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

TL;DR

This paper reports the first ATLAS measurement of the inclusive W+jets cross section in pp collisions at √s = 7 TeV, based on ~1.3 pb^-1. It provides cross sections and ratios as functions of jet multiplicity and jet p_T, corrected to particle level and compared to NLO pQCD predictions and LO event generators. The results show good agreement with NLO predictions for up to two jets, while multi-jet production is well described by LO generators normalized to NNLO W cross sections, illustrating the reliability of perturbative QCD and Monte Carlo tools in W+jets at 7 TeV. The analysis defines a restricted phase space to avoid extrapolations and enables precise tests of QCD and background modeling for SM processes and Higgs searches.

Abstract

This Letter reports on a first measurement of the inclusive W+jets cross section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC, with the ATLAS detector. Cross sections, in both the electron and muon decay modes of the W boson, are presented as a function of jet multiplicity and of the transverse momentum of the leading and next-to-leading jets in the event. Measurements are also presented of the ratio of cross sections sigma(W+ \ge n) / sigma(W+ \ge n-1) for inclusive jet multiplicities n=1-4. The results, based on an integrated luminosity of 1.3 pb-1, have been corrected for all known detector effects and are quoted in a limited and well-defined range of jet and lepton kinematics. The measured cross sections are compared to particle-level predictions based on perturbative QCD. Next-to-leading order calculations, studied here for n \le 2, are found in good agreement with the data. Leading-order multiparton event generators, normalized to the NNLO total cross section, describe the data well for all measured jet multiplicities.

Figures (5)

• Figure 1: Results of fitting the signal and background templates to the $E_{\mathrm{T}}^{\mathrm{miss}}$ distribution in the electron (left) and muon (right) channels for the 1-jet bin. In the electron channel, the fit was performed for $E_{\mathrm{T}}^{\mathrm{miss}}$$>$ 10 GeV. All templates were from simulated events, except for the QCD background template in the electron channel which was obtained from the data.
• Figure 2: Summary of the systematic uncertainties on the cross section measurement shown as a function of jet multiplicity in the electron channel (left) and leading-jet $p_{\mathrm{T}}$ in the muon channel (right). The jet energy scale uncertainty includes the uncertainty on $E_{\mathrm{T}}^{\mathrm{miss}}$. The main contribution to the "sum of other uncertainties" in the electron channel comes from the QCD background (especially at high jet multiplicities), the electron identification efficiency and the electron energy scale. For the muon channel, the main contribution is from the muon reconstruction efficiency.
• Figure 3: $W$+jets cross-section results as a function of corrected jet multiplicity. Left: electron channel. Right: muon channel. The cross sections are quoted in a limited and well-defined kinematic region, described in the text. For the data, the statistical uncertainties are shown by the vertical bars, and the combined statistical and systematic uncertainties are shown by the hashed regions. Note that the uncertainties are correlated from bin to bin. Also shown are predictions from PYTHIA, ALPGEN, SHERPA and MCFM, and the ratio of theoretical predictions to data (PYTHIA is not shown in the ratio). The theoretical uncertainties are shown only for MCFM, which provides NLO predictions for $N_{\rm{jet}} \le 2$ and a LO prediction for $N_{\rm{jet}} = 3$.
• Figure 4: $W$+jets cross-section ratio results as a function of corrected jet multiplicity. Left: electron channel. Right: muon channel. The cross sections are quoted in a limited and well-defined kinematic region, described in the text. For the data, the statistical uncertainties are shown by the vertical bars, and the combined statistical and systematic uncertainties are shown by the hashed regions. Also shown are theoretical predictions from PYTHIA, ALPGEN, SHERPA, and MCFM. The theoretical uncertainties are shown only for MCFM, which provides NLO predictions for $N_{\rm{jet}} \le 2$ and a LO prediction for $N_{\rm{jet}} = 3$.
• Figure 5: $W$+jets cross-section as a function of the $p_{\mathrm{T}}$ of the two leading jets in the event. The $p_{\mathrm{T}}$ of the leading jet is shown for events with $\ge 1$ jet while the $p_{\mathrm{T}}$ of the next-to-leading jet is shown for events with $\ge 2$ jets. Left: electron channel. Right: muon channel. The cross sections are quoted in a limited and well-defined kinematic region, described in the text. For the data, the statistical uncertainties are shown by the vertical bars, and the combined statistical and systematic uncertainties are shown by the hashed regions. Also shown are theoretical predictions from PYTHIA, ALPGEN, SHERPA and MCFM, and the ratio of theoretical predictions to data (PYTHIA is not shown in the ratio). The theoretical uncertainties are shown only for MCFM, which provides NLO predictions for $N_{\rm{jet}} \le 2$ and a LO prediction for $N_{\rm{jet}} = 3$.