View
218
Download
0
Embed Size (px)
Citation preview
Introduction to Single-Top
Single-Top Cross Section Measurements at ATLAS
Patrick Ryan (Michigan State University)[email protected]
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.
Background to Single-Top
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 || < 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 R = 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
t-channel Cross Section at 14 TeV
Cut-based analysis: Require b-jet pT > 50 GeV to remove low-pT W + Jets. Require || > 2.5 for hardest light jet to remove ttbar (main background). Results of these cuts are shown in Table 1 for 1fb-
1.
Multivariate analysis: Use Boosted Decision Tree (BDT) to remove ttbar instead of cut on Jet ||. 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.
Wt-channel Cross Section at 14 TeV
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.
Single-Top at 7 TeV and 10 TeV
Single-Top Production at 14 TeV
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.
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.
t-cha
nnel
Figure 1: Single-top production in the s, t, and Wt -channels
s-ch
anne
lW
t-cha
nnel
2
22
2 a
nd 2
3
2
2, 2
3
, and
2
4
Single-Top Event Pre-Selection
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.
/Statistical
/Systematic
/Total
Cut-based 1 fb-1
5.0% 45% 45%
BDT1 fb-1
5.7% 22% 23%
Cut-based10 fb-1
1.6% 22% 22%
BDT10 fb-1
1.8% 10% 10%
Events Pre-selection
b-jet pT > 50 GeV
Non-b-jet || > 2.5
Signal 6,191 4,412 1,460
Background 50,656 35,472 3,906
The single-top cross section is proportional to |fLVtb|2 (where fL is 1 in the SM).
Main systematics are ISR/FSR, background cross section, and luminosity.
/Statistical
/Systematic
/Total
BDT 1 fb-1 21% 48% 52%
BDT 10 fb-1 6.6% 19% 20%
Table 1: Results of t-channel cut-based analysis.
Table 5: Results of Wt-channel cut-based analysis.
Table 2: Uncertainties for t-channel analysis.
Table 4: Uncertainties for s-channel analysis.
Table 6: Uncertainties for Wt-channel analysis.
Events 2 jets 3 jets 4 jets
Signal 58.0 20.9 6.6
Background 165.6 45.1 15.6
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
Number of Events Number of Background
Events
Signal Efficiency Luminosity
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.
Events 1 fb-1
Signal 15.4
Bkg 82.7
Figure 3: Likelihood function for ttbar lep + jets
Table 3: Results of s-channel multivariate analysis
Main uncertainties are data statistics, b-tagging, ISR/FSR, and bkg cross sections.
/Statistical
/Systematic
/Total
Likelihood1 fb-1
64% 95% 115%
Likelihood10 fb-1
20% 48% 52%
14 TeV Summary
For evidence (3) or discovery (5): - t-channel: 5 with 1 fb-1
- s-channel: 3 with 30 fb-1
- Wt-chan: 3 with 1 fb-1, 5 with 10 fb-1
Systematics are the limiting factor for the single-top measurement and have a strong MC dependence in the current analysis.
= 10.65 pb
= 246 pb
= 66.5 pb
Trigger Selection
Figure 2: Electron trigger efficiencies.
Select high 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.
Figure 4: Single-top cross sections as a function of ECM