Measurements of fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets in $pp$ collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector

TL;DR

This paper reports measurements of fiducial cross-sections for top-antitop quark pair production with additional b-jets at 8 TeV using the ATLAS detector, providing data that aligns well with theoretical predictions.

Contribution

First measurement of $t\bar{t}$ production with one or two additional b-jets at 8 TeV, using novel selection methods and data-driven background normalization.

Findings

Measured cross-section times branching ratio for $t\bar{t}$ with ≥1 b-jet: 950 ± 70 (stat.) +240/-190 (syst.) fb.

Measured cross-section times branching ratio for $t\bar{t}$ with ≥2 b-jets: 19.3 ± 3.5 (stat.) ± 5.7 (syst.) fb.

Ratio of $t\bar{t}$ with two additional b-jets to all $t\bar{t}$ with two jets: 1.30 ± 0.33 (stat.) ± 0.28 (syst.)%.

Abstract

Fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets are reported, using an integrated luminosity of 20.3 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for $t\bar{t}$ events with at least one additional $b$-jet is measured to be 950 $\pm$ 70 (stat.) $^{+240}_{-190}$ (syst.) fb in the lepton-plus-jets channel and 50 $\pm$ 10 (stat.) $^{+15}_{-10}$ (syst.) fb in the $e \mu$ channel. The cross-section times branching ratio for events with at least two additional $b$-jets is measured to be 19.3 $\pm$ 3.5 (stat.) $\pm$ 5.7 (syst.) fb in the dilepton channel ($e \mu$,\,$\mu\mu$, and \,$ee$) using a method based on tight selection criteria, and 13.5 $\pm$ 3.3 (stat.) $\pm$ 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 $\pm$ 0.33 (stat.) $\pm$ 0.28 (syst.)\% for the ratio of $t\bar{t}$ production with two additional $b$-jets to $t\bar{t}$ production with any two additional jets. All measurements are in good agreement with recent theory predictions.