Measurements of the associated production of a W boson and a charm quark in proton-proton collisions at s = 8 TeV

Abstract

Measurements of the associated production of a W boson and a charm ( $\text{c}$ ) quark in proton-proton collisions at a centre-of-mass energy of 8 $\text{TeV}$ are reported. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7 ${\text{fb}}^{-1}$ collected by the CMS detector at the LHC. The W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm quark jets are selected using distinctive signatures of charm hadron decays. The product of the cross section and branching fraction $\sigma (\text{pp}\to \text{W}+\text{c}+\text{X})B(\text{W}\to \ell \nu )$ , where $\ell =\text{e}$ or $\mu$ , and the cross section ratio $\sigma (\text{pp}\to {\text{W}}^{+}+\overline{\text{c}}+\text{X})/\sigma (\text{pp}\to {\text{W}}^{-}+\text{c}+\text{X})$ are measured in a fiducial volume and differentially as functions of the pseudorapidity and of the transverse momentum of the lepton from the W boson decay. The results are compared with theoretical predictions. The impact of these measurements on the determination of the strange quark distribution is assessed.

Fig. 1

Leading order diagrams for the associated production of a W boson and a charm (anti)quark

Fig. 2

Distributions after $\text{OS--SS}$ subtraction of the impact parameter significance, IPS, (left) and $p_{\text{T}}$ (right), of the muon inside the $\text{c jet}$ for events in the SL sample, summing up the contributions of the two W boson decay channels. The IPS distribution is shown after all selection requirements except the one on this variable. The last bin of the distribution includes all events with $\text{IPS}>7.5$. The $p_{\text{T}}$ distribution includes the selection requirement $\text{IPS}>1.0$ for the $\text{W}\to \mathrm{\mu }\mathrm{\nu }$ channel. The contributions of the various processes are estimated with the simulated samples. Vertical bars on data points represent statistical uncertainty in the data. The hatched areas represent the sum in quadrature of statistical and systematic uncertainties in the MC simulation. The ratio of data to simulation is shown in the lower panels. The uncertainty band in the ratio includes the statistical uncertainty in the data, and the statistical and systematic uncertainties in the MC simulation

Fig. 3

Distributions after $\text{OS--SS}$ subtraction of the secondary-vertex displacement significance (left) and corrected secondary-vertex mass (right). For each distribution all selection requirements are applied except the one on the displayed variable. The last bin of each plot includes all events beyond the bin. The contributions from all processes are estimated with the simulated samples. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of statistical and systematic uncertainties in the MC simulation. The ratio of data to simulation is shown in the lower panels. The uncertainty band in the ratio includes the statistical uncertainty in the data, and the statistical and systematic uncertainties in the MC simulation

Fig. 4

Comparison of the theoretical predictions for $\sigma (\text{W}+\text{c})$ (left) and $\sigma ({\text{W}}^{+}+\overline{\text{c}})/\sigma ({\text{W}}^{-}+\text{c})$ (right) computed with $\mathrm{M}\mathrm{C}\mathrm{F}\mathrm{M}$ and several sets of PDFs with the current experimental measurements

Fig. 5

Differential cross sections, $\text{d}\sigma (\text{W}+\text{c})/\text{d}|{\eta }^{\ell }|$ (upper) and $\text{d}\sigma (\text{W}+\text{c})/\text{d}{p}_{\text{T}}^{\ell }$ (lower). The data points are the combination of the results with the four different samples: SL and SV samples in $\text{W}\to \text{e}\mathrm{\nu }$ and $\text{W}\to \mathrm{\mu }\mathrm{\nu }$ events. Theoretical predictions at NLO computed with $\mathrm{M}\mathrm{C}\mathrm{F}\mathrm{M}$ and four different NLO PDF sets are also shown. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The error bars in the $\mathrm{M}\mathrm{C}\mathrm{F}\mathrm{M}$ predictions include PDF, ${\alpha }_{\text{S}}$, and scale uncertainties. The inset in the lower plot, $\text{d}\sigma (\text{W}+\text{c})/\text{d}{p}_{\text{T}}^{\ell }$, zooms into the measurement-prediction comparison for the last bin, $100<p_{\text{T}}^{\ell }<200\text{GeV}$. Predictions from MadGraph using the PDF set MSTW2008NNLO are also presented

Fig. 6

Cross section ratio, $R_{\text{c}}^{\pm }$, as functions of $|{\eta }^{\ell }|$ (upper) and $p_{\text{T}}^{\ell }$ (lower). The data points are the combination of the results from the SL and SV samples in $\text{W}\to \text{e}\mathrm{\nu }$ and $\text{W}\to \mathrm{\mu }\mathrm{\nu }$ events. Theoretical predictions at NLO computed with $\mathrm{M}\mathrm{C}\mathrm{F}\mathrm{M}$ and four different NLO PDF sets are also shown. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The error bars in the $\mathrm{M}\mathrm{C}\mathrm{F}\mathrm{M}$ predictions include PDF, ${\alpha }_{\text{S}}$, and scale uncertainties. Predictions from MadGraph using the PDF set MSTW2008NNLO are also presented

Fig. 7

The strange quark distribution (upper left) and the strangeness suppression factor (upper right) as a function of x at the factorization scale of $m_{\text{W}}^2$. The corresponding relative total uncertainties are compared in the lower plots (strange quark distribution, lower left, and strangeness suppression factor, lower right). The results from the QCD analysis, shown as a filled area, use as input the combination of the inclusive deep inelastic scattering (DIS) cross sections [71], the CMS measurements of the lepton charge asymmetry in W boson production at $\sqrt{s}=7$ and 8$\text{TeV}$ [72, 73], and the CMS measurements of $\text{W}+\text{c}$ production at $\sqrt{s}=7$ [5],  8 (this analysis) and 13$\text{TeV}$ [6]. The $\text{W}+\text{c}$ measurement at $\sqrt{s}=8\text{TeV}$ is not used for the fit shown in hatched style

Fig. 8

The strange quark distribution (left) and the strangeness suppression factor (right) as a function of x at the factorization scale of $m_{\text{W}}^2$. The results of the current analysis are shown together with those from the global NLO PDFs, ABMP16 and NNPDF3.1 in the upper plot, and CT18 and MSHT20 in the lower one. This QCD analysis uses as input the combination of the inclusive deep inelastic scattering (DIS) cross sections [71], the CMS measurements of the lepton charge asymmetry in W boson production at $\sqrt{s}=7$ and 8$\text{TeV}$ [72, 73], and the CMS measurements of $\text{W}+\text{c}$ production at $\sqrt{s}=7$ [5],  8 (this analysis) and 13$\text{TeV}$ [6]