CMS-PAS-SMP-19-004

CMS-PAS-SMP-19-004
Measurement of the associated production of a Z boson with charm or bottom quark jets in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
August 2019

Abstract: Ratios of cross sections, $\sigma$ (Z + c jets)/ $\sigma$ (Z + jets), $\sigma$ (Z + b jets)/ $\sigma$ (Z + jets), and $\sigma$ (Z + c jets)/ $\sigma$ (Z + b jets) in the associated production of a Z boson with at least one charm or bottom quark jet are measured in pp collisions at $\sqrt{s}=$ 13 TeV using 35.9 fb $^{-1}$ of data collected by the CMS experiment at the CERN LHC. The fiducial volume of the measurement is defined by jet transverse momentum $p_{\mathrm{T}} >$ 30 GeV and pseudorapidity $\|{\eta}\| <$ 2.4. The Z bosons are selected within the mass range of 71 and 111 GeV requiring leptons (electrons or muons) with $p_{\mathrm{T}} >$ 25 GeV and $\|{\eta}\| <$ 2.4. Results on the inclusive and differential cross section ratios as functions of jet and Z boson transverse momentum are compared with predictions from leading and next-to-leading order perturbative QCD calculations. These are the first of a kind measurements at 13 TeV, and the measurement precision exceeds that of the current theoretical predictions.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRD 102 (2020) 032007. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Comparison of c jet (left), b jet (right) M $_{\text {SV}}$ in data and MC simulation. The correction to account for the difference between the b jets in ${\mathrm{t} {}\mathrm{\bar{t}}}$ and Z + b jets processes is applied to the data distribution in the right panel.
png pdf	Figure 1-a: Comparison of c jet (left), b jet (right) M $_{\text {SV}}$ in data and MC simulation. The correction to account for the difference between the b jets in ${\mathrm{t} {}\mathrm{\bar{t}}}$ and Z + b jets processes is applied to the data distribution in the right panel.
png pdf	Figure 1-b: Comparison of c jet (left), b jet (right) M $_{\text {SV}}$ in data and MC simulation. The correction to account for the difference between the b jets in ${\mathrm{t} {}\mathrm{\bar{t}}}$ and Z + b jets processes is applied to the data distribution in the right panel.
png pdf	Figure 2: Post-fit M $_{\text {SV}}$ distributions for electron (left) and muon (right) channels in the inclusive measurement.
png pdf	Figure 2-a: Post-fit M $_{\text {SV}}$ distributions for electron (left) and muon (right) channels in the inclusive measurement.
png pdf	Figure 2-b: Post-fit M $_{\text {SV}}$ distributions for electron (left) and muon (right) channels in the inclusive measurement.
png pdf	Figure 3: Unfolded, particle-level MG5_aMC and parton-level MCFM R(c/j) cross section ratios versus jet (left) and Z boson (right) transverse momentum. The vertical error bars for the data points are statistical while the hatched band represents the total uncertainties. The predictions are slightly shifted along the x-axis for readability in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 3-a: Unfolded, particle-level MG5_aMC and parton-level MCFM R(c/j) cross section ratios versus jet (left) and Z boson (right) transverse momentum. The vertical error bars for the data points are statistical while the hatched band represents the total uncertainties. The predictions are slightly shifted along the x-axis for readability in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 3-b: Unfolded, particle-level MG5_aMC and parton-level MCFM R(c/j) cross section ratios versus jet (left) and Z boson (right) transverse momentum. The vertical error bars for the data points are statistical while the hatched band represents the total uncertainties. The predictions are slightly shifted along the x-axis for readability in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 4: Unfolded, particle-level MG5_aMC and parton-level MCFM R(b/j) cross section ratio versus jet (left) and Z boson (right) transverse momentum. The predictions are slightly shifted along the x-axis in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 4-a: Unfolded, particle-level MG5_aMC and parton-level MCFM R(b/j) cross section ratio versus jet (left) and Z boson (right) transverse momentum. The predictions are slightly shifted along the x-axis in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 4-b: Unfolded, particle-level MG5_aMC and parton-level MCFM R(b/j) cross section ratio versus jet (left) and Z boson (right) transverse momentum. The predictions are slightly shifted along the x-axis in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 5: Unfolded, particle-level MG5_aMC and parton-level MCFM R(c/b) cross section ratio versus jet (left) and Z boson (right) transverse momentum. The vertical error bars in data are statistical while the hatched band represents the total uncertainties. The predictions are slightly shifted along the x-axis to ease readability in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 5-a: Unfolded, particle-level MG5_aMC and parton-level MCFM R(c/b) cross section ratio versus jet (left) and Z boson (right) transverse momentum. The vertical error bars in data are statistical while the hatched band represents the total uncertainties. The predictions are slightly shifted along the x-axis to ease readability in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.
png pdf	Figure 5-b: Unfolded, particle-level MG5_aMC and parton-level MCFM R(c/b) cross section ratio versus jet (left) and Z boson (right) transverse momentum. The vertical error bars in data are statistical while the hatched band represents the total uncertainties. The predictions are slightly shifted along the x-axis to ease readability in the upper plots and their total PDF and QCD scales (renormalization and factorization) uncertainties are shown as error bands in the ratio plots.

Tables
png pdf	Table 1: Numbers of events after the Z + jets selection in the electron and muon channels. The uncertainties are statistical only.
png pdf	Table 2: Numbers of events after the Z + HF jets selection in the electron and muon channels. The uncertainties are statistical only.
png pdf	Table 3: The SF $_\text {c}$ , SF $_\text {b}$ scale factor fit results for electron, muon, and combined channels in jet ${p_{\mathrm {T}}}$ bins. The first and second uncertainty values correspond to the statistical and systematic contributions, respectively. The fraction of the observed number of Z + c jets and Z + b jets in the total Z + jets events are shown in the parentheses and are derived by applying the scale factors to the corresponding MC events.
png pdf	Table 4: The SF $_\text {c}$ , SF $_\text {b}$ scale factor fit results for electron, muon, and combined channels in Z ${p_{\mathrm {T}}}$ bins. The first and second uncertainty values correspond to the statistical and systematic contributions, respectively. The fraction of the observed number of Z + c jets and Z + b jets in the total Z + jets events with the total uncertainty are shown in the parentheses and are derived by applying the scale factors to the corresponding MC events.
png pdf	Table 5: Systematic uncertainties in the scale factor measurements. The indicated ranges correspond to the various jet and Z ${p_{\mathrm {T}}}$ bins.
png pdf	Table 6: The systematic uncertainties in the cross section ratio measurements. The indicated ranges correspond to the various jet and Z ${p_{\mathrm {T}}}$ bins.
png pdf	Table 7: The cross section ratios measured in the electron and muon channels, along with the combined results. The first and second uncertainty values correspond to the statistical and systematic contributions, respectively.
png pdf	Table 8: The unfolded cross section ratios in the electron and muon channels, along with the combined results. The first and second uncertainty values correspond to the statistical and systematic contributions, respectively.
png pdf	Table 9: The predicted cross section ratios from NLO and LO MG5_aMC. The first and second sets of uncertainties correspond to PDF and QCD scales (renormalization and factorization) variations, respectively.
png pdf	Table 10: The cross section ratios measured in the electron and muon channels together with the combined results in jet ${p_{\mathrm {T}}}$ bins.
png pdf	Table 11: The cross section ratios measured in the electron and muon channels together with the combined results in Z ${p_{\mathrm {T}}}$ bins.

Summary

Ratios of cross sections,

$\sigma$ (Z + c jets )/

$\sigma$ (Z + jets ),

$\sigma$ (Z + b jets )/

$\sigma$ (Z + jets ), and

$\sigma$ (Z + c jets )/

$\sigma$ (Z + b jets ) in the associated production of a Z boson with at least one charm or bottom quark jet have been measured in proton-proton collisions at

$\sqrt s=$ 13 TeV using 35.9 fb

$^{-1}$ of data collected by the CMS experiment at the LHC. The fiducial volume of the measurement is defined by jet transverse momentum

${p_{\mathrm{T}}} >$ 30 GeV and pseudorapidity

$|\eta| <$ 2.4. The Z bosons are selected within the mass range of 71 and 111 GeV requiring leptons (electrons or muons) with

${p_{\mathrm{T}}} >$ 25 GeV and

$|\eta| <$ 2.4. The measured values are

$\sigma$ (Z + c jet )/

$\sigma$ (Z + jets )

$=$ 0.102

$\pm$ 0.002

$\pm$ 0.009,

$\sigma$ (Z + b jet )/

$\sigma$ (Z + jets )

$=$ 0.0633

$\pm$ 0.0004

$\pm$ 0.0015, and

$\sigma$ (Z + c jet )/

$\sigma$ (Z + b jet )

$=$ 1.62

$\pm$ 0.03

$\pm$ 0.15. Results on the inclusive and differential cross section ratios as functions of jet and Z boson transverse momentum are compared to predictions from leading and next-to-leading order perturbative QCD calculations. These are the first of a kind measurements at 13 TeV, and the measurement precision exceeds that of the current theoretical predictions.

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