Measurement of associated Z + charm production in proton-proton collisions at $\sqrt{s} = $ 8 TeV

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 $p_T^l > 20$ GeV, $|\eta_l| < 2.1$, $71 < m_{\ell\ell} < 111$ GeV, and heavy flavour jets with $p_T^{\text{jet}} > 25$ GeV and $|\eta_\text{jet}| < 2.5$. The Z+c production cross section is measured to $\sigma(pp \to Z + c + X) \mathcal{B}(Z \to \ell^+\ell^-) = 8.8 \pm 0.5$ (stat) $\pm 0.6$ (syst) pb. The ratio of the Z+c and Z+b production cross sections is measured to be $\sigma(pp \to Z + c + X)/\sigma(pp \to Z + b + X) = 2.0 \pm 0.2$ (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.