Single-Top Cross Section Measurements at ATLAS

1. Single-Top Cross Section Measurements at ATLAS Patrick Ryan (Michigan State University)Patrick.Ryan@cern.ch Cross Section and Uncertainties Introduction to Single-Top Simulation of Monte Carlo Samples s-channel Cross Section 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 Current. The measurement of the single-top cross section is obtained with: Experimental Uncertainties (1fb-1/10fb-1) - Jet Energy Scale (± 5% / ±1%) - b-tagging Likelihood (± 5%) - Lumi (±20% for 1 fb-1, ± 5% for 10 fb-1) - Pile-up (not included) Theoretical Uncertainties: - Gluon Radiation - Background cross sections - PDFs - b-quark Fragmentation Cut-based Analysis: Require 2-jet events to reject ttbar. Require both jets to be b-jets to reject W + jets and QCD. Cuts on angle between jets, total jet pT, and Missing ET + pT. After selection, there are 25 signal and 251 background events. Multivariate Analysis: Require above cuts then discriminate between signal and background using a likelihood function. Figure 4 shows an example and Table 4 shows the results of likelihood selection. Number of Events in Data Number of Background Events Signal Efficiency Luminosity Table 1: Monte Carlo samples and their properties The t-channel and ttbar samples include NLO calculations and negative weights were applied to account for divergences. Single-Top Event Pre-Selection The three single-top processes share a common pre-selection. Only single-top events with an electron or muon in the final state are included in this study and the muon and electron channels are exclusive. Single-top events with hadrons in the final state from W or tau decay are difficult to distinguish from the background due to the lack of a lepton. Lepton Requirements: - Muons & electrons are reconstructed if: - ET > 10 GeV and |h| < 2.5 - ET in 0.2 cone around axis < 6 GeV - 1 muon or 1 electron with pT > 30 GeV - Veto events with more than 1 lepton - Veto events with electrons in the calorimeter gaps of 1.37 < |h| < 1.52. 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 - Triangle cut to remove QCD Single-Top Production Single-top quarks are produced via the electroweak interaction. At leading order there are three production processes: s-channel, t-channel, and Wt-channel. These processes are shown in Figure 1. Note that each process contains a W-t-b vertex. t-channel Cross Section Table 4: Results of s-channel multivariate analysis Figure 4: Likelihood function for ttbar  lep + jets 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 but this cut not very effective. Results of these cuts are shown below for 1fb-1 of luminosity. Uncertainties are shown in Table 5. s-channel Table 5: Uncertainties for s-channel analysis. 2  2 Wt-channel Cross Section Cut-based Analysis: Require one b-jet with pT > 50 GeV. Reject events with more than 1 b-jet with pT > 35 GeV to remove ttbar. Results of these cuts are shown below for 1fb-1 of luminosity. Table 2: Results of t-channel cut-based analysis. Multivariate Analysis: Use Boosted Decision Tree (BDT) to remove ttbar instead of cut on Jet |h|. BDT Output is shown in Figure 3. The minimum is a 0.6, which corresponds to S/B = 1.3. t-channel 2  2 and 2  3 s(x): Signal Distribution b(x): Background Distribution Table 5: Results of Wt-channel cut-based analysis. Multivariate Analysis: Use Boosted Decision tree (BDT) to remove ttbar events. Results are shown below for 1 fb-1 of luminosity. Figure 3: BDT Output Wt-channel Trigger Selection 2  2, 2  3, and 2 4 Statistical and systematic uncertainties for 1 fb-1 & 10 fb-1 of luminosity are shown in Table 3. Triggersselectevents with high pT muons and electrons, which 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 Trigger efficiencies are shown in Figure 2. Pre-selection + trigger efficiency: 5.9-7.1% for muon channel and 5.2-5.9% for electron channel. Figure 1: Single-top production in the s, t, and Wt -channels Table 6: Results of Wt-channel cut-based analysis. Background to Single Top Statistical and systematic uncertainties for 1 fb-1 & 10 fb-1 of luminosity are shown below. Top pair production, which has a cross section 3 times higher than the combined single-top cross section, is the dominant background. The single high-pT lepton, 2 b-jets, and missing ET of of semi-leptonic top pair decay is most likely to mimic single-top. W/Z + jets processes have cross sections many order of magnitudes higher than the single-top cross sections. Di-boson events and QCD can also contribute to the background. Table 3: Uncertainties for t-channel analysis. The single-top cross section is proportional to |fLVtb|2 (where fL is 1 in the Standard Model). |Vtb|2 is obtained by dividing the measured cross section by the theoretical cross section. Table 7: Uncertainties for Wt-channel analysis. Summary A 5s measurement of single-top is attainable with a few fb-1 for t and Wt-channels and approx 10 fb-1 for s-channel. Figure 2: Trigger Efficiencies for single-top events.