CMS-SMP-17-014 ; CERN-EP-2018-282 | ||
Measurement of associated production of a W boson and a charm quark in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
CMS Collaboration | ||
26 November 2018 | ||
Eur. Phys. J. C 79 (2019) 269 | ||
Abstract: Measurements are presented of associated production of a W boson and a charm quark (W+c) in proton-proton collisions at a center-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 35.7 fb$^{-1}$ collected by the CMS experiment at the CERN LHC. The W bosons are identified by their decay into a muon and a neutrino. The charm quarks are tagged via the full reconstruction of ${{\mathrm{D}^{*}(2010)^{\pm}}}$ mesons that decay via ${{\mathrm{D}^{*}(2010)^{\pm}}} \to {\mathrm{D^0}} \pi^{\pm} \to \mathrm{K}^{\mp} \pi^{\pm} \pi^{\pm}$. A cross section is measured in the fiducial region defined by the muon transverse momentum ${p_{\mathrm{T}}}^{\mu} > $ 26 GeV, muon pseudorapidity $|\eta^{\mu}| < 2.4$, and charm quark transverse momentum ${p_{\mathrm{T}}}^{\mathrm{c}} > $ 5 GeV. The inclusive cross section for this kinematic range is $\sigma(\mathrm{W}+\mathrm{c})= $ 1026 $\pm$ 31 (stat)$^{+76}_{-72}$ (syst) pb. The cross section is also measured differentially as a function of the pseudorapidity of the muon from the W boson decay. These measurements are compared with theoretical predictions and are used to probe the strange quark content of the proton. | ||
Links: e-print arXiv:1811.10021 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Dominant contributions to ${{\mathrm {W}}{+} {\mathrm {c}}}$ production at the LHC at leading order in pQCD. |
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Figure 1-a:
Dominant contribution to ${{\mathrm {W}^+} {+}{\mathrm {\bar{c}}}}$ production at the LHC at leading order in pQCD. |
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Figure 1-b:
Dominant contribution to ${{\mathrm {W}^-} {+}{\mathrm {c}}}$ production at the LHC at leading order in pQCD. |
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Figure 2:
Distributions of the reconstructed invariant mass of $ {{\mathrm {K}}^{\mp}} {\pi ^\mathrm {{\pm}}}$ candidates (left) in the range $ | {\Delta m({\mathrm {D}}^*, {\mathrm {D^0}})} -0.1454 | < $ 0.001 GeV, and the reconstructed mass difference ${\Delta m({\mathrm {D}}^*, {\mathrm {D^0}})}$ (right). The SS combinations are subtracted. The data (filled circles) are compared to MC simulation with contributions from different processes shown as histograms of different shades. |
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Figure 2-a:
Distribution of the reconstructed invariant mass of $ {{\mathrm {K}}^{\mp}} {\pi ^\mathrm {{\pm}}}$ candidates in the range $ | {\Delta m({\mathrm {D}}^*, {\mathrm {D^0}})} -0.1454 | < $ 0.001 GeV. The SS combinations are subtracted. The data (filled circles) are compared to MC simulation with contributions from different processes shown as histograms of different shades. |
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Figure 2-b:
Distribution of the reconstructed mass difference ${\Delta m({\mathrm {D}}^*, {\mathrm {D^0}})}$. The SS combinations are subtracted. The data (filled circles) are compared to MC simulation with contributions from different processes shown as histograms of different shades. |
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Figure 3:
Number of events after OS$-$SS subtraction for data (filled circles) and MC simulation (filled histograms) as a function of $ {{| \eta ^{{\mu}} |}} $. |
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Figure 4:
Inclusive fiducial cross section $ {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})} $ and the cross section ratio $ {\sigma ({{\mathrm {W^+}}{+} {\overline {\mathrm {c}}}})} / {\sigma ({{\mathrm {W^-}}{+} {\mathrm {c}}})} $ at 13 TeV. The data are represented by a line with the statistical (total) uncertainty shown by a light (dark) shaded band. The measurements are compared to the NLO QCD prediction using several PDF sets, represented by symbols of different types. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars depict the total theoretical uncertainty, including the PDF and the scale variation uncertainty. |
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Figure 4-a:
Inclusive fiducial cross section $ {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})} $ at 13 TeV. The data are represented by a line with the statistical (total) uncertainty shown by a light (dark) shaded band. The measurements are compared to the NLO QCD prediction using several PDF sets, represented by symbols of different types. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars depict the total theoretical uncertainty, including the PDF and the scale variation uncertainty. |
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Figure 4-b:
Cross section ratio $ {\sigma ({{\mathrm {W^+}}{+} {\overline {\mathrm {c}}}})} / {\sigma ({{\mathrm {W^-}}{+} {\mathrm {c}}})} $ at 13 TeV. The data are represented by a line with the statistical (total) uncertainty shown by a light (dark) shaded band. The measurements are compared to the NLO QCD prediction using several PDF sets, represented by symbols of different types. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars depict the total theoretical uncertainty, including the PDF and the scale variation uncertainty. |
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Figure 5:
Left: differential cross sections of $ {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})} $ production at 13 TeV measured as a function $ {{| \eta ^{{\mu}} |}} $. The data are presented by filled circles with the statistical (total) uncertainties shown by vertical error bars (light shaded bands). The measurements are compared to the QCD predictions calculated with MCFM at NLO using different PDF sets, presented by symbols of different style. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars represent theoretical uncertainties, which include PDF and scale variation uncertainty. Right: $ {\sigma ({\mathrm {W}}{+} {{\mathrm {D}^{*}(2010)^{\pm}}})} $ differential production cross sections presented as a function of $ {{| \eta ^{{\mu}} |}} $. The data (filled circles) are shown with their total (outer error bars) and statistical (inner error bars) uncertainties and are compared to the predictions of the signal MC generated with MadGraph 5_amc@nlo and using NNPDF3.0nlo to describe the proton structure. PDF uncertainties and scale variations are accounted for and added in quadrature (shaded band). |
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Figure 5-a:
Differential cross sections of $ {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})} $ production at 13 TeV measured as a function $ {{| \eta ^{{\mu}} |}} $. The data are presented by filled circles with the statistical (total) uncertainties shown by vertical error bars (light shaded bands). The measurements are compared to the QCD predictions calculated with MCFM at NLO using different PDF sets, presented by symbols of different style. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars represent theoretical uncertainties, which include PDF and scale variation uncertainty. |
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Figure 5-b:
$ {\sigma ({\mathrm {W^{-}}}{+} {{\mathrm {D}^{*}(2010)^{+}}})} $ differential production cross sections presented as a function of $ {{| \eta ^{{\mu}} |}} $. The data (filled circles) are shown with their total (outer error bars) and statistical (inner error bars) uncertainties and are compared to the predictions of the signal MC generated with MadGraph 5_amc@nlo and using NNPDF3.0nlo to describe the proton structure. PDF uncertainties and scale variations are accounted for and added in quadrature (shaded band). |
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Figure 5-c:
Differential cross sections of $ {\sigma ({{\mathrm {W^{+}}}{+} {\mathrm {\bar{c}}}})} $ production at 13 TeV measured as a function $ {{| \eta ^{{\mu}} |}} $. The data are presented by filled circles with the statistical (total) uncertainties shown by vertical error bars (light shaded bands). The measurements are compared to the QCD predictions calculated with MCFM at NLO using different PDF sets, presented by symbols of different style. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars represent theoretical uncertainties, which include PDF and scale variation uncertainty. |
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Figure 5-d:
$ {\sigma ({\mathrm {W^{+}}}{+} {{\mathrm {D}^{*}(2010)^{-}}})} $ differential production cross sections presented as a function of $ {{| \eta ^{{\mu}} |}} $. The data (filled circles) are shown with their total (outer error bars) and statistical (inner error bars) uncertainties and are compared to the predictions of the signal MC generated with MadGraph 5_amc@nlo and using NNPDF3.0nlo to describe the proton structure. PDF uncertainties and scale variations are accounted for and added in quadrature (shaded band). |
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Figure 5-e:
Differential cross sections of $ {\sigma ({{\mathrm {W^{-}}}{+} {\mathrm {c}}})} $ production at 13 TeV measured as a function $ {{| \eta ^{{\mu}} |}} $. The data are presented by filled circles with the statistical (total) uncertainties shown by vertical error bars (light shaded bands). The measurements are compared to the QCD predictions calculated with MCFM at NLO using different PDF sets, presented by symbols of different style. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars represent theoretical uncertainties, which include PDF and scale variation uncertainty. |
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Figure 5-f:
Left: differential cross sections of $ {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})} $ production at 13 TeV measured as a function $ {{| \eta ^{{\mu}} |}} $. The data are presented by filled circles with the statistical (total) uncertainties shown by vertical error bars (light shaded bands). The measurements are compared to the QCD predictions calculated with MCFM at NLO using different PDF sets, presented by symbols of different style. All used PDF sets are evaluated at NLO, except for ATLASepWZ16, which is obtained at NNLO. The error bars represent theoretical uncertainties, which include PDF and scale variation uncertainty. Right: $ {\sigma ({\mathrm {W}}{+} {{\mathrm {D}^{*}(2010)^{\pm}}})} $ differential production cross sections presented as a function of $ {{| \eta ^{{\mu}} |}} $. The data (filled circles) are shown with their total (outer error bars) and statistical (inner error bars) uncertainties and are compared to the predictions of the signal MC generated with MadGraph 5_amc@nlo and using NNPDF3.0nlo to describe the proton structure. PDF uncertainties and scale variations are accounted for and added in quadrature (shaded band). |
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Figure 6:
The s quark distribution (upper) and the strangeness suppression factor (lower) as functions of $x$ at the factorization scale of 1.9 GeV$^2$ (left) and $ {m^2_{{\mathrm {W}}}} $ (right). The results of the current analysis are presented with the fit uncertainties estimated by the Hessian method (hatched band) and using MC replicas (shaded band). |
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Figure 6-a:
The s quark distribution as a function of $x$ at the factorization scale of 1.9 GeV$^2$. The results of the current analysis are presented with the fit uncertainties estimated by the Hessian method (hatched band) and using MC replicas (shaded band). |
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Figure 6-b:
The s quark distribution as a function of $x$ at the factorization scale of $ {m^2_{{\mathrm {W}}}} $. The results of the current analysis are presented with the fit uncertainties estimated by the Hessian method (hatched band) and using MC replicas (shaded band). |
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Figure 6-c:
Strangeness suppression factor as a function of $x$ at the factorization scale of 1.9 GeV$^2$. The results of the current analysis are presented with the fit uncertainties estimated by the Hessian method (hatched band) and using MC replicas (shaded band). |
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Figure 6-d:
Strangeness suppression factor as a function of $x$ at the factorization scale of $ {m^2_{{\mathrm {W}}}} $. The results of the current analysis are presented with the fit uncertainties estimated by the Hessian method (hatched band) and using MC replicas (shaded band). |
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Figure 7:
The strangeness suppression factor as a function of $x$ at the factorization scale of 1.9 GeV$^2$ (left) and $ {m^2_{{\mathrm {W}}}} $ (right). The results of the current analysis (hatched band) are compared to ABMP16nlo (dark shaded band) and ATLASepWZ16nnlo (light shaded band) PDFs. |
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Figure 7-a:
The strangeness suppression factor as a function of $x$ at the factorization scale of 1.9 GeV$^2$. The results of the current analysis (hatched band) are compared to ABMP16nlo (dark shaded band) and ATLASepWZ16nnlo (light shaded band) PDFs. |
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Figure 7-b:
The strangeness suppression factor as a function of $x$ at the factorization scale of $ {m^2_{{\mathrm {W}}}} $. The results of the current analysis (hatched band) are compared to ABMP16nlo (dark shaded band) and ATLASepWZ16nnlo (light shaded band) PDFs. |
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Figure 8:
The distributions of s quarks (upper panel) in the proton and their relative uncertainty (lower panel) as a functions of $x$ at the factorization scale of 1.9 GeV$^2$ (left) and $ {m^2_{{\mathrm {W}}}} $ (right). The result of the current analysis (filled band) is compared to the result of Ref. [11] (dashed band). The PDF uncertainties resulting from the fit are shown. |
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Figure 8-a:
The distributions of s quarks (upper panel) in the proton and their relative uncertainty (lower panel) as a functions of $x$ at the factorization scale of 1.9 GeV$^2$. The result of the current analysis (filled band) is compared to the result of Ref. [11] (dashed band). The PDF uncertainties resulting from the fit are shown. |
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Figure 8-b:
The distributions of s quarks (upper panel) in the proton and their relative uncertainty (lower panel) as a functions of $x$ at the factorization scale of $ {m^2_{{\mathrm {W}}}} $. The result of the current analysis (filled band) is compared to the result of Ref. [11] (dashed band). The PDF uncertainties resulting from the fit are shown. |
Tables | |
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Table 1:
Systematic uncertainties [%] in the inclusive and differential W+c cross section measurement in the fiducial region of the analysis. The total uncertainty corresponds to the sum of the individual contributions in quadrature. The contributions listed in the top part of the table cancel in the ratio $ {\sigma ({{\mathrm {W^+}}{+} {\overline {\mathrm {c}}}})} / {\sigma ({{\mathrm {W^-}}{+} {\mathrm {c}}})} $. |
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Table 2:
Inclusive cross sections of W+c and W+${\mathrm {D}^{*}(2010)^{\pm}}$ production in the fiducial range of the analysis. The correction factor $\mathcal {C}$ accounts for the acceptance and efficiency of the detector. |
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Table 3:
Number of signal events, correction factors $\mathcal {C}$, accounting for the acceptance and efficiency of the detector and the differential cross sections in each $ {{| \eta ^{{\mu}} |}} $ range for W+c (upper), ${{\mathrm {W^+}}{+} {\overline {\mathrm {c}}}}$ (middle) and ${{\mathrm {W^-}}{+} {\mathrm {c}}}$ (lower). |
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Table 4:
The NLO predictions for $ {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})} $, obtained with MCFM [16,17,18]. The uncertainties account for PDF and scale variations. |
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Table 5:
Theoretical predictions for $ {{\mathrm {d}} {\sigma ({{\mathrm {W}}{+} {\mathrm {c}}})}} / {{\mathrm {d}} {{| \eta ^{{\mu}} |}}} $ calculated with MCFM at NLO for different PDF sets. The relative uncertainties due to PDF and scale variations are shown. |
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Table 6:
The partial $ {\chi ^2} $ per number of data points, $n_{\mathrm {dp}}$, and the global $ {\chi ^2} $ per number of degree of freedom, $n_{\mathrm {dof}}$, resulting from the PDF fit. |
Summary |
Associated production of W bosons with charm quarks in proton-proton collisions at ${\sqrt{s}} = $ 13 TeV is measured using the data collected by the CMS experiment in 2016 and corresponding to an integrated luminosity of 35.7 fb$^{-1}$. The W boson is detected via the presence of a high-${p_{\mathrm{T}}}$ muon and missing transverse momentum, suggesting the presence of a neutrino. The charm quark is identified via the full reconstruction of the ${{{\mathrm{D}^{*}(2010)^{\pm}}}} $ meson decaying to ${\mathrm{D^0}} {\pi_{\text{slow}}^{\pm}} \to {\mathrm{K}^{\mp}} \pi^{\pm} {\pi_{\text{slow}}^{\pm}} $. Since in W+c production the W boson and the c quark have opposite charge, contributions from background processes, mainly c quark production from gluon splitting, are largely removed by subtracting the events in which the charges of the W boson and of the ${{{\mathrm{D}^{*}(2010)^{\pm}}}}$ meson have the same sign. The fiducial cross sections are measured in the kinematic range of the muon transverse momentum ${p_{\mathrm{T}}}Muon > $ 26 GeV, pseudorapidity $|{\eta^{\mu}}| < $ 2.4, and transverse momentum of the charm quark ${p_{\mathrm{T}}}^{\mathrm{c}} > $ 5 GeV. The fiducial cross section of ${\mathrm{W}{+}{{\mathrm{D}^{*}(2010)^{\pm}}}}$ production is measured in the kinematic range ${p_{\mathrm{T}}}^{\mu} > $ 26 GeV, $|{\eta^{\mu}}| < $ 2.4, transverse momentum of the ${{{\mathrm{D}^{*}(2010)^{\pm}}}}$ meson ${p_{\mathrm{T}}}^{\mathrm{D}^{*}} > $ 5 GeV and $|{\eta^{\mathrm{D}^*}}| < $ 2.4, and compared to the Monte Carlo prediction. The measurements are performed inclusively and in five bins of $|{\eta^{\mu}}| $. The obtained values for the inclusive fiducial W+c cross section and for the cross section ratio are: ${\sigma(\mathrm{W^{+}}{+}\mathrm{\bar{c}}) }/{\sigma(\mathrm{W^{-}}{+}\mathrm{c})} = $ 0.968 $\pm$ 0.055 (stat)$^{+0.015}_{-0.028}$ (syst). The measurements are in good agreement with the theoretical predictions at next-to-leading order (NLO) for different sets of parton distribution functions (PDF), except for the one using the ATLASepWZ16nnlo PDF. To illustrate the impact of these measurements in the determination of the strange quark distribution in the proton, the data is used in a QCD analysis at NLO together with inclusive DIS measurements and earlier results from CMS on W+c production and the lepton charge asymmetry in W boson production. The strange quark distribution and the strangeness suppression factor $r_{\mathrm{s}}(x,\mu_f^2)=(\mathrm{s}+\mathrm{\bar{s}})/(\mathrm{\bar{u}}+\mathrm{\bar{d}})$ are determined and agree with earlier results obtained in neutrino scattering experiments. The results do not support the hypothesis of an enhanced strange quark contribution in the proton quark sea reported by ATLAS [14]. |
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Compact Muon Solenoid LHC, CERN |