Introduction to Single-Top

1. Single-Top Cross Section Measurements at ATLAS Patrick Ryan (Michigan State University)Patrick.Ryan@cern.ch Introduction to Single-Top Single-Top Event Pre-Selection t-channel Cross Section at 14 TeV Wt-channel Cross Section at 14 TeV The measurement of the single-top cross section provides a direct measurement of the CKM Matrix Element |Vtb| and permits verification of Standard Model electroweak coupling. The single-top quark transmits its polarization to its decay products and can provide insight into W-t-b couplings. The single-top quark could also lead to observations of new fields, mediators, and particles which noticeably couple only to heavy fermions. Examples include the Standard Model neutral Higgs, the minimal SUSY charged Higgs, and Flavor Changing Neutral Currents. The three single-top processes share a common pre-selection. Only single-top events with an isolated and high-pT electron or muon in the final state are included in this study. Single-top events with only hadrons in the final state are not considered. The muon and electron channels are exclusive. Lepton requirements: - Muons & electrons are reconstructed if: - ET > 10 GeV and |h| < 2.5 - Isolation ET < 6 GeV in 0.2 cone - 1 muon or 1 electron with pT > 30 GeV - Veto events with more than 1 lepton Jet requirements: - Reconstruct jets with - A cone algorithm with DR = 0.4 - ET > 15 GeV. - Jet multiplicity between 2 and 4 - At least 2 jets with pT > 30 GeV - At least 1 b-tagged jet Other requirements: - Missing ET > 25 GeV Cut-based analysis: Require b-jet pT > 50 GeV to remove low-pT W + Jets. Require |h| > 2.5 for hardest light jet to remove ttbar (main background). Results of these cuts are shown in Table 1 for 1fb-1. Cut-based analysis: Require one b-jet with pT > 50 GeV. Reject events with more than 1 b-jet (found utilizing a looser weight cut) with pT > 35 GeV to remove ttbar. Multivariate analysis: 4 BDTs developed against ttbar (lepton + di-lepton), W + Jets, and t-channel. BDT thresholds set by minimizing total uncertainty. Results are shown below for 1 fb-1 of luminosity. Number of Events Number of Background Events Signal Efficiency Luminosity Table 1: Results of t-channel cut-based analysis. Multivariate analysis: Use Boosted Decision Tree (BDT) to remove ttbar instead of cut on Jet |h|. Variables giving a good S/B separation were input into BDT. Cut on BDT output to maximize ttbar separation and S/B. Main systematics are Jet Energy Scale, ISR/FSR, and luminosity. Single-Top Production at 14 TeV Table 5: Results of Wt-channel cut-based analysis. Single-top quarks are produced via the electroweak interaction. At leading order there are 3 production processes; s-channel, t-channel, and Wt-channel. These are shown in Figure 1. Note that each process contains a W-t-b vertex. Main systematics are ISR/FSR, background cross section, and luminosity. s = 10.65 pb Table 6: Uncertainties for Wt-channel analysis. 14 TeV Summary For evidence (3s) or discovery (5s): - t-channel: 5s with 1 fb-1 - s-channel: 3s with 30 fb-1 - Wt-chan: 3s with 1 fb-1, 5s with 10 fb-1 Systematics are the limiting factor for the single-top measurement and have a strong MC dependence in the current analysis. Trigger Selection s-channel Selecthigh pT muons and electrons, which could indicate W decay. Events satisfying any of the following triggers are accepted: - Muon with pT > 20 GeV - Isolated Electron with pT > 25 GeV - Electron with pT > 60 GeV Electron trigger efficiencies are shown in Figure 2. 2  2 Table 2: Uncertainties for t-channel analysis. The single-top cross section is proportional to |fLVtb|2 (where fL is 1 in the SM). s = 246 pb Single-Top at 7 TeV and 10 TeV t-channel 2  2 and 2  3 Combined single-top cross sections: - At 14 TeV: 323 pb - At 10 TeV: 161 pb - At 7 TeV: 76 pb 7,600 single-top events are expected with 100 pb-1 of 7 TeV data. Similar to number of events in Tevatron observation. Goals: 100 pb-1 of lumi insufficient to measure cross sections but can set limits on all single-top processes. Could rule out Standard Model t-channel at 95%. Data-driven techniques: Use as much as possible because theory has large uncertainties. Estimate background using orthogonal samples for ttbar and W + Jets. Maximize signal sensitivity: Use multi-variate techniques to extract signal. s-channel Cross Section Cut-based analysis: Require 2 jets to reject ttbar and both jets to be b-jets to reject W + Jets and QCD. Cuts on angle btw jets, total jet pT, and Missing ET + pT. Multivariate analysis: Require above cuts then discriminate between signal and background using a likelihood function (LF). Input variables to LF chosen according to discrimination power and thresholds set by minimizing uncertainty. There is a set of LFs for each background. s = 66.5 pb Wt-channel 2  2, 2  3, and 2 4 Background to Single-Top Figure 1: Single-top production in the s, t, and Wt -channels Top pair production is the dominant background, with a cross section 3 times higher than that of combined single-top. The single high-pT lepton, 2 b-jets, and missing ET of semi-leptonic top pair decay is most likely to mimic single-top. W + Jets processes have cross sections many orders of magnitude higher than the single-top cross sections. Di-boson events contribute minimally. QCD will be estimated by data driven methods and is not considered in these studies. Contamination depends on the selections specific to the analyses. Figure 4: Single-top cross sections as a function of ECM Cross Section and Uncertainties The cross section will be calculated with: Experimental uncertainties (1fb-1/10fb-1) - Jet Energy Scale (± 5% / ±1%) - b-tagging Likelihood (± 5% / ± 3%) - Luminosity (±5% / ±3%) Theoretical uncertainties: - Background cross sections - ISR / FSR - PDF and b-quark Fragmentation Figure 2: Electron trigger efficiencies. Table 3: Results of s-channel multivariate analysis Figure 3: Likelihood function for ttbar  lep + jets Main uncertainties are data statistics, b-tagging, ISR/FSR, and bkg cross sections. Table 4: Uncertainties for s-channel analysis.