Measurement of associated Z + charm production in proton-proton collisions at s = 8 TeV

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 $\text{TeV}$ are presented. The analysis uses a data sample corresponding to an integrated luminosity of 19.7 ${\text{fb}}^{-1}$ , 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 $p_T^{\ell }>20\text{GeV}$ , $|{\eta }^{\ell }|<2.1$ , $71<m_{\ell \ell }<111\text{GeV}$ , and heavy flavour jets with $p_T^{\text{jet}}>25\text{GeV}$ and $|{\eta }^{\text{jet}}|<2.5$ . The $Z+c$ production cross section is measured to be $\sigma (pp\to Z+c+X)B(Z\to {\ell }^{+}{\ell }^{-})=8.8\pm 0.5\text{(stat)}\pm 0.6\text{(syst)}\text{pb}$ . The ratio of the $Z+c$ and $Z+b$ production cross sections is measured to be $\sigma (pp\to Z+c+X)/\sigma (pp\to Z+b+X)=2.0\pm 0.2\text{(stat)}\pm 0.2\text{(syst)}$ . 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.

Keywords: CMS; Heavy flavour; Physics; SMP; Standard model physics; Vector boson.

Fig. 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

Fig. 2

The invariant mass distribution of three-prong secondary vertices for events selected in the ${\mathrm{D}}^{\pm }$ 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 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

Fig. 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

Fig. 4

Transverse momentum distribution of the $\mathrm{c}$-tagged jet (left) and number of reconstructed secondary vertices (right), normalized to unity, in simulated $\mathrm{W}+\mathrm{c}$ and $\mathrm{Z}+\mathrm{c}$ samples and in $\mathrm{W}+\mathrm{c}$ data events. The $\mathrm{W}+\mathrm{c}$ distributions are presented after the $\mathrm{OS}-\mathrm{SS}$ subtraction. Vertical bars represent the statistical uncertainties

Fig. 5

Distributions of the corrected secondary-vertex mass (left plot) and JP discriminant (${\mathrm{D}}^{\pm }$ and ${\mathrm{D}}^{\ast }{(2010)}^{\pm }$ modes in the middle and right plots), normalized to unity, in simulated $\mathrm{W}+\mathrm{c}$ and $\mathrm{Z}+\mathrm{c}$ samples, and in $\mathrm{W}+\mathrm{c}$ data events. The $\mathrm{W}+\mathrm{c}$ distributions are presented after the $\mathrm{OS}-\mathrm{SS}$ subtraction. Events with $M_{\text{vertex}}^{\text{corr}}>8\text{GeV}$ are included in the last bin of the corrected secondary-vertex mass distribution. Vertical bars represent the statistical uncertainties

Fig. 6

Distribution of the corrected secondary-vertex mass normalized to unity from simulated $\mathrm{Z}+\mathrm{b}$ and $\mathrm{e}\mu$-$\mathrm{t}\overline{\mathrm{t}}$ data (described in the text) events. Vertical bars represent the statistical uncertainties. The last bin of the distribution includes events with $M_{\text{vertex}}^{\text{corr}}>8\text{GeV}$

Fig. 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 $M_{\text{vertex}}^{\text{corr}}>8\text{GeV}$ are included in the last bin of the distribution. The shape of the $\mathrm{Z}+\mathrm{c}$ and $\mathrm{Z}+\mathrm{b}$ 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 ($\mathrm{Z}+\mathrm{c}$ from $\mathrm{W}+\mathrm{c}$ data events and $\mathrm{Z}+\mathrm{b}$ from simulation)

Fig. 8

Background-subtracted distributions of the JP discriminant in the dielectron (left) and dimuon (right) channels for $\mathrm{Z}+\text{jets}$ events with a ${\mathrm{D}}^{\pm }\to {\mathrm{K}}^{\mp }{\pi }^{\pm }{\pi }^{\pm }$ candidate. The shape of the $\mathrm{Z}+\mathrm{c}$ and $\mathrm{Z}+\mathrm{b}$ 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 ($\mathrm{Z}+\mathrm{c}$ from $\mathrm{W}+\mathrm{c}$ data events and $\mathrm{Z}+\mathrm{b}$ from simulation)

Fig. 9

Background-subtracted distributions of the JP discriminant in the dielectron (left) and dimuon (right) channels for $\mathrm{Z}+\text{jets}$ 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 $\mathrm{Z}+\mathrm{c}$ and $\mathrm{Z}+\mathrm{b}$ 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 ($\mathrm{Z}+\mathrm{c}$ from $\mathrm{W}+\mathrm{c}$ data events and $\mathrm{Z}+\mathrm{b}$ from simulation)

Fig. 10

Contributions to the systematic uncertainty in the measured $\mathrm{Z}+\mathrm{c}$ cross section and in the $(\mathrm{Z}+\mathrm{c})/(\mathrm{Z}+\mathrm{b})$ cross section ratio. The first three blocks in the graph show the uncertainties in the $\mathrm{Z}+\mathrm{c}$ cross section in the three decay modes, semileptonic (SL), ${\mathrm{D}}^{\pm }$, and ${\mathrm{D}}^{\ast }{(2010)}^{\pm }$, calculated from the combination of the dimuon and dielectron $\mathrm{Z}$ boson decay channels. The fourth block shows the systematic uncertainties in the combined (Comb.) $\mathrm{Z}+\mathrm{c}$ cross section. The last block presents the systematic uncertainty in the $(\mathrm{Z}+\mathrm{c})/(\mathrm{Z}+\mathrm{b})$ 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

Fig. 11

Differential $\mathrm{Z}+\mathrm{c}$ cross section and $(\mathrm{Z}+\mathrm{c})/(\mathrm{Z}+\mathrm{b})$ cross section ratio as a function of the transverse momentum of the $\mathrm{Z}$ boson (top) and the transverse momentum of the jet (bottom). The combination of the results in the dielectron and dimuon channels is presented. The $\mathrm{Z}+\mathrm{c}$ differential cross section is shown on the left and the $(\mathrm{Z}+\mathrm{c})/(\mathrm{Z}+\mathrm{b})$ 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