 11 ± 1 pb σNLO = 66 ± 2 pb

Larger cross-section at LHC From Tevatron to LHC t-channel Larger cross-section at LHC t-channel : 2 pb -> 240 pb (x120) s-channel : 1 pb -> 11 pb (x11) Wt : 0.1 pb -> 66 pb (x660) Top pair : 7 pb -> 833 pb (x120) W+jets : ~2 nb -> ~20 nb (x10)  s-channel Tevatron claimed a 3-sigma observation of (s+t channels) in 2007. Complex multivariate analyses (NN, BDT, ME) No discovery yet !!!  Wt  S/B ratio similar for t-channel S/B worse by a factor 10 for s-channel S/B improved by a factor 6 for Wt BUT HIGHER STATISTICS

Associated Wt production Single top signatures In all channels : 1 high pT (~70 GeV) b-jet 1 W boson from the top quark decay t-channel single top 1 or 2 extra forward jet (b-jet often very forward) Leptonic decay of W only : BR(W->(τ->)e/μ) = 25.4% -> 1 high-pT e or μ + missing ET s-channel single top 1 second central b-jet Leptonic decay of W only : high-pT e or μ + missing ET Associated Wt production 1 second W boson - lepton + jet channel (1 had W, 1 lep W) : BR = 34% 2 jets, 1 lepton, missing ET - di-lepton channel (2 leptonic Ws) : BR = 6.4% 2 leptons, some missing ET, no extra jets not considered yet

Main backgrounds Top pairs (σ≈800pb ) in both lepton+jets and di-lepton decay. -> Dominant background to all analyses W + jets, associated production W+light jets(σ≈O(100) nb) W+bb+jets (σ≈O(0.1) nb) Multijet events with a fake lepton - jet identifed as an electron - muon from heavy flavour or pion decay, reconstructed outside the jet cone. - tt -> l + jets Wbb

Event Selection Common preselection : Selecting top-like events Single electron or muon (20 GeV threshold) trigger 1 isolated electron or muon, pT>30 GeV, |η|<2.5 no extra lepton with pT>10 GeV, |η|<2.5 missing ET > 20 GeV 1 b-tagged jet, pT > 30 GeV, |η|<2.5 (b-tag eff ~60%, rej ~ 100) at least 1 extra jet, pT > 30 GeV, |η|<5 Kills most of the Multijet and W+jet background Leptonic W Top quark The issue of multijet events No credible simulation of mutlijet with fake isolated lepton -> need real data. Check that Tevatron’s “triangular cut” should do the job - cut in MET vs Δφ(Lepton, MET) plane - kills events with fake MET from mis-id - only suppress a few % of the signal

Analysis specific selection t-channel Harder b-jet cut : pT > 50 GeV -> against Wbb Forward light jet (cut based analysis only): hardest untagged jet with |η|>2.5 -> against tt Second b-tagged jet with pT > 30 GeV Veto on extra jet with pT > 15 GeV Topological cut (cut based analysis only) 0.5<ΔR(b1,b2)<4 80 < HT(jets) < 220 GeV (scalar sum of pTs) 60 <lepton pT < 130 GeV s-channel Wt Harder b-jet cut : pT > 50 GeV Veto extra b-jet, using looser b-tag (eff~90%, rej~2) -> against tt (improve S/B by a factor 3) 2 to 4 jets (incl b’s) Hadronic W mass cut (cut based analysis only): if at least 2 light jets : 50 <Mjj< 125 GeV

Number of Selected Events : 1fb-1 Signal 1460 24.8 639 Other ST 148 39.5 1418 Top pairs 2816 145.1 3022 W+jets 942 66.4 3384 Total Bkg 3906 251 7824 S/B 0.37 0.1 0.08 S/√B 23.4 1.6 7.2 t-channel s-channel Wt Statistical significance ok for t-channel and Wt Poor S/B separation for s-channel and Wt Everything gets far worse once systematics enter !!!

Multivariate analysis Combining information of several (poorly) discriminating variables to improve separation Cut on discriminant = surface in var space Many methods exist : Likelihood, ANN, DT, Fisher… Boosted decision trees (t-channel and Wt) Decision Tree : At each node find the variable and the cut that gives the best separation and split the sample into 2 new nodes. Boosting : Average over several trees obtained by reweighting wrongly separated events. Use TMVA implementation Likelihood ratio (s-channel) Compute the likelihood ratio S/(S+B) assuming uncorrlated variables. L= Π Psignal(xi) / (Π Psignal(xi) + Π Pbruit(xi) ) Pdf are derived from MC histograms

Multivariate analysis (2) For each analysis, several discriminants are trained - 1 discriminant per principal background - different sets of discriminating variables - split analyses in separate channels (electron/muon, jet multiplicity) t-channel : 1 BDT against top pairs s-channel : 5 LH against : tt->l+j, tt->tau+j, tt->ll, W+jets, t-channel Wt : 4 BDT against : tt->lj, tt->ll, W+jets, t-channel LH s-ch vs tt->tau+j BDT Wt vs tt->l+j

Cross section error estimation Poisson likelihood - Bi : # of background events - αi : the signal acceptance. - L : integrated luminosity. - Di : Expected data = Bi+ αi L σth - σ : cross section to fit Random shift of Bi, αi L : gaussian errors Random Di following a Poisson distribution Errors from a same source are 100% correlated between MC samples and channels Errors from different sources remains uncorrelated Error source D B α L Data statistics  MC Statistics Luminosity Backgrounds XS B-tagging, JES… Minimisation obviously gives the input cross section. Use MC approach to estimate cross section uncertainty. Estimates cross-section pdf -> Variance gives the error

XS Errors and sensitivities Stat error 5.7% 64% 20.6% MC statistics 7.9% 29% 15.6% Luminosity 8.8% 31% 20% B-tag efficiency 6.6% 44% 16% Jet energy scale 9.9% 25% 11% Lepton ID 0.7% 6% 2.6% Trigger 1.7% 2% Theory (xs, pdf,ISR/FSR…) 13.5% 74% 35% Total 1 (10)fb-1 22% (10%) 95% (48%) 48% (19%) t-channel s-channel Wt For 1 fb-1

Summary Single top studies from the CSC book focused on - Separate studies of each 3 channels : s, t and associated production. - Establishing and optimizing selections for single-top events in early data (<10fb-1), including multivariate analyses. - Estimating the systematics uncertainties one can expect and the impact on the mesured cross-section. 5 sigma incompability with the null hypothesis (ie discovery) should be attained for: - t-channel production with 1fb-1 (Δσ/σ = 22%) -> leads to a mesurement of |Vtb| with Δ |Vtb|/ |Vtb| = 12% - Wt production with 10fb-1 (Δσ/σ = 19%) s-channel discovery will require more statistics (>30 fb-1) but some systematics will have been reduced when this luminosity is reached . Once single top signal is established : - Top properties in single top channels (in paticular spin correlation) - Extension to new physiscs search (deviation in cross-section, modified kinematics) -> Charged Higgses, W’…