Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

Measuring inclusive b-jet and bbbar-dijet cross-sections in 7 TeV pp collisions with ATLAS tests perturbative QCD predictions. The analysis employs two tagging strategies—lifetime-based secondary vertices and muon-based semileptonic decays—to obtain cross-sections as functions of jet and dijet kinematics, and compares them to NLO predictions from POWHEG+Pythia and MC@NLO+Herwig. The results show good agreement with POWHEG+Pythia for the inclusive and bbbar-dijet channels, while MC@NLO+Herwig underperforms for the inclusive cross-section in central high-pT regions; the muon-based cross-section provides a consistent cross-check. Overall, the study validates pQCD expectations at 7 TeV and demonstrates robust b-jet measurements with complementary tagging methods.

Abstract

The inclusive and dijet production cross-sections have been measured for jets containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The measurements use data corresponding to an integrated luminosity of 34 pb^-1. The b-jets are identified using either a lifetime-based method, where secondary decay vertices of b-hadrons in jets are reconstructed using information from the tracking detectors, or a muon-based method where the presence of a muon is used to identify semileptonic decays of b-hadrons inside jets. The inclusive b-jet cross-section is measured as a function of transverse momentum in the range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet cross-section is measured as a function of the dijet invariant mass in the range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets and the angular variable chi in two dijet mass regions. The results are compared with next-to-leading-order QCD predictions. Good agreement is observed between the measured cross-sections and the predictions obtained using POWHEG + Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet cross-section. However, it does not reproduce the measured inclusive cross-section well, particularly for central b-jets with large transverse momenta.

Figures (8)

• Figure 1: Examples of template fits to the measured $p_{\rm T}\xspace^{\rm rel}$ distribution, before and after applying the requirement of $L/\sigma_L > 5.85$. The error bars represent the data statistical errors. The differences between the data and the sum of the templates are covered by the systematic uncertainties on the template shapes.
• Figure 2: Examples of purity fits in the inclusive and dijet measurements. The error shown for the $b$-fraction is the uncertainty on the fit parameter. For the inclusive measurement the statistical uncertainty on the sum of the templates, indicated by the shaded area, is taken into account in the fit. In the dijet measurement the templates are parameterized, the uncertainty on the parameterization is taken into account as a systematic uncertainty and not shown here.
• Figure 3: Inclusive double-differential $b$-jet cross-section as a function of $p_{\rm T}$ for the different rapidity ranges. The data are compared to the predictions of Pythia, POW-HEG and MC-@NLO. The leading-order Pythia prediction is scaled $(\times 0.67)$ to the measured integrated cross-section.
• Figure 4: Differential $b$-jet cross-section as a function of $p_{\rm T}$ for $b$-jets with $|y| < 2.1$. The data are compared to the predictions of Pythia, POW-HEG and MC-@NLO. In the region $30 < p_{\rm T}\xspace < 140$ GeV the muon-based cross-section measurement is also shown. For the muon-based measurement only the POW-HEG prediction is shown.
• Figure 5: Ratio of the measured cross-sections to the theory predictions of POW-HEG and MC-@NLO. In the region where the lifetime-based measurement overlaps with the muon $p_{\rm T}\xspace^{\rm rel}$ measurement both results are shown. The top plot shows the full rapidity acceptance, while the four smaller plots show the comparison for each of the rapidity ranges separately. The data points show both the statistical uncertainty (dark colour) and the combination of the statistical and systematic uncertainty (light colour). The shaded regions around the theoretical predictions reflect the statistical uncertainty only. Systematic uncertainties in the NLO predictions are discussed in the text.