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| CMS-PAS-SMP-21-005 | ||
| Measurement of the production cross section of a W boson in association with a charm quark in proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| July 2022 | ||
| Abstract: The strange quark content of the proton is probed through the measurement of the production cross section of a W boson and a charm (c) quark in proton-proton collisions at a centre-of-mass energy of 13 TeV. The analysis uses a data sample corresponding to a total integrated luminosity of 138 fb$^{-1}$ collected by the CMS detector at the LHC. W bosons are identified through their leptonic decays to an electron or a muon, and a neutrino. Charm jets are tagged by the presence of a muon or a secondary vertex inside the jet. The W+c production cross section and the cross section ratio $ \sigma({\rm W^+\! {+}\bar{c}}) / \sigma({\rm W^-\! {+}c})$ are measured inclusively and differentially as functions of the transverse momentum and the pseudorapidity of the lepton from the W boson decay. The measurements, with improved precision from previous CMS studies, are compared with theoretical predictions. | ||
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These preliminary results are superseded in this paper, Submitted to EPJC. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Leading order diagrams for the associated production of a W boson and a charm quark. The electric charges of the W boson and the c quark have opposite sign. |
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Figure 2:
Distributions after OS-SS subtraction of the $ {p_{\mathrm {T}}} $ of the muon inside the c 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 two W boson decay channels. 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:
Distributions after OS-SS subtraction of the $ {p_{\mathrm {T}}} $ of the muon inside the c 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 two W boson decay channels. 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-b:
Distributions after OS-SS subtraction of the $ {p_{\mathrm {T}}} $ of the muon inside the c 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 two W boson decay channels. 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 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 two W boson decay channels. 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. 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:
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 two W boson decay channels. 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. 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-b:
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 two W boson decay channels. 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. 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 4:
Comparison of the measured fiducial $ {\sigma ({\mathrm{W} \,\mathrm{c}})}$ cross section unfolded to the particle level with the predictions from the MadGraph5_aMC@NLO simulation using two different PDF sets (NNPDF3.0 and NNPDF3.1). Two different tunes (CUETP8M1 and CP5) for the parton showering, hadronization and underlying event modelling in PYTHIA 8 are also used. |
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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 MadGraph5_aMC@NLO simulation. |
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Figure 5-a:
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 MadGraph5_aMC@NLO simulation. |
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Figure 5-b:
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 MadGraph5_aMC@NLO simulation. |
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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 MCFM NLO calculations using different PDF sets. |
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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 PDF sets. |
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Figure 7-a:
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 PDF sets. |
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Figure 7-b:
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 PDF sets. |
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Figure 8:
Comparison of the experimental measurement of $ {R_c^{\pm}}$ with the MCFM NLO calculations using different PDF sets. |
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Figure 9:
Measured cross section ratio $ {R_c^{\pm}}$ as a function of the absolute value of $\eta ^\ell $ (left) and $ {p_{\mathrm {T}}} ^\ell $ (right), compared with the MCFM NLO calculations using different PDF sets. |
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Figure 9-a:
Measured cross section ratio $ {R_c^{\pm}}$ as a function of the absolute value of $\eta ^\ell $ (left) and $ {p_{\mathrm {T}}} ^\ell $ (right), compared with the MCFM NLO calculations using different PDF sets. |
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Figure 9-b:
Measured cross section ratio $ {R_c^{\pm}}$ as a function of the absolute value of $\eta ^\ell $ (left) and $ {p_{\mathrm {T}}} ^\ell $ (right), compared with the MCFM NLO calculations using different PDF sets. |
| Tables | |
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Table 1:
Simulated flavour composition (in percentage) of the SL sample after selection and OS-SS subtraction. $ {\mathrm{W} + \mathrm{ Q \bar{Q} }}$ stands for the sum of the contributions of ${\mathrm{W} +\mathrm{ c \bar{c} }}$ and ${\mathrm{W} + \mathrm{ b \bar{b} }}$. |
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Table 2:
Simulated flavour composition (in percentage) of the SV sample after selection and OS-SS subtraction. $ {\mathrm{W} + \mathrm{ Q \bar{Q} }}$ stands for the sum of the contributions of ${\mathrm{W} +\mathrm{ c \bar{c} }}$ and ${\mathrm{W} + \mathrm{ b \bar{b} }}$. |
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Table 3:
Summary of the selection requirements for the four selection channels of the analysis. |
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Table 4:
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 5:
Measured production cross sections $ {\sigma ({\mathrm{W} \,\mathrm{c}})}$ unfolded to the particle level in the four channels (electron and muon W decay modes, SL and SV charm tagging modes) together with statistical (first) and systematic (second) uncertainties. Event yields (with statistical uncertainties) after OS-SS and remaining background subtraction, and acceptance times efficiency values ($\mathcal {C}$) are also given. |
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Table 6:
Measured production cross sections $ {\sigma ({\mathrm{W} \,\mathrm{c}})}$ unfolded to the parton level in the four channels (electron and muon W decay modes, SL and SV charm tagging modes) together with statistical (first) and systematic (second) uncertainties. Event yields (with statistical uncertainties) after OS-SS and remaining background subtraction, and acceptance times efficiency values (${\cal C}$) are also given. |
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Table 7:
Predictions for $ {\sigma ({\mathrm{W} \,\mathrm{c}})}$ production from MCFM at NLO 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 $\alpha _{\rm {s}}$. The total uncertainty is given in the last column. The last row in the table gives the experimental results presented in this document. |
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Table 8:
Measured production cross section ratio $ {R_c^{\pm}}$ in the four channels (electron and muon W decay modes, SL and SV charm tagging modes). Statistical (first) and systematic (second) uncertainties are also given. |
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Table 9:
Theoretical predictions for $ {R_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 $\alpha _{\rm {s}}$. The total uncertainty is given in the last column. The last row in the table gives the experimental results presented in this note. |
| Summary |
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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 is studied with a data sample collected by the CMS experiment corresponding to an integrated luminosity of 138 fb$^{-1}$ . The W+c 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 a function of $|{\eta^\ell}|$ and ${p_{\mathrm{T}}}^\ell$. The cross section ratio for the processes $\mathrm{W^{+}}{+}\bar{c}$ and $\mathrm{W^{-}}{+}c$ is measured as well. The measured fiducial $\sigma(\mathrm{W{+}c})$ production cross section unfolded to the particle level is: The cross section measurement unfolded to the parton level yields: The measured $\sigma(\mathrm{W^{+}{+}\bar{c}})/{\sigma(\mathrm{W^{-}{+}c})}$ cross section ratio is: The measurements are compared with theoretical predictions. The particle level measurements are compared with the predictions of the MadGraph5_aMC@NLO MC generator. The parton level measurements and the cross section ratio measurement are compared with analytical calculations from the MCFM program using different NLO PDF sets. The predicted fiducial cross section is generally higher (up to around 10%) than the measured $\sigma(\mathrm{W{+}c})$, and modest deviations are observed in the shapes of the differential cross sections. Inclusion of these measurements in future PDF fits should improve the modeling of the strange quark parton distribution function. |
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