Single-Top Cross Section Measurements at ATLAS

Patrick Ryan (Michigan State University)Patrick.Ryan@cern.ch

Introduction to Single-Top

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

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

Cross Section Uncertainties: NData was generated randomly according to Poisson distribution. NBkg and Signal were varied for each systematic source by a random value determined by a Gaussian distribution.

t-channel Cross Section

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) but this cut is not very effective. Results of these cuts are shown in Table 3 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. The BDT output of 0.6 (shown in Figure 3) minimizes total uncertainty and corresponds to S/B = 1.3.

Wt-channel Cross Section

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.

s-channel Cross Section

Summary

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.

Single-Top Production

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.

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.

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Figure 1: Single-top production in the s, t, and Wt -channels

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Single-Top Event Pre-Selection

Simulation of Monte Carlo Samples

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 magnitudes 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.

Process Generator Matrix Element

Theoretical [pb]

x BR [pb]

t-channel PYTHIA AcerMC 246 69

s-channel PYTHIA AcerMC 10.65 3.3

Wt-channel PYTHIA AcerMC 66.5 26.7

ttbar HERWIG MC@NLO 833 461.0

W + Jets PYTHIA ALPGEN + MCFM

- 17,189

The listed cross sections are theoretical and do not correspond to generator + ME. MCFM was used to derive K-factors in order to scale LO to NLO for W + Jets.

Number of Events Number of Background

Events

Signal Efficiency Luminosity

/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

Figure 3: BDT Output

Main systematics are Jet Energy Scale, ISR/FSR, and luminosity.

The single-top cross section is proportional to |fLVtb|2 (where fL is 1 in the SM).

/Statistical

/Systematic

/Total

Likelihood1 fb-1

64% 95% 115%

Likelihood10 fb-1

20% 48% 52%

Main uncertainties are data statistics, b-tagging, ISR/FSR, and bkg cross sections.

Events 1 fb-1

Signal 15.4

Bkg 82.7

Figure 4: Likelihood function for ttbar lep + jets

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: Monte Carlo samples and their properties

Table 3: Results of t-channel cut-based analysis.

Table 5: Results of s-channel multivariate analysis

Table 7: Results of Wt-channel cut-based analysis.

Table 4: Uncertainties for t-channel analysis.

Table 6: Uncertainties for s-channel analysis.

Table 8: Uncertainties for Wt-channel analysis.

Figure 2: Trigger Efficiencies for single-top events.

Trigger Selection

Triggers select events with 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

Trigger efficiencies are shown in Figure 2. Results of pre-selection + trigger are shown in Table 2.

Events 2 jets 3 jets 4 jets

Signal 58.0 20.9 6.6

Background 165.6 45.1 15.6

Muon Channel Electron Channel

Process (%) N (1 fb-1) (%) N (1 fb-1)

t-channel 5.9% 3143 5.2% 2787

s-channel 7.1% 166 5.8% 136

Wt-channel 6.8% 1314 5.6% 1091

Table 2: Results of pre-selection and trigger