Measurement of associated Z + charm production in pp collisions at $\sqrt{s} = 8~\mathrm{TeV}$

A study of the associated production of a Z boson and one charm-quark jet (${\rm Z + c}$) 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 $19.7~{\rm fb}^{-1}$, 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 ${\rm Z + c}$ production cross section is measured to be $\sigma({\rm pp} \rightarrow {\rm Z + c + X}) = 8.6 \pm 0.5~({\mathrm {stat.}}) \pm 0.7~({\mathrm {syst.}})~{\mathrm {pb}}$. 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 ${\rm Z + c}$ and ${\rm Z + b}$ production cross sections is measured to be $\sigma({\rm pp} \rightarrow {\rm Z + c + X})/\sigma({\rm pp} \rightarrow {\rm Z+b + X}) = 2.0 \pm 0.2~({\mathrm {stat.}}) \pm 0.2~({\mathrm {syst.}})$. The ${\rm Z + c}$ 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.