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| CMS-PAS-SMP-18-013 | ||
| Measurement of the associated production of a W boson and a charm quark at $\sqrt{s}= $ 8 TeV | ||
| CMS Collaboration | ||
| July 2019 | ||
| Abstract: A measurement of the associated production of a W boson and a charm quark is presented. The analysis uses a data sample corresponding to a total integrated luminosity of 19.7 fb$^{-1}$ collected by the CMS detector at the LHC at a centre-of-mass energy of 8 TeV. W bosons are identified through their leptonic decays to an electron or a muon and a neutrino. Charm jets are selected using semileptonic and other inclusive decays of charm hadrons. The cross section $\sigma({\rm pp} \rightarrow {\rm W + c + X}) {\cal B}({\rm W}\rightarrow \ell \nu)$ (where $\ell =$ e and $\mu$), and the cross section ratio $\sigma({\rm pp} \rightarrow {\rm W^+ + \bar{c} + X})/\sigma({\rm pp} \rightarrow {\rm W^- + c + X})$ are measured, both inclusively and differentially as a function of the absolute value of the pseudorapidity and the transverse momentum of the lepton from the W boson decay. The measurements are compared with theoretical predictions. | ||
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Links:
CDS record (PDF) ;
inSPIRE record ;
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These preliminary results are superseded in this paper, Submitted to EPJC. The superseded preliminary plots can be found here.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 of the $ {p_{\mathrm {T}}} $ (left) and IPS (right) of the muon inside the c-jet for events in the SL sample, adding the two W decay channels. The last bin in the IPS distribution includes all events with IPS > 7.5. The contributions of the various processes, after OS-SS subtraction, are estimated with the simulated samples. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. The ratio data/MC is shown at the bottom. |
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Figure 2-a:
Distribution of the $ {p_{\mathrm {T}}} $ of the muon inside the c-jet for events in the SL sample, adding the two W decay channels. The contributions of the various processes, after OS-SS subtraction, are estimated with the simulated samples. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. The ratio data/MC is shown at the bottom. |
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Figure 2-b:
Distribution of the IPS of the muon inside the c-jet for events in the SL sample, adding the two W decay channels. The last bin in the distribution includes all events with IPS > 7.5. The contributions of the various processes, after OS-SS subtraction, are estimated with the simulated samples. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. The ratio data/MC is shown at the bottom. |
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Figure 3:
Distributions of the flight distance significance (left) and secondary vertex mass (right) for events in the SV sample summing the contributions of the two W decay channels. 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 statistical uncertainty in the MC simulation. The ratio data/MC is shown at the bottom. |
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Figure 3-a:
Distribution of the flight distance significance for events in the SV sample summing the contributions of the two W decay channels. 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 statistical uncertainty in the MC simulation. The ratio data/MC is shown at the bottom. |
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Figure 3-b:
Distribution of the secondary vertex mass for events in the SV sample summing the contributions of the two W decay channels. 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 statistical uncertainty in the MC simulation. The ratio data/MC is shown at the bottom. |
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Figure 4:
Total cross section (left) and cross section ratio (right) data/prediction comparison. |
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Figure 4-a:
Total cross section ratio data/prediction comparison. |
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Figure 4-b:
Cross section ratio data/prediction comparison. |
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Figure 5:
Differential cross sections, $ {{\mathrm {d}}\sigma ({\mathrm{W} + \mathrm{c}})/ {\mathrm {d}} {| \eta |}}$ as a function of $|\eta ^\ell |$, and $ {{\mathrm {d}}\sigma ({\mathrm{W} + \mathrm{c}})/ {\mathrm {d}}{{p_{\mathrm {T}}}}}$ as a function of $ {p_{\mathrm {T}}} ^\ell $, compared with the theoretical predictions. Theoretical predictions at NLO are computed with MCFM using four different PDF sets. Kinematic selection follows the experimental requirements: $ {p_{\mathrm {T}}} ^{\rm jet} > $ 25 GeV, $|\eta ^{\rm jet}| < $ 2.5, $ {p_{\mathrm {T}}} ^\ell > $ 30 GeV, and $|\eta ^{\ell}| < $ 2.1. The data points are the average of the results with the four different samples: semileptonic and secondary vertex samples in the $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ samples and $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ samples. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility of the predictions. The error bars in the MCFM predictions include PDF+$\alpha _{\rm s}$+ scale uncertainties. |
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Figure 5-a:
Differential cross section, $ {{\mathrm {d}}\sigma ({\mathrm{W} + \mathrm{c}})/ {\mathrm {d}}{{p_{\mathrm {T}}}}}$ as a function of $ {p_{\mathrm {T}}} ^\ell $, compared with the theoretical predictions. Theoretical predictions at NLO are computed with MCFM using four different PDF sets. Kinematic selection follows the experimental requirements: $ {p_{\mathrm {T}}} ^{\rm jet} > $ 25 GeV, $|\eta ^{\rm jet}| < $ 2.5, $ {p_{\mathrm {T}}} ^\ell > $ 30 GeV, and $|\eta ^{\ell}| < $ 2.1. The data points are the average of the results with the four different samples: semileptonic and secondary vertex samples in the $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ samples and $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ samples. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility of the predictions. The error bars in the MCFM predictions include PDF+$\alpha _{\rm s}$+ scale uncertainties. |
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Figure 5-b:
Differential cross sections, $ {{\mathrm {d}}\sigma ({\mathrm{W} + \mathrm{c}})/ {\mathrm {d}} {| \eta |}}$ as a function of $|\eta ^\ell |$, and $ {{\mathrm {d}}\sigma ({\mathrm{W} + \mathrm{c}})/ {\mathrm {d}}{{p_{\mathrm {T}}}}}$ as a function of $ {p_{\mathrm {T}}} ^\ell $, compared with the theoretical predictions. Theoretical predictions at NLO are computed with MCFM using four different PDF sets. Kinematic selection follows the experimental requirements: $ {p_{\mathrm {T}}} ^{\rm jet} > $ 25 GeV, $|\eta ^{\rm jet}| < $ 2.5, $ {p_{\mathrm {T}}} ^\ell > $ 30 GeV, and $|\eta ^{\ell}| < $ 2.1. The data points are the average of the results with the four different samples: semileptonic and secondary vertex samples in the $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ samples and $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ samples. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility of the predictions. The error bars in the MCFM predictions include PDF+$\alpha _{\rm s}$+ scale uncertainties. |
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Figure 6:
Cross section ratio, $\sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})/\sigma ({\mathrm{W^{-}} + \mathrm{c}})$, as a function of $|\eta ^\ell |$ and $ {p_{\mathrm {T}}} ^\ell $. The data points are the average of the results from the semileptonic and secondary vertex samples in the $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ samples and $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ samples. Theoretical predictions at NLO computed with MCFM and four different PDF sets are also shown. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility of the predictions. The error bars in the MCFM predictions include PDF+$\alpha _{\rm s}$+ scale uncertainties. |
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Figure 6-a:
Cross section ratio, $\sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})/\sigma ({\mathrm{W^{-}} + \mathrm{c}})$, as a function of $|\eta ^\ell |$. The data points are the average of the results from the semileptonic and secondary vertex samples in the $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ samples and $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ samples. Theoretical predictions at NLO computed with MCFM and four different PDF sets are also shown. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility of the predictions. The error bars in the MCFM predictions include PDF+$\alpha _{\rm s}$+ scale uncertainties. |
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Figure 6-b:
Cross section ratio, $\sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})/\sigma ({\mathrm{W^{-}} + \mathrm{c}})$, as a function of $ {p_{\mathrm {T}}} ^\ell $. The data points are the average of the results from the semileptonic and secondary vertex samples in the $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ samples and $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ samples. Theoretical predictions at NLO computed with MCFM and four different PDF sets are also shown. Symbols showing the theoretical expectations are slightly displaced in the horizontal axis for better visibility of the predictions. The error bars in the MCFM predictions include PDF+$\alpha _{\rm s}$+ scale uncertainties. |
| Tables | |
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Table 1:
Flavour composition after selection and OS-SS subtraction, for the electron and muon decay channels of the W boson. |
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Table 2:
Flavour composition after selection, including OS-SS subtraction, for the electron and muon W decay channels. |
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Table 3:
Measured production cross sections $ {\sigma ({\mathrm{W} + \mathrm{c}})}$ in the SL (top) and SV (bottom) channels for the $ {\mathrm{W} {\rightarrow}\mathrm{e} \nu}$ and $ {\mathrm{W} {\rightarrow} {\mu \nu}}$ decays separately. Statistical (first error) and systematic (second error) uncertainties are also given. |
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Table 4:
Measured production cross sections $\sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})$ and $\sigma ({\mathrm{W^{-}} + \mathrm{c}})$ in the SL (top) and SV (bottom) channels for the W-boson electron and muon decays separately. Statistical (first error) and systematic (second error) uncertainties are also given. |
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Table 5:
Relative contribution on $\sigma _{W+c}$ of each systematic uncertainty for all channels. |
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Table 6:
Measured differential cross section as a function of the absolute value of $|\eta ^\ell |$. Statistical (first error) and systematic (second error) uncertainties are also given. |
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Table 7:
Measured differential cross section as a function of $ {p_{\mathrm {T}}} ^\ell $. Statistics (first error) and systematic (second error) uncertainties are also given. |
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Table 8:
Measured ratios $\sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})/\sigma ({\mathrm{W^{-}} + \mathrm{c}})$ as a function of the absolute value of the pseudorapidity of the lepton from the W-boson decay. Statistical (first error) and systematic (second error) uncertainties are also given. |
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Table 9:
Measured ratios $\sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})/\sigma ({\mathrm{W^{-}} + \mathrm{c}})$ as a function of the transverse momentum of the lepton from the W-boson decay. Statistical (first error) and systematic (second error) uncertainties are also given. |
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Table 10:
Predictions for $ {\sigma ({\mathrm{W} + \mathrm{c}})}$ from MCFM at NLO. Kinematic selection follows the experimental requirements: $ {p_{\mathrm {T}}} ^{\ell} > $ 30 GeV, $|\eta ^{\ell}| < $ 2.1, $ {p_{\mathrm {T}}} ^{\rm jet} > $ 25 GeV, and $|\eta ^{\rm jet}| < $ 2.5. Partons are clustered into jets using an anti-$k_{\rm T}$ algorithm with a distance parameter of 0.5. For every PDF set, the central value of the prediction is given, together with the relative uncertainty as prescribed from the PDF set, and the uncertainties associated with scale variations and with the value of $\alpha _{\rm {s}}$. The last row in the table gives the experimental results presented in this document. |
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Table 11:
Theoretical predictions for $ {R_c^{\pm}}\equiv \sigma ({\mathrm{W^{+}} + \mathrm{\bar{c}}})/\sigma ({\mathrm{W^{-}} + \mathrm{c}})$ calculated with MCFM at NLO. Kinematic selection follows the experimental requirements: $ {p_{\mathrm {T}}} ^{\ell} > $ 30 GeV, $|\eta ^{\ell}| < $ 2.1, $ {p_{\mathrm {T}}} ^{\rm jet} > $ 25 GeV, and $|\eta ^{\rm jet}| < $ 2.5. Partons are clustered into jets using an anti-$k_{\rm T}$ algorithm with a distance parameter of 0.5. For every PDF set, the central value of the prediction is given, together with the relative uncertainty as prescribed from the PDF set, and the uncertainties associated with scale variations and with the value of $\alpha _{\rm {s}}$. The last row in the table gives the experimental results presented in this document. |
| Summary |
| The associated production of a W boson with a charm quark in proton-proton collisions at a centre-of-mass energy of 8 TeV is studied with a data sample collected by the CMS experiment corresponding to an integrated luminosity of 19.7 $\pm$ 0.5 fb$^{-1}$ . The W+c process is identified by the presence of an isolated and high transverse momentum lepton (electron or muon) coming from the decay of the W boson and the reconstruction of inclusive (semileptonic and hadronic) final states from the decay of charm hadrons. 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. Inclusive and differential (as a function of $\eta^\ell$ and ${p_{\mathrm{T}}}^\ell$) cross section measurements are performed with four different data samples (electron and muon W boson decay channels and reconstruction of semileptonic and hadronic decays of charm hadrons). The cross section ratio for the processes $\mathrm{W^{+}}$+c and $\mathrm{W^{-}}$+c is measured. Cross sections and cross sections ratios are found to be consistent in the four different channels and are combined. |
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