Measurement of associated W + charm production in pp collisions at $ \sqrt{s} $ = 7 TeV

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 fb −1 , 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 p jet T > 25 GeV, |η jet | < 2.5, for two different thresholds for the transverse momentum of the lepton from the W-boson decay, and in the pseudorapidity range |η | < 2.1. Hadronic and inclusive semileptonic decays of charm hadrons are used to measure the following total cross sections: σ(pp → W + c + X) × B(W → ν) = 107.7 ± 3.3 (stat.) ± 6.9 (syst.) pb (p T > 25 GeV) and σ(pp → W + c + X)×B(W → ν) = 84.1±2.0 (stat.)±4.9 (syst.) pb (p T > 35 GeV), and the cross section ratios σ(pp → W + +c + X)/σ(pp → W − + c + X) = 0.954 ± 0.025 (stat.) ± 0.004 (syst.) (p T > 25 GeV) and σ(pp → W + +c + X)/σ(pp → W − + c + X) = 0.938 ± 0.019 (stat.) ± 0.006 (syst.) (p T > 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. Keywords: Hadron-Hadron Scattering IntroductionThe 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 (Q 2 ∼(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). 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]. More precise knowledge of the PDFs is essential for many present and future precision analyses, such as the measurement of the W-boson mass [6]. An asymmetry between the strange quark and antiquark PDFs has also been proposed as an explanation of the NuTeV anomaly [5], making it crucial to measure observables related to this asymmetry with high precision.W + c production receives contributions at a few percent level from the processes dg → W − + c and dg → W + + c, which are Cabibbo suppressed [7]. Overall, the W − + c yield is expected to be slightly larger than the W + + c yield at the LHC because of the participation of down valence quarks in the ini...

QCD analysis of the ATLAS and CMS W± and Z cross-section measurements and implications for the strange sea density

“…This observation was supported by analysis of the ATLAS W þ c data [9]. However, the CMS W þ c data [10] favor a somewhat smaller strangeness.…”

Multiboson interactions at the LHC

“…Studies of the W γ final state have been published by ATLAS (Aad et al, 2011b and CMS (Chatrchyan et al, 2011c(Chatrchyan et al, , 2014c at 7 TeV using data samples with integrated luminosities of up to 5 fb −1 , where the W boson was observed in the leptonic final state with the charged lepton being either an electron or a muon and the photon was required to be isolated. Both experiments provide inclusive diboson cross sections, and ATLAS additionally provides exclusive cross sections where central jet activity has been vetoed.…”

“…The production of Zγ pairs in final states with an oppositely charged electron or muon pair and an isolated photon has been studied by ATLAS (Aad et al, 2011b and CMS (Chatrchyan et al, 2011c(Chatrchyan et al, , 2014c SM Zγ production arises from photons radiated from initial state quarks or radiative Z boson decays to charged leptons as well as fragmentation of final state quarks and gluons into photons. ZγZ and Zγγ anomalous triple gauge couplings h V 3 , h V 4 (V = Z, γ) are constrained by comparing their effect on the photon E T spectrum with the observed spectrum.…”

Progress in the Determination of the Partonic Structure of the Proton

“…A much more direct handle on strangeness at the LHC comes from W production with an associated charm-tagged jet, where the dominant partonic subprocesses ares g → W +c and s g → W − c. A first preliminary measurement has been made by CMS [146] of the cross-section ratios R c ≡ σ(W + c)/σ(W + jets), probing the strange content of the proton relative to other light-quark flavors, and R ± c ≡ σ(W + +c)/σ(W − + c), potentially probing the strange asymmetry. With more precise measurements to come, including differential distributions, the W +charm process should enable powerful constraints to be made on the s ands distributions [147].…”