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| CMS-PAS-SMP-15-009 | ||
| Measurement of associated Z + charm production in pp collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| July 2016 | ||
| Abstract: A study of the associated production of a Z boson and one charm-quark jet (Z+c) in pp collisions at a center-of-mass energy of 8 TeV is presented. The analysis is conducted with a data sample corresponding to an integrated luminosity of 19.7 fb$^{-1}$, collected by the CMS detector at the CERN LHC. The Z-boson candidates are identified through their decay into a pair 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 for heavy flavour jets in the kinematic region $p_{\rm T}^{{\rm jet}} > $ 25 GeV, ${\mid\eta^{ {\rm jet}}\mid} < $ 2.5. The Z+c production cross section is measured to be $\sigma({\rm pp} \rightarrow {\rm Z+c + X}) =$ 8.6 $\pm$ 0.5 (stat) $\pm$ 0.7 (syst) pb. The relative production of a Z boson and at least one c- or b-quark jet is analysed in terms of cross sections ratio. The ratio of the Z+c and Z+b production cross sections is measured to be $\sigma({\rm pp} \rightarrow {\rm Z+c + X})/\sigma({\rm pp} \rightarrow {\rm Z+b + X}) =$ 2.0 $\pm$ 0.2 (stat) $\pm$ 0.2 (syst). The Z+c production cross section and the cross sections ratio are also measured differentially as a function of transverse momentum of the Z boson and of the heavy flavour jet. Measurements are compared with several theoretical predictions. | ||
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These preliminary results are superseded in this paper, EPJC 78 (2018) 287. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1-a:
Transverse momentum distribution of the c-tagged jet (a) and number of reconstructed secondary vertices (b) in simulated W+c and Z+c samples, and in W+c data events. Events with no reconstructed IVF vertices have at least one reconstructed vertex with the SSV vertex algorithm. The W+c distributions are presented after $ {\mathrm {OS}-\mathrm {SS}}$ subtraction. |
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Figure 1-b:
Transverse momentum distribution of the c-tagged jet (a) and number of reconstructed secondary vertices (b) in simulated W+c and Z+c samples, and in W+c data events. Events with no reconstructed IVF vertices have at least one reconstructed vertex with the SSV vertex algorithm. The W+c distributions are presented after $ {\mathrm {OS}-\mathrm {SS}}$ subtraction. |
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Figure 2-a:
Distributions of the corrected secondary vertex mass (a) and JP discriminant ($ {\mathrm {D^{\pm }}}$ mode in the (b) plot and $ {{\mathrm {D}^{\ast \pm }(2010)}}$ mode in the (c) plot), normalized to unity, in simulated W+c and Z+c, and in W+c data events. The W+c distributions are presented after $ {\mathrm {OS}-\mathrm {SS}}$ subtraction. |
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Figure 2-b:
Distributions of the corrected secondary vertex mass (a) and JP discriminant ($ {\mathrm {D^{\pm }}}$ mode in the (b) plot and $ {{\mathrm {D}^{\ast \pm }(2010)}}$ mode in the (c) plot), normalized to unity, in simulated W+c and Z+c, and in W+c data events. The W+c distributions are presented after $ {\mathrm {OS}-\mathrm {SS}}$ subtraction. |
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Figure 2-c:
Distributions of the corrected secondary vertex mass (a) and JP discriminant ($ {\mathrm {D^{\pm }}}$ mode in the (b) plot and $ {{\mathrm {D}^{\ast \pm }(2010)}}$ mode in the (c) plot), normalized to unity, in simulated W+c and Z+c, and in W+c data events. The W+c distributions are presented after $ {\mathrm {OS}-\mathrm {SS}}$ subtraction. |
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Figure 3:
Distributions of the corrected secondary vertex mass normalized to unity from simulated Z+b and data $\mathrm{ e } \mu $-${\mathrm{ t } \mathrm{ \bar{t} } } $ events. |
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Figure 4-a:
Corrected secondary vertex mass distributions in the dielectron (a) and dimuon (b) channels. 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. |
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Figure 4-b:
Corrected secondary vertex mass distributions in the dielectron (a) and dimuon (b) channels. 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. |
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Figure 5-a:
Distributions of the JP discriminant in the dielectron (a) and dimuon (b) channels for $ {\mathrm{ Z } + \text {jets}}$ events with a $ {\mathrm {D^{\pm }}}\to {\mathrm {D^0}}\pi ^\pm _{\rm s} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm _{\rm s}$ 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. |
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Figure 5-b:
Distributions of the JP discriminant in the dielectron (a) and dimuon (b) channels for $ {\mathrm{ Z } + \text {jets}}$ events with a $ {\mathrm {D^{\pm }}}\to {\mathrm {D^0}}\pi ^\pm _{\rm s} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm _{\rm s}$ 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. |
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Figure 6-a:
Distributions of the JP discriminant in the dielectron (a) and dimuon (b) channels for $ {\mathrm{ Z } + \text {jets}}$ events with a $ {{\mathrm {D}^{\ast \pm }(2010)}}\to {\mathrm {D^0}}\pi ^\pm _{\rm s} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm _{\rm s}$ 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. |
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Figure 6-b:
Distributions of the JP discriminant in the dielectron (a) and dimuon (b) channels for $ {\mathrm{ Z } + \text {jets}}$ events with a $ {{\mathrm {D}^{\ast \pm }(2010)}}\to {\mathrm {D^0}}\pi ^\pm _{\rm s} \to \mathrm{K} ^\mp \pi ^\pm \pi ^\pm _{\rm s}$ 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. |
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Figure 7:
Contributions to the systematic uncertainty in the measured Z+c cross section and in the $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio. The first three bins in the graphic show the uncertainties in the Z+c cross section in the three decay modes, semileptonic , $ {\mathrm {D^{\pm }}}$, and $ {{\mathrm {D}^{\ast \pm }(2010)}}$, calculated from the combination in the dimuon and dielectron $\mathrm{ Z } $-boson decay channels. The fourth bin shows the systematic uncertainties in the combined Z+c cross section. The last bin presents the systematic uncertainty in the $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio measured in the semileptonic mode. The uncertainty from every source is added on top of the already displayed ones according to its contribution to the total uncertainty. |
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Figure 8-a:
Differential Z+c cross section and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio as a function of the transverse momentum of the $\mathrm{ Z } $ boson (a,b) and the transverse momentum of the jet (c,d). The combination of the results in the dielectron and dimuon channels are shown. The Z+c differential cross section is shown in the (a,c) plots and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio is shown in the (b,d) plots. Statistical uncertainties in the data are shown as error bars. The solid rectangles indicate the total (statistical plus systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. |
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Figure 8-b:
Differential Z+c cross section and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio as a function of the transverse momentum of the $\mathrm{ Z } $ boson (a,b) and the transverse momentum of the jet (c,d). The combination of the results in the dielectron and dimuon channels are shown. The Z+c differential cross section is shown in the (a,c) plots and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio is shown in the (b,d) plots. Statistical uncertainties in the data are shown as error bars. The solid rectangles indicate the total (statistical plus systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. |
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Figure 8-c:
Differential Z+c cross section and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio as a function of the transverse momentum of the $\mathrm{ Z } $ boson (a,b) and the transverse momentum of the jet (c,d). The combination of the results in the dielectron and dimuon channels are shown. The Z+c differential cross section is shown in the (a,c) plots and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio is shown in the (b,d) plots. Statistical uncertainties in the data are shown as error bars. The solid rectangles indicate the total (statistical plus systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. |
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Figure 8-d:
Differential Z+c cross section and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio as a function of the transverse momentum of the $\mathrm{ Z } $ boson (a,b) and the transverse momentum of the jet (c,d). The combination of the results in the dielectron and dimuon channels are shown. The Z+c differential cross section is shown in the (a,c) plots and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio is shown in the (b,d) plots. Statistical uncertainties in the data are shown as error bars. The solid rectangles indicate the total (statistical plus systematic) experimental uncertainty. Statistical and systematic uncertainties in the theoretical predictions are shown added in quadrature. |
| Tables | |
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Table 1:
Cross section $ {\sigma ( \mathrm{Z+c})}$, and cross sections ratio $ {\sigma ( \mathrm{Z+c})}/ {\sigma ( {\mathrm{ Z } + \mathrm{ b } })}$ in the three categories of this analysis and in the two Z-boson decay channels. $N^{\rm signal}_{ \mathrm{Z+c}}$ and $N^{\rm signal}_{ {\mathrm{ Z } + \mathrm{ b } }}$ are the yields of Z+c and Z+b events extracted from the fit to the corrected secondary vertex mass (semileptonic mode) or JP discriminant ($ {\mathrm {D^{\pm }}}$ and $ {{\mathrm {D}^{\ast \pm }(2010)}}$ 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 the measured cross sections and cross sections ratio where the first uncertainty is statistical and the second one is the estimated systematic uncertainty from the sources discussed in the text. |
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Table 2:
Differential cross section $ {d\sigma ( \mathrm{Z+c})/ {\mathrm {d}}{ {p_{\mathrm {T}}} ^{\mathrm{ Z } }} }$, and cross section ratio $( {d\sigma ( \mathrm{Z+c})/ {\mathrm {d}}{ {p_{\mathrm {T}}} ^{\mathrm{ Z } }} })/( {d\sigma ( {\mathrm{ Z } + \mathrm{ b } })/ {\mathrm {d}}{ {p_{\mathrm {T}}} ^{\mathrm{ Z } }} })$ in the semileptonic mode and in the two Z-boson decay channels. $N^{\rm signal}_{ \mathrm{Z+c}}$ and $N^{\rm signal}_{ {\mathrm{ Z } + \mathrm{ b } }}$ are the yields of Z+c and Z+b events extracted from the fit. All uncertainties quoted in the table are statistical except for the measured cross sections and cross sections ratio where the first uncertainty is statistical and the second one is the estimated systematic uncertainty from the sources discussed in the text. |
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Table 3:
Differential cross section $ {d\sigma ( \mathrm{Z+c})/ {\mathrm {d}}{ {p_{\mathrm {T}}} ^{\text {jet}}} }$, and cross section ratio $( {d\sigma ( \mathrm{Z+c})/ {\mathrm {d}}{ {p_{\mathrm {T}}} ^{\text {jet}}} })/( {d\sigma ( {\mathrm{ Z } + \mathrm{ b } })/ {\mathrm {d}}{ {p_{\mathrm {T}}} ^{\text {jet}}} })$ in the semileptonic mode and in the two Z-boson decay channels. $N^{\rm signal}_{ \mathrm{Z+c}}$ and $N^{\rm signal}_{ {\mathrm{ Z } + \mathrm{ b } }}$ are the yields of Z+c and Z+b events extracted from the fit. All uncertainties quoted in the table are statistical except for the measured cross sections and cross sections ratio where the first uncertainty is statistical and the second one is the estimated systematic uncertainty from the sources discussed in the text. |
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Table 4:
Differential Z+c cross section and $ \mathrm{Z+c}/ {\mathrm{ Z } + \mathrm{ b } }$ cross sections ratio. The first block presents the differential measurements as a function of the transverse momentum of the Z boson. The second block shows the cross section and ratio as a function of the transverse momentum of the jet with heavy flavour content. The first uncertainty is the statistical and the second one is the systematic uncertainty arising from the sources discussed in the text. |
| Summary |
| The cross section of the production of a Z boson associated with at least one jet originated by a c-quark in pp collisions at a center-of-mass energy of 8 TeV is measured with a data sample corresponding to an integrated luminosity of 19.7 $\pm$ 0.5 fb$^{-1}$. The cross sections ratio of the production of a Z boson and at least one c- or b-quark jet is also determined. The measurements are performed in the kinematic region with two leptons with $p_{\mathrm{T}}^{\ell} >$ 20 GeV, pseudorapidity $|\eta^{\ell}| < $ 2.1, and dilepton invariant mass 71 $ < m_{\ell\ell} < $ 111 GeV, and a HF-quark jet with $p_{\mathrm{T}}^{\text jet} > $ 25 GeV, $ | \eta^{\text{jet}} | < $ 2.5, and separated from the leptons from the Z-boson candidate by a distance $\Delta R (\text{jet},\ell) >$ 0.5. The measured Z+c production cross section is $\sigma( \mathrm{ pp \to Zc }) =$ 8.6 $\pm$ 0.5 (stat) $\pm$ 0.7 (syst) pb, and the cross sections ratio is $\sigma(\mathrm{ pp \to Zc })/\sigma(\mathrm{ pp \to Zb }) =$ 2.0 $\pm$ 0.2 (stat) $\pm$ 0.2 (syst). Both, the Z+c production cross section and the cross sections ratio are measured inclusively and differentially as a function of transverse momentum of the Z boson and of the heavy flavour jet. The measurements are in agreement with the LO predictions from MadGraph and NLO predictions from aMCatNLO. Predictions from the MCFM program are lower than the measured Z+c cross section, both inclusive and differentially. A better description is reached in terms of the Z+c/Z+b cross sections ratio. |
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Compact Muon Solenoid LHC, CERN |
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