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Neural Network b-Tagger Top Cross-Section Measurement

t t Production Cross-Section Measurement Using the Neural Network b-Tagger

Introduction
Using SecVtx, and the NN b-tagger to reduce background, we ascertain a cross-section of 8.5 ± 0.06 (syst) ± 1.0 (stat), which is consistent with the measurement 8.4 ± 0.6 (stat.) ± 1.1 (syst.) determined with SecVtx alone. This offers evidence the NN b-tagger is behaving appropriately and usefully. Events for this measurement are taken from the lepton + jets channel (695 pb-1). Please refer to the NN b-tagger public homepage for more information relevant to this analysis.

Event Selection
• Leptons: pT > 20GeV/c, ET > 20GeV, well isolated
• Jets: ET > 15GeV, |η| < 2.0, (for the actual measurement, >3 jets are required, but all jet multiplicities are examined.)
• Vetoed: Dilepton events, Z0, photon conversions, cosmic rays
• At least one SecVtx tag passing the NN b-tagger

Relevant Backgrounds
• Mistags (light jets identified as b by SecVtx)
• W + heavy flavor
• Non-W (QCD)
• WW, WZ, ZZ, Z→ττ, single top

NN Operating Point
b-l network cut at 0.182, b-c network cut at 0.242; result: 90% b efficiency, 50% c efficiency, 35% l efficiency.

Method
W+Njet events passing the NN b-tagger are fed into each network. The output distributions are fitted for fractions of b, c and l using each network's output distribution for that jet type. Also, we input the events into a prescription called "Method 2", which calculates fractional jet composition as well and was used in the standard cross-section measurement using SecVtx. The fractions calculated with the neural networks and Method 2 agree. [show tables, maybe plots]. To obtain the measurement reported here, the W+jets events passing the NN b-tagger are inputted into Method 2 which decomposes the backgrounds into the species in the table below [incl table].

Background Estimate
Njets 1 jet 2 jets 3 jets 4 jets
Pretag 68183 10647 1713 468
Mistag 100.1 ± 20.6 41.7 ± 8.6 12.3 ± 2.5 3.0 ± 0.6
Wbb 179.2 ± 55.4 100.4 ± 29.7 22.1 ± 5.8 4.1 ± 1.1
Wcc 37.3 ± 10.7 28.2 ± 8.5 5.6 ± 1.7 1.12 ± 0.35
Wc 137.4 ± 34.6 26.6 ± 6.8 2.7 ± 0.7 0.4 ± 0.1
Single Top 13.0 ± 1.3 20.7 ± 2.1 4.5 ± 0.5 0.79 ± 0.09
Diboson 7.3 ± 1.2 11.9 ± 1.9 3.0 ± 0.5 0.81 ± 0.15
Non-W (QCD) 59.6 ± 10.0 29.5 ± 5.1 8.8 ± 1.7 3.96 ± 0.85
Total Bkg 533.9 ± 79.2 259.2 ± 41.0 58.9 ± 8.2 14.2 ± 1.9
Top 6.6 ± 0.9 48.1 ± 6.7 105.2 ± 14.7 130.2 ± 18.2
Top + Bkg 540.5 ± 79.2 307.3 ± 41.5 164.1 ± 16.8 144.4 ± 18.3
Data 623 332 166 145

Top events form the majority of the W + 3 and 4 jet bins, and the data agree with the expected top + background predictions made by Method 2. This agreement is not as good for W + 1 and 2 jets, but still agree within about one sigma.

Without NN b-taggingAfter NN b-tagging

Results
The cross-section is:
σt t = Nobs - Nbg
------------
εt t × L

where Nobs and Nbg are the quantity of observed events and expected background, εt t is the signal acceptance, and L is the luminosity integrated with respect to time. Using the b-tagger to vet events, the calculated cross-section is 8.5 ± 0.06 (syst) ± 1.0 (stat).


Sampling of Kinematic Distributions
Lepton ETLepton Eta

Lepton ET distribution. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Lepton eta distribution. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Lepton PhiMissing ET

Lepton phi distribution. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Missing ET. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Phi of Missing ETET of 3rd leading jet.

Phi of missing ET. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

ET distribution of 3rd leading jet. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Eta of 3rd leading jet.Phi of 3rd leading jet.

Eta distibution of 3rd leading jet. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Phi distibution of 3rd leading jet. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

HT, >3 jets binW Mass

Number of events in >3 jets bin as a function of HT. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.

Mass on the W boson. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb-1.