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| CMS-SMP-15-009 ; CERN-EP-2017-257 | ||
| Measurement of associated Z+charm production in proton-proton collisions at $\sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 6 November 2017 | ||
| Eur. Phys. J. C 78 (2018) 287 | ||
| Abstract: A study of the associated production of a Z boson and a charm quark jet (Z+c), and a comparison to production with a b quark jet (Z+b), in pp collisions at a centre-of-mass energy of 8 TeV are presented. The analysis uses a data sample corresponding to an integrated luminosity of 19.7 fb$^{-1}$, collected with the CMS detector at the CERN LHC. The Z boson candidates are identified through their decays into pairs of electrons or muons. Jets originating from heavy flavour quarks are identified using semileptonic decays of c or b flavoured hadrons and hadronic decays of charm hadrons. The measurements are performed in the kinematic region with two leptons with $p_{\rm T}^{\ell} > $ 20 GeV, ${|\eta^{\ell}|} < $ 2.1, 71 $ < m_{\ell\ell} < $ 111 GeV, and heavy flavour jets with $p_{\rm T}^{{\rm jet}} > $ 25 GeV and ${|\eta^{{\rm jet}}|} < $ 2.5. The Z+c production cross section is measured to be $\sigma({\mathrm{p}}{\mathrm{p}} \rightarrow \mathrm{Z} + \mathrm{c} + \mathrm{X}) {\cal B}(\mathrm{Z} \rightarrow \ell^+\ell^-) = $ 8.8 $\pm$ 0.5 (stat) $\pm$ 0.6 (syst) pb. The ratio of the Z+c and Z+b production cross sections is measured to be $\mathrm{(Z+c)/(Z+b)} = $ 2.0 $\pm$ 0.2 (stat) $\pm$ 0.2 (syst). The Z+c production cross section and the cross section ratio are also measured as a function of the transverse momentum of the Z boson and of the heavy flavour jet. The measurements are compared with theoretical predictions. | ||
| Links: e-print arXiv:1711.02143 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Transverse momentum distribution of the selected muon-inside-a-jet for events with an identified muon among the jet constituents, in the dielectron (left) and dimuon (right) 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. |
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Figure 1-a:
Transverse momentum distribution of the selected muon-inside-a-jet for events with an identified muon among the jet constituents, in the dielectron channel. 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. |
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Figure 1-b:
Transverse momentum distribution of the selected muon-inside-a-jet for events with an identified muon among the jet constituents, in the dimuon channel. 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. |
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Figure 2:
The invariant mass distribution of three-prong secondary vertices for events selected in the D mode, in the dielectron (left) and dimuon (right) channels. The mass assigned to each of the three tracks is explained in the text. The contributions from all processes are estimated with the simulated samples. The two dashed, vertical lines indicate the mass range of the signal region. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. |
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Figure 2-a:
The invariant mass distribution of three-prong secondary vertices for events selected in the D mode, in the dielectron channel. The mass assigned to each of the three tracks is explained in the text. The contributions from all processes are estimated with the simulated samples. The two dashed, vertical lines indicate the mass range of the signal region. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. |
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Figure 2-b:
The invariant mass distribution of three-prong secondary vertices for events selected in the D mode, in the dimuon channel. The mass assigned to each of the three tracks is explained in the text. The contributions from all processes are estimated with the simulated samples. The two dashed, vertical lines indicate the mass range of the signal region. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. |
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Figure 3:
The invariant mass distribution of the three-track system composed of a two-prong secondary vertex and a primary particle for events selected in the $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ mode, in the dielectron (left) and dimuon (right) channels. The mass assigned to each of the three tracks is explained in the text. The contributions from all processes are estimated with the simulated samples. The two dashed, vertical lines mark the mass range of the signal region. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. |
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Figure 3-a:
The invariant mass distribution of the three-track system composed of a two-prong secondary vertex and a primary particle for events selected in the $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ mode, in the dielectron channel. The mass assigned to each of the three tracks is explained in the text. The contributions from all processes are estimated with the simulated samples. The two dashed, vertical lines mark the mass range of the signal region. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. |
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Figure 3-b:
The invariant mass distribution of the three-track system composed of a two-prong secondary vertex and a primary particle for events selected in the $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ mode, in the dimuon channel. The mass assigned to each of the three tracks is explained in the text. The contributions from all processes are estimated with the simulated samples. The two dashed, vertical lines mark the mass range of the signal region. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the statistical uncertainty in the MC simulation. |
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Figure 4:
Transverse momentum distribution of the c-tagged jet (left) and number of reconstructed secondary vertices (right), normalized to unity, in simulated W+c and Z+c samples and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Vertical bars represent the statistical uncertainties. |
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Figure 4-a:
Transverse momentum distribution of the c-tagged jet, normalized to unity, in simulated W+c and Z+c samples and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Vertical bars represent the statistical uncertainties. |
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Figure 4-b:
Number of reconstructed secondary vertices, normalized to unity, in simulated W+c and Z+c samples and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Vertical bars represent the statistical uncertainties. |
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Figure 5:
Distributions of the corrected secondary-vertex mass (left plot) and JP discriminant (D and $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ modes in the middle and right plots), normalized to unity, in simulated W+c and Z+c samples, and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the corrected secondary-vertex mass distribution. Vertical bars represent the statistical uncertainties. |
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Figure 5-a:
Distribution of the JP discriminant for the $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ mode, normalized to unity, in simulated W+c and Z+c samples, and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the corrected secondary-vertex mass distribution. Vertical bars represent the statistical uncertainties. |
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Figure 5-b:
Distributions of the corrected secondary-vertex mass (left plot) and JP discriminant (D and $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ modes in the middle and right plots), normalized to unity, in simulated W+c and Z+c samples, and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the corrected secondary-vertex mass distribution. Vertical bars represent the statistical uncertainties. |
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Figure 5-c:
Distributions of the corrected secondary-vertex mass (left plot) and JP discriminant (D and $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ modes in the middle and right plots), normalized to unity, in simulated W+c and Z+c samples, and in W+c data events. The W+c distributions are presented after the OS-SS subtraction. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the corrected secondary-vertex mass distribution. Vertical bars represent the statistical uncertainties. |
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Figure 6:
Distribution of the corrected secondary-vertex mass normalized to unity from simulated Z+b and $\mathrm{e} \mu $-${\mathrm{t} {}\mathrm{\bar{t}}} $ data (described in the text) events. Vertical bars represent the statistical uncertainties. The last bin of the distribution includes events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV. |
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Figure 7:
Corrected secondary-vertex mass distributions, after background subtraction, in the dielectron (left) and dimuon (right) channels for events selected in the semileptonic mode. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the distribution. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 7-a:
Corrected secondary-vertex mass distributions, after background subtraction, in the dielectron channel for events selected in the semileptonic mode. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the distribution. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 7-b:
Corrected secondary-vertex mass distributions, after background subtraction, in the dimuon channel for events selected in the semileptonic mode. Events with $M_{\rm vertex}^{\rm corr} > $ 8 GeV are included in the last bin of the distribution. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 8:
Background-subtracted distributions of the JP discriminant in the dielectron (left) and dimuon (right) channels for Z+jets events with a $ \mathrm{D}^{\pm} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm $ candidate. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 8-a:
Background-subtracted distributions of the JP discriminant in the dielectron channel for Z+jets events with a $ \mathrm{D}^{\pm} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm $ candidate. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 8-b:
Background-subtracted distributions of the JP discriminant in the dimuon channel for Z+jets events with a $ \mathrm{D}^{\pm} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm $ candidate. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 9:
Background-subtracted distributions of the JP discriminant in the dielectron (left) and dimuon (right) channels for Z+jets events with a $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}\to {{{\mathrm{D^0}}}}\pi ^\pm \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm $ candidate. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 9-a:
Background-subtracted distributions of the JP discriminant in the dielectron channel for Z+jets events with a $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}\to {{{\mathrm{D^0}}}}\pi ^\pm \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm $ candidate. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 9-b:
Background-subtracted distributions of the JP discriminant in the dimuon channel for Z+jets events with a $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}\to {{{\mathrm{D^0}}}}\pi ^\pm \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm $ candidate. The shape of the Z+c and Z+b contributions is estimated as explained in the text. Their normalization is adjusted to the result of the signal extraction fit. Vertical bars on data points represent the statistical uncertainty in the data. The hatched areas represent the sum in quadrature of the statistical uncertainties of the templates describing the two contributions (Z+c from W+c data events and Z+b from simulation). |
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Figure 10:
Contributions to the systematic uncertainty in the measured Z+c cross section and in the (Z+c)/(Z+b) cross section ratio. The first three blocks in the graph show the uncertainties in the Z+c cross section in the three decay modes, semileptonic (SL), D, and $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$, calculated from the combination of the dimuon and dielectron Z boson decay channels. The fourth block shows the systematic uncertainties in the combined (Comb.) Z+c cross section. The last block presents the systematic uncertainty in the (Z+c)/(Z+b) cross section ratio measured in the semileptonic mode. For every block, the height of the hatched bars indicates the contribution from the different sources of systematic uncertainty. The last, solid bar shows their sum in quadrature. |
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Figure 11:
Differential Z+c cross section and (Z+c)/(Z+b) cross section ratio as a function of the transverse momentum of the Z boson (top) and the transverse momentum of the jet (bottom). The combination of the results in the dielectron and dimuon channels is presented. The Z+c differential cross section is shown on the left and the (Z+c)/(Z+b) cross section ratio is on the right. Statistical uncertainties in the data are shown as crosses. The solid rectangles indicate the total (statistical and systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. Symbols showing the theoretical expectations are slightly displaced from the bin centre in the horizontal axis for better visibility of the predictions. |
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Figure 11-a:
Differential Z+c cross section as a function of the transverse momentum of the Z boson. The combination of the results in the dielectron and dimuon channels is presented. The Z+c differential cross section is shown on the left and the (Z+c)/(Z+b) cross section ratio is on the right. Statistical uncertainties in the data are shown as crosses. The solid rectangles indicate the total (statistical and systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. Symbols showing the theoretical expectations are slightly displaced from the bin centre in the horizontal axis for better visibility of the predictions. |
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Figure 11-b:
(Z+c)/(Z+b) cross section ratio as a function of the transverse momentum of the jet. The combination of the results in the dielectron and dimuon channels is presented. The Z+c differential cross section is shown on the left and the (Z+c)/(Z+b) cross section ratio is on the right. Statistical uncertainties in the data are shown as crosses. The solid rectangles indicate the total (statistical and systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. Symbols showing the theoretical expectations are slightly displaced from the bin centre in the horizontal axis for better visibility of the predictions. |
png pdf |
Figure 11-c:
Differential Z+c cross section as a function of the transverse momentum of the Z boson. The combination of the results in the dielectron and dimuon channels is presented. The Z+c differential cross section is shown on the left and the (Z+c)/(Z+b) cross section ratio is on the right. Statistical uncertainties in the data are shown as crosses. The solid rectangles indicate the total (statistical and systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. Symbols showing the theoretical expectations are slightly displaced from the bin centre in the horizontal axis for better visibility of the predictions. |
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Figure 11-d:
(Z+c)/(Z+b) cross section ratio as a function of the transverse momentum of the jet. The combination of the results in the dielectron and dimuon channels is presented. The Z+c differential cross section is shown on the left and the (Z+c)/(Z+b) cross section ratio is on the right. Statistical uncertainties in the data are shown as crosses. The solid rectangles indicate the total (statistical and systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. Symbols showing the theoretical expectations are slightly displaced from the bin centre in the horizontal axis for better visibility of the predictions. |
| Tables | |
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Table 1:
Cross section $ {\sigma (\mathrm{Z+c})}{\cal B}$ and cross section ratio $ {\sigma (\mathrm{Z+c})}/ {\sigma (\mathrm{Z+b})}$ in the three categories of this analysis and in the two Z boson decay channels. The $N^{\rm signal}_{\mathrm{Z+c}}$ and $N^{\rm signal}_{\mathrm{Z+b}}$ are the yields of Z+c and Z+b events, respectively, extracted from the fit to the corrected secondary-vertex mass (semileptonic mode) or JP discriminant (D and $ {{{\mathrm{D}^{\ast}(2010)^{\pm}}}}$ modes) distributions. The factors ${\cal C}$ that correct the selection inefficiencies are also given. They include the relevant branching fraction for the corresponding channel. All uncertainties quoted in the table are statistical, except for those of the measured cross sections and cross section ratios where the first uncertainty is statistical and the second is the estimated systematic uncertainty from the sources discussed in the text. |
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Table 2:
Differential cross section $ {{\mathrm {d}}\sigma (\mathrm{Z+c})/ {\mathrm {d}}{{{p_{\mathrm {T}}} ^{ \mathrm {Z}}}}}{\mathcal B}$ and cross section ratio $({{\mathrm {d}}\sigma (\mathrm{Z+c})/ {\mathrm {d}}{{{p_{\mathrm {T}}} ^{ \mathrm {Z}}}}})/({{\mathrm {d}}\sigma (\mathrm{Z+b})/ {\mathrm {d}}{{{p_{\mathrm {T}}} ^{ \mathrm {Z}}}}})$ in the semileptonic mode and in the two Z boson decay channels. The $N^{\rm signal}_{\mathrm{Z+c}}$ and $N^{\rm signal}_{\mathrm{Z+b}}$ are the yields of Z+c and Z+b events, respectively, extracted from the fit. All uncertainties quoted in the table are statistical, except for those of the measured cross sections and cross section ratios, where the first uncertainty is statistical and the second is the estimated systematic uncertainty from the sources discussed in the text. |
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Table 3:
Differential cross section $ {{\mathrm {d}}\sigma (\mathrm{Z+c})/ {\mathrm {d}}{{p_{\mathrm {T}}} ^{jet}}}{\mathcal B}$ and cross section ratio $({{\mathrm {d}}\sigma (\mathrm{Z+c})/ {\mathrm {d}}{{p_{\mathrm {T}}} ^{jet}}})/({{\mathrm {d}}\sigma (\mathrm{Z+b})/ {\mathrm {d}}{{p_{\mathrm {T}}} ^{jet}}})$ in the semileptonic mode and in the two Z boson decay channels. The $N^{\rm signal}_{\mathrm{Z+c}}$ and $N^{\rm signal}_{\mathrm{Z+b}}$ are the yields of Z+c and Z+b events, respectively, extracted from the fit. All uncertainties quoted in the table are statistical, except for those of the measured cross sections and cross section ratios, where the first uncertainty is statistical and the second is the estimated systematic uncertainty from the sources discussed in the text. |
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Table 4:
Differential Z+c cross section and (Z+c)/(Z+b) cross section ratio. The first column presents the $ {p_{\mathrm {T}}} $ range for each bin. Column 2 presents the cross section and column 3 the ratio. The differential measurements as a function of the transverse momentum of the Z boson (jet with heavy flavour content) are given in the upper (lower) part of the table. The first uncertainty is statistical and the second is the systematic uncertainty arising from the sources discussed in the text. |
| Summary |
|
The associated production of a Z boson with at least one charm quark jet in proton-proton collisions at a centre-of-mass energy of 8 TeV was studied with a data sample corresponding to an integrated luminosity of 19.7 $\pm$ 0.5 fb$^{-1}$. It was compared to the production of a Z boson with at least one b quark jet. Selection of event candidates relies on the identification of semileptonic decays of c or B hadrons with a muon in the final state and through the reconstruction of exclusive decay channels of D and D* mesons. The Z boson is identified through its decay into an $\mathrm{e}^+\mathrm{e}^-$ or $\mu^+\mu^-$ pair. The cross section for the production of a Z boson associated with at least one c quark jet is measured. The measurement is performed in the kinematic region with two leptons with transverse momentum ${p_{\mathrm{T}}}^{\ell} > $ 20 GeV, pseudorapidity $|\eta^{\ell}| < $ 2.1, dilepton invariant mass 71 $ < m_{\ell\ell} < $ 111 GeV and a jet with ${p_{\mathrm{T}}}^{\text{jet}} > $ 25 GeV, $ | {\eta^{\text{jet}}} | < $ 2.5, separated from the leptons of the Z boson candidate by a distance $\Delta R ({\text{jet}},\ell) > $ 0.5. The Z+c production cross sections measured in all the analysis categories are fully consistent, and the combined value is $\sigma(\mathrm{pp \to Z+c + X} ) {\mathcal B}(\mathrm{Z} \rightarrow \ell^+\ell^-) = $ 8.8 $\pm$ 0.5 (stat) $\pm$ 0.6 (syst) pb. This is the first measurement at the LHC of Z+c production in the central pseudorapidity region. The cross section ratio for the production of a Z boson and at least one c and at least one b quark jet is measured in the same kinematic region and is $\sigma(\mathrm{pp \to Z+c + X} )/\sigma(\mathrm{pp \to Z+b + X} ) = $ 2.0 $\pm$ 0.2 (stat) $\pm$ 0.2 (syst). The size of the sample selected in the semileptonic channel allows for the first differential measurements of the Z+c cross section at the LHC. The Z+c cross section and $(Z+c)/(Z+b)$ cross section ratio are measured as a function of the transverse momentum of the Z boson and of the heavy flavour jet. The measurements are in agreement with the leading order predictions from MadGraph and next-to-leading-order predictions from MadGraph5+MCatNLO. Predictions from the MCFM program are lower than the measured Z+c cross section and $(Z+c)/(Z+b)$ cross section ratio, both inclusively and differentially. This difference can be explained by the absence of parton shower development and nonperturbative effects in the MCFM calculation. Measurements in the highest ${p_{\mathrm{T}}}^{\mathrm{Z}}$ (${p_{\mathrm{T}}}^{\text{jet}}$) region analyzed, 60 $ < {p_{\mathrm{T}}}^{\mathrm{Z}} ({p_{\mathrm{T}}}^{\text{jet}}) < $ 200 GeV, would be sensitive to the existence of an intrinsic charm component inside the proton if this IC component were large enough to induce a significant enhancement in the Z+c production cross section. However, our measurements of the Z+c cross section and $(Z+c)/(Z+b)$ cross section ratio are consistent with predictions using PDF sets with no IC component. |
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