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_{ \rm T}^{ \ell} > $ 20 GeV, ${ |\eta^{ \ell}|} < $ 2.1, 71 $ < m_{ \ell\ell} < $ 111 GeV, and heavy flavour jets with $p_{ \rm T}^{ { \rm jet}} > $ 25 GeV and ${ |\eta^{ { \rm jet}}|} < $ 2.5. The Z + c production cross section is measured to be $\sigma({ \mathrm{ p}}{ \mathrm{ p}} \rightarrow \mathrm{ Z} + \mathrm{ c} + \mathrm{ X}) { \cal B}(\mathrm{ Z} \rightarrow \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({ \mathrm{ p}}{ \mathrm{ p}} \rightarrow \mathrm{ Z} + \mathrm{ c} + \mathrm{ X}) / \sigma({ \mathrm{ p}}{ \mathrm{ p}} \rightarrow \mathrm{ Z} + \mathrm{ b} + \mathrm{ 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.
Categories: hep-ex