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t t
Production Cross-Section Measurement Using the Neural Network b-Tagger
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-tagging | After NN b-tagging |
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σt t = | Nobs - Nbg |
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εt t × L |
Lepton ET | Lepton Eta |
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![]() 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 Phi | Missing 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 ET | ET 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 bin | W 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. |