Measurement of associated Z + charm production in proton-proton collisions at √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 ^ℓ_T > 20GeV, |^ℓ| < 2.1, 71 < _ℓℓ < 111 GeV, and heavy flavour jets with p^jet_T > 25 GeV and |^jet| < 2.5 . The Z+c production cross section is measured to be (pp→Z+c+)Br(Z→ℓ+ℓ−)=8.8±0.5(stat)±0.6(syst)pb . The ratio of the Z+c and Z+b production cross sections is measured to be (pp→Z+c+)/(pp→Z+b+)=2.0±0.2(stat)±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.