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CMS-SMP-21-005 ; CERN-EP-2023-105
Measurement of the production cross section for a W boson in association with a charm quark in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
Eur. Phys. J. C 84 (2024) 27
Abstract: The strange quark content of the proton is probed through the measurement of the production cross section for a W boson and a charm (c) quark in proton-proton collisions at a center-of-mass energy of 13 TeV. The analysis uses a data sample corresponding to a total integrated luminosity of 138 fb$^{-1}$ collected with 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 jets are tagged using the presence of a muon or a secondary vertex inside the jet. The Wc production cross section and the cross section ratio $ R_\mathrm{c}^{\pm} = \sigma(\mathrm{W^{+}}\,+\,\bar{\mathrm{c}})/\sigma(\mathrm{W^{-}}\,+\,\mathrm{c}) $ are measured inclusively and differentially as functions of the transverse momentum and the pseudorapidity of the lepton originating from the W boson decay. The precision of the measurements is improved with respect to previous studies, reaching 1% in $ R_\mathrm{c}^{\pm} $. The measurements are compared with theoretical predictions up to next-to-next-to-leading order in perturbative quantum chromodynamics.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Leading order Feynman diagrams for the associated production of a W boson and a charm quark. The electric charges of the W boson and c quark have opposite signs.

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Figure 2:
Distributions after OS-SS subtraction of the $ p_{\mathrm{T}} $ of the muon inside the $ \mathrm{c}\text{ jet} $ (left) and the $ p_{\mathrm{T}} $ of the lepton from the W decay (right) for events in the SL sample, summing up the contributions of the W boson decay channels to electrons and muons. The contributions of the various processes are estimated with the simulated samples. The statistical uncertainty in the data is smaller than the size of the data dots. 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.

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Figure 2-a:
Distribution after OS-SS subtraction of the $ p_{\mathrm{T}} $ of the muon inside the $ \mathrm{c}\text{ jet} $ for events in the SL sample, summing up the contributions of the W boson decay channels to electrons and muons. The contributions of the various processes are estimated with the simulated samples. The statistical uncertainty in the data is smaller than the size of the data dots. 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 panel. The uncertainty band in the ratio includes the statistical uncertainty in the data, and the statistical and systematic uncertainties in the MC simulation.

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Figure 2-b:
Distribution after OS-SS subtraction of the $ p_{\mathrm{T}} $ of the lepton from the W decay for events in the SL sample, summing up the contributions of the W boson decay channels to electrons and muons. The contributions of the various processes are estimated with the simulated samples. The statistical uncertainty in the data is smaller than the size of the data dots. 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 panel. The uncertainty band in the ratio includes the statistical uncertainty in the data, and the statistical and systematic uncertainties in the MC simulation.

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Figure 3:
Distributions after OS-SS subtraction of the corrected SV mass (left) and SV transverse momentum divided by the jet transverse momentum (right) for events in the SV sample, summing up the contributions of the W boson decay channels to electrons and muons. The contributions from all processes are estimated with the simulated samples. The statistical uncertainty in the data is smaller than the size of the data dots for most of the data points. 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.

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Figure 3-a:
Distribution after OS-SS subtraction of the corrected SV mass for events in the SV sample, summing up the contributions of the W boson decay channels to electrons and muons. The contributions from all processes are estimated with the simulated samples. The statistical uncertainty in the data is smaller than the size of the data dots for most of the data points. 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 panel. The uncertainty band in the ratio includes the statistical uncertainty in the data, and the statistical and systematic uncertainties in the MC simulation.

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Figure 3-b:
Distribution after OS-SS subtraction of the SV transverse momentum divided by the jet transverse momentum for events in the SV sample, summing up the contributions of the W boson decay channels to electrons and muons. The contributions from all processes are estimated with the simulated samples. The statistical uncertainty in the data is smaller than the size of the data dots for most of the data points. 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 panel. The uncertainty band in the ratio includes the statistical uncertainty in the data, and the statistical and systematic uncertainties in the MC simulation.

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Figure 4:
Comparison of the measured fiducial $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ cross section unfolded to the particle level with the predictions from the MadGraph-5_aMC@NLO simulation using two different PDF sets (NLO NNPDF3.0 and NNLO NNPDF3.1). Two different tunes (CUETP8M1 and CP5) for the parton showering, hadronization and underlying event modeling in PYTHIAeight are also used. Horizontal error bars indicate the total uncertainty in the predictions.

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Figure 5:
Measured differential cross sections $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}|\eta^\ell| $ (left) and $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}{p_{\mathrm{T}}^\ell} $ (right) unfolded to the particle level, compared with the predictions of the MadGraph-5_aMC@NLO simulation. Two different PDF sets (NLO NNPDF3.0 and NNLO NNPDF3.1) are used. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panels. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 5-a:
Measured differential cross section $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}|\eta^\ell| $, unfolded to the particle level, compared with the predictions of the MadGraph-5_aMC@NLO simulation. Two different PDF sets (NLO NNPDF3.0 and NNLO NNPDF3.1) are used. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 5-b:
Measured differential cross section $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}{p_{\mathrm{T}}^\ell} $, unfolded to the particle level, compared with the predictions of the MadGraph-5_aMC@NLO simulation. Two different PDF sets (NLO NNPDF3.0 and NNLO NNPDF3.1) are used. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 6:
Comparison of the experimental measurement of $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $, unfolded to the parton level, with the predictions from the NLO QCD MCFM calculations using different NLO PDF sets. Horizontal error bars indicate the total uncertainty in the predictions.

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Figure 7:
Measured differential cross sections $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}|\eta^\ell| $ (left) and $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}{p_{\mathrm{T}}^\ell} $ (right) unfolded to the parton level, compared with the predictions from the MCFM NLO calculations using different NLO PDF sets. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panels. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 7-a:
Measured differential cross section $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}|\eta^\ell| $, unfolded to the parton level, compared with the predictions from the MCFM NLO calculations using different NLO PDF sets. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 7-b:
Measured differential cross section $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}{p_{\mathrm{T}}^\ell} $, unfolded to the parton level, compared with the predictions from the MCFM NLO calculations using different NLO PDF sets. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 8:
Comparison of the experimental measurement of $ R_\mathrm{c}^{\pm} $ with the NLO QCD MCFM calculations using different NLO PDF sets. Horizontal error bars indicate the total uncertainty in the predictions.

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Figure 9:
Measured cross section ratio $ R_\mathrm{c}^{\pm} $ as a function of the absolute value of $ \eta^\ell $ (left) and $ p_{\mathrm{T}}^\ell $ (right), compared with the NLO QCD MCFM calculations using different NLO PDF sets. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panels. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 9-a:
Measured cross section ratio $ R_\mathrm{c}^{\pm} $ as a function of the absolute value of $ \eta^\ell $, compared with the NLO QCD MCFM calculations using different NLO PDF sets. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratio of data to prediction is shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 9-b:
Measured cross section ratio $ R_\mathrm{c}^{\pm} $ as a function of the absolute value of $ p_{\mathrm{T}}^\ell $, compared with the NLO QCD MCFM calculations using different NLO PDF sets. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratio of data to prediction is shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 10:
Comparison of the experimental measurement of $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ with the OS-SS LO, NLO, and NNLO QCD predictions, and NLO EW corrections. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Horizontal error bars indicate the total uncertainty in the predictions.

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Figure 11:
Comparison of the measured differential cross sections $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}|\eta^\ell| $ (left) and $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}{p_{\mathrm{T}}^\ell} $ (right) with the OS-SS LO, NLO, and NNLO QCD predictions, and NLO EW corrections. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panels. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 11-a:
Comparison of the measured differential cross section $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}|\eta^\ell| $ with the OS-SS LO, NLO, and NNLO QCD predictions, and NLO EW corrections. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratio of data to prediction is shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 11-b:
Comparison of the measured differential cross section $ \mathrm{d}\sigma(\mathrm{W}\,+\,\mathrm{c})/\mathrm{d}{p_{\mathrm{T}}^\ell} $ with the OS-SS LO, NLO, and NNLO QCD predictions, and NLO EW corrections. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratio of data to prediction is shown in the lower panel. The uncertainty in the ratio includes the uncertainties in both data and prediction.

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Figure 12:
Comparison of the experimental measurement of $ R_\mathrm{c}^{\pm} $ with the OS-SS LO, NLO and NNLO QCD predictions. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Horizontal error bars indicate the total uncertainty in the predictions.

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Figure 13:
Comparison of the measured differential cross section ratio $ R_\mathrm{c}^{\pm} $ as a function of the absolute value of $ \eta^\ell $ (left) and $ p_{\mathrm{T}}^\ell $ (right) with the OS-SS LO, NLO, and NNLO QCD predictions. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratios of data to predictions are shown in the lower panels. The uncertainty in the ratio includes the uncertainties in the data and prediction.

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Figure 13-a:
Comparison of the measured differential cross section ratio $ R_\mathrm{c}^{\pm} $ as a function of the absolute value of $ \eta^\ell $ with the OS-SS LO, NLO, and NNLO QCD predictions. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratio of data to prediction is shown in the lower panel. The uncertainty in the ratio includes the uncertainties in the data and prediction.

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Figure 13-b:
Comparison of the measured differential cross section ratio $ R_\mathrm{c}^{\pm} $ as a function of the absolute value of $ p_{\mathrm{T}}^\ell $ with the OS-SS LO, NLO, and NNLO QCD predictions. The NNLO QCD NNPDF3.1 PDF set is used for computing all the predictions. CMPP stands for the authors of the calculations [15]. Error bars on data points include statistical and systematic uncertainties. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility. The ratio of data to prediction is shown in the lower panel. The uncertainty in the ratio includes the uncertainties in the data and prediction.
Tables

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Table 1:
Data and background event yields (with statistical uncertainties) after selection and OS-SS subtraction for the SL channels (electron and muon W decay modes).

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Table 2:
Simulated signal and background composition (in percentage) of the SL sample after selection and OS-SS subtraction. The $ \mathrm{W} + {\mathrm{Q}\bar{\mathrm{Q}}} $ stands for the sum of the contributions of $ \mathrm{W} + \mathrm{c} \bar{\mathrm{c}} $ and $ \mathrm{W} + \mathrm{b} \bar{\mathrm{b}} $.

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Table 3:
Data and background event yields (with statistical uncertainties) after selection and OS-SS subtraction for the SV channels (electron and muon W decay modes).

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Table 4:
Simulated signal and background composition (in percentage) of the SV sample after selection and OS-SS subtraction. The $ \mathrm{W} + {\mathrm{Q}\bar{\mathrm{Q}}} $ stands for the sum of the contributions of $ \mathrm{W} + \mathrm{c} \bar{\mathrm{c}} $ and $ \mathrm{W} + \mathrm{b} \bar{\mathrm{b}} $.

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Table 5:
Summary of the selection requirements for the four selection channels of the analysis.

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Table 6:
Summary of the main systematic uncertainties, in percentage of the measured fiducial cross section, for the four selection channels of the analysis.

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Table 7:
Measured production cross sections $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ unfolded to the particle level in the four selection channels together with statistical (first) and systematic (second) uncertainties. The acceptance times efficiency values ($ \mathcal{C} $) are also given.

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Table 8:
Measured production cross sections $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ unfolded to the parton level in the four selection channels together with statistical (first) and systematic (second) uncertainties. The acceptance times efficiency values ($ {\cal C} $) are also given.

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Table 9:
Predictions for $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ production from MCFM at NLO in QCD for the phase space of the analysis. For every PDF set, the central value of the prediction is given, together with the uncertainty as prescribed from the PDF set, and the uncertainties associated with the scale variations and with the value of $ \alpha_{\mathrm{s}} $. The total uncertainty is given in the last column. The last row in the table gives the experimental result presented in this paper.

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Table 10:
Measured production cross section ratio $ R_\mathrm{c}^{\pm} $ in the four selection channels. Statistical (first) and systematic (second) uncertainties are also given.

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Table 11:
Theoretical predictions for $ R_\mathrm{c}^{\pm} $ calculated with MCFM at NLO. The kinematic selection follows the experimental requirements. For every PDF set, the central value of the prediction is given, together with the uncertainty as prescribed from the PDF set, and the uncertainties associated with the scale variations and with the value of $ \alpha_{\mathrm{s}} $. The total uncertainty is given in the last column. The last row in the table gives the experimental result presented in this paper.

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Table 12:
Predictions for $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ in the phase space of the analysis. For each QCD and EW order, the central values of the OS, SS and OS-SS predictions are given, together with the statistical, scales, PDF, and total uncertainties of the OS-SS prediction. All values are given in pb. The last row in the table gives the experimental result presented in this paper.

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Table 13:
Theoretical predictions for $ R_\mathrm{c}^{\pm} $. For each QCD order, the central values are given, together with the MC statistical, scales, PDF, and total uncertainties. The last row in the table gives the experimental result presented in this paper.
Summary
The associated production of a W boson with a charm quark (Wc) in proton-proton (pp) collisions at a center-of-mass energy of 13 TeV was studied with a data sample collected by the CMS experiment corresponding to an integrated luminosity of 138 fb$^{-1}$. The Wc process is selected based on the presence of a high transverse momentum lepton (electron or muon), coming from a W boson decay, and a jet with the signature of a charm hadron decay. Charm hadron decays are identified either by the presence of a muon inside a jet or by reconstructing a secondary decay vertex within the jet. Measurements are combined from the four different channels: electron and muon W boson decay channels, muon and secondary vertex charm identification channels.

Cross section measurements, within a fiducial region defined by the kinematics of the lepton from the W boson decay and the jet originated by the charm quark ($ p_{\mathrm{T}}^{\ell} > $ 35 GeV, $ |\eta^{\ell}| < $ 2.4, $ p_{\mathrm{T}}^{\mathrm{c}\text{ jet} } > $ 30 GeV, $ |\eta^{\mathrm{c}\text{ jet} }| < $ 2.4), are unfolded to the particle and parton levels. Cross sections are also measured differentially, as functions of $ |\eta^\ell| $ and $ p_{\mathrm{T}}^\ell $. The cross section ratio for the processes $ \mathrm{W^+}\,+\,\bar{\mathrm{c}} $ and $ \mathrm{W^-}\,+\,\mathrm{c} $ is measured as well, achieving the highest precision in this measurement to date.

The measured fiducial $ \sigma(\mathrm{W}\,+\,\mathrm{c}) $ production cross section unfolded to the particle level is:

$\sigma( \mathrm{p}\mathrm{p} \to \mathrm{W}\,+\,\mathrm{c} )\mathcal{B}(W \to \ell\nu) = $ 148.7 $\pm$ 0.4 (stat) $\pm$ 5.6 (syst) pb.


The cross section measurement unfolded to the parton level yields:

$\sigma( \mathrm{p}\mathrm{p} \to \mathrm{W}\,+\,\mathrm{c} )\mathcal{B}(W \to \ell\nu) = $ 163.4 $\pm$ 0.5 (stat) $\pm$ 6.2 (syst) pb.


The measured $ \sigma(\mathrm{W^+}\,+\,\bar{\mathrm{c}})/\sigma(\mathrm{W^-}\,+\,\mathrm{c}) $ cross section ratio is:

${\sigma( \mathrm{p}\mathrm{p} \to \mathrm{W^+}\,+\,\bar{\mathrm{c}} )}/{\sigma( \mathrm{p}\mathrm{p} \to \mathrm{W^-}\,+\,\mathrm{c} )} = $ 0.950 $\pm$ 0.005 (stat) $\pm$ 0.010 (syst).


The measurements are compared with theoretical predictions. The particle level measurements are compared with the predictions of the MadGraph-5_aMC@NLO MC generator. The parton level cross section measurements are compared with NLO QCD calculations from the MCFM program using different PDF sets and with recently available NNLO QCD calculations including NLO EW corrections. The predicted fiducial cross section and cross section ratio are consistent with the measurements within uncertainties. The NNLO QCD and NLO EW corrections improve the agreement between the predicted and measured cross sections. Despite the improvement in precision of the cross section ratio measurement compared with previous studies, discrimination between predictions using symmetric or asymmetric strange quark and antiquark PDFs would require a further reduction of experimental and theoretical uncertainties. The theoretical uncertainty is dominated by the PDF uncertainties. The inclusion of the cross section measurements in future PDF fits should improve the modeling of the strange parton distribution function of the proton.
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Compact Muon Solenoid
LHC, CERN