Measurement of the cross section for the production of a W boson in association with b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This study measures the fiducial cross section for W boson production in association with one or two b-jets in 7 TeV pp collisions using the ATLAS detector, based on 35 pb^-1 of data. It employs b-jet tagging via secondary vertices and data-driven background estimates, extracting the signal with a template fit to the secondary-vertex mass and correcting for detector effects. The result, 10.2 pb (with statistical and systematic uncertainties), is larger than the NLO 5FNS prediction but consistent within ~1.5σ after non-perturbative corrections, illustrating both the predictive power and remaining tensions in heavy-flavor W production modeling. The analysis provides important cross-checks for QCD in heavy-flavor final states and informs backgrounds relevant to Higgs and top-quark studies at the LHC.

Abstract

A measurement is presented of the cross section for the production of a W boson with one or two jets, of which at least one must be a b-jet, in pp collisions at sqrt(s)=7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb^-1 of data collected with the ATLAS detector at the LHC. The W+b-jet cross section is defined for jets reconstructed with the anti-kt clustering algorithm with transverse momentum above 25 GeV and rapidity within +/-2.1. The b-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay channel of the W boson and is found to be 10.2 +/- 1.9 (stat) +/- 2.6 (syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent with, the measured value.

Figures (5)

• Figure 1: (top) $E_{\mathrm{T}}^{\mathrm{miss}}$ distribution in the electron channel in the combined 1- and 2-jet bin without applying the $E_{\mathrm{T}}^{\mathrm{miss}}$ selection criterion. (bottom) $m_{T}$ distribution in the muon channel in the 1-jet bin without applying the $m_{T}$ selection criterion after applying the $b$-tagging requirement. Non-multi-jet contributions are normalized to their MC predictions.
• Figure 2: $m_{SV}$ distributions for the $b$-tagged jet in data and MC, where the $W$+jets samples are normalized to the results of the maximum likelihood fit and non-$W$+jets backgrounds are normalized to the estimates as given in the text, in the 1-jet bin in the electron channel (top) and the muon channel (bottom). The stack order is the same as in the legend.
• Figure 3: $m_{SV}$ distributions for the $b$-tagged jet in data and MC, where the $W$+jets samples are normalized to the results of the maximum likelihood fit and non-$W$+jets backgrounds are normalized to the estimates as given in the text, in the 2-jet bin in the electron channel (top) and the muon channel (bottom). The stack order is the same as in the legend.
• Figure 4: Invariant mass of the $W$+$b$-jet system in the electron channel. The neutrino $p_z$ is obtained by imposing the $W$ invariant mass and using the smallest in absolute value of the two solutions. The $W$+jets samples are normalized to the results of the maximum likelihood fit and non-$W$+jets backgrounds are normalized to the estimates as given in the text.
• Figure 5: Measured fiducial cross section with the statistical (inner error bar) and statistical plus systematic (outer error bar) uncertainty in the electron, muon, and combined electron plus muon channel. The cross section is given in the 1, 2, and 1+2 jet exclusive bins. The measurements are compared with NLO ref:4FNS5FNS predictions. The yellow (shaded) band represents the total uncertainty on the prediction obtained by combining in quadrature the renormalisation and factorization scale, PDF set, and non-perturbative correction uncertainties. The leading order predictions from Alpgen interfaced with Herwig and Jimmy are given for $b$-jets generated only by the matrix element and by the matrix element and the parton shower. The prediction from Pythia is also shown.