Abstract
Measurements are presented of the associated production of a W boson and a charm-quark jet (W + c) in pp collisions at a center-of-mass energy of 7 TeV. The analysis is conducted with a data sample corresponding to a total integrated luminosity of 5 inverse femtobarns, collected by the CMS detector at the LHC. W boson candidates are identified by their decay into a charged lepton (muon or electron) and a neutrino. The W + c measurements are performed for charm-quark jets in the kinematic region pt[jet] > 25 GeV, abs(eta) < 2.5, for two different thresholds for the transverse momentum of the lepton from the W-boson decay, and in the pseudorapidity range abs(eta[ell]) < 2.1. Hadronic and inclusive semileptonic decays of charm hadrons are used to measure the following total cross sections: sigma(pp to W + c + X) times B(W to ell nu) = 107.7 +/- 3.3 (stat.) +/- 6.9 (syst.) pb (pt[ell] > 25 GeV) and sigma(pp to W + c + X) times B(W to ell nu) = 84.1 +/- 2.0 (stat.) +/- 4.9 (syst.) pb (pt[ell] > 35 GeV), and the cross section ratios sigma(pp to W+ + c + X)/sigma(pp to W- + c + X) = 0.954 +/- 0.025 (stat.) +/- 0.004 (syst.) (pt[ell] > 25 GeV) and sigma(pp to W+ + c bar + X)/sigma(pp to W- + c + X) = 0.938 +/- 0.019 (stat.) +/- 0.006 (syst.) (pt[ell] > 35 GeV). Cross sections and cross section ratios are also measured differentially with respect to the absolute value of the pseudorapidity of the lepton from the W-boson decay. These are the first measurements from the LHC directly sensitive to the strange quark and antiquark content of the proton. Results are compared with theoretical predictions and are consistent with the predictions based on global fits of parton distribution functions.
Highlights
The study of associated production of a W boson and a charm (c) quark at hadron colliders provides direct access to the strange-quark content of the proton at an energy scale of the order of the W-boson mass (Q2∼(100 GeV)2) [1,2,3]
Recent work [4] indicates that precise measurements of this process at the Large Hadron Collider (LHC) may significantly reduce the uncertainties in the strange quark and antiquark parton distribution functions (PDFs) and help resolve existing ambiguities and limitations of low-energy neutrino deep-inelastic scattering (DIS) data [5]
An asymmetry between the strange quark and antiquark PDFs has been proposed as an explanation of the NuTeV anomaly [5], making it crucial to measure observables related to this asymmetry with high precision
Summary
The study of associated production of a W boson and a charm (c) quark at hadron colliders (hereafter referred to as W + c production) provides direct access to the strange-quark content of the proton at an energy scale of the order of the W-boson mass (Q2∼(100 GeV)2) [1,2,3]. This sensitivity is due to the dominance of sg → W− +c and sg → W+ +c contributions at the hard-scattering level (figure 1).
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