Neural Network b-Tagger Top Cross-Section Measurement Public Webpage

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tt Production Cross-Section Measurement Using the Neural Network b-Tagger
H. Bachacou¹, Y. Kusakabe², P. Lujan¹, T. Masubuchi³, M. McFarlane¹, A. Taffard⁴, W. Yao¹
¹LBNL, ²Waseda,³Tsukuba,⁴UIUC

Public Note

Introduction
Using SecVtx, and the NN b-tagger to reduce background, we ascertain a cross-section of 8.5 ± 0.6 (stat) ± 1.0 (syst), 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
• Electrons: E_T > 20GeV, isolated
• Muons: p_T > 20GeV/c, isolated
• Jets: E_T > 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
All W + jet events passing the NN b-tagger are fed into the b-l neural network so that their combined output distribution is fitted according to the distributions for known b, c and l jets, to estimate the constituent fractions thereof. This process is repeated using the b-c network. We then input the W + jet events into a prescription called "Method 2", which also calculates fractional jet composition and was used in the standard cross-section measurement using SecVtx. The fractions calculated with the neural networks and Method 2 agree.

To obtain the measurement reported here, the W + jets events passing the NN b-tagger are inputted into Method 2 to yield the background decomposition below.

Background Estimates
N_jets	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

Method 2 output totals are scaled to data totals for the W + 3, 4 jet bins. The W + 1, 2 jet bins, which serve as a control, agree within one one standard deviation. If this were not the case, the 3 and 4 jet bins could not be trusted. Plots representing the table above are below:

Without NN b-tagging	After NN b-tagging

Results
The cross-section is defined as:

σ_tt =	N_obs - N_bg

	ε_tt × L

where N_obs and N_bg are the quantity of observed events and expected background, ε_tt is the signal acceptance, and L is the luminosity integrated with respect to time. Using the W + >3 jets bin and the NN b-tagger to vet events, the cross-section calculation gives 8.5 ± 0.6 (stat) ± 1.0 (syst).

Sampling of Kinematic Distributions

CLICK TO DOWNLOAD .eps FILE
Lepton E_T	Lepton Eta
Lepton E_T 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 E_T
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 E_T. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb^-1.
Phi of Missing E_T	E_T of 3^rd leading jet.
Phi of missing E_T. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb^-1.	E_T distribution of 3^rd 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 3^rd leading jet.	Phi of 3^rd leading jet.
Eta distibution of 3^rd 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 3^rd leading jet. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb^-1.
H_T, >3 jets bin	W Mass
Number of events in >3 jets bin as a function of H_T. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb^-1.	Mass of the W boson. tt contribution is added to the predicted background with the normalization to the measured cross-section of 8.5 pb^-1.
CLICK TO DOWNLOAD .eps FILE