Measurement of the associated Z-boson and b-jet production in proton proton collisions at s = 7 TeV with the CMS detector

This thesis presents the measurement of the associated Z boson and b-jet production using proton-proton collisions delivered by the LHC at a centre-of-mass energy of sqrt{s},=,7 TeV and with data recorded by the CMS detector in 2011, corresponding to an integrated luminosity of 2.2,fb^{-1}. The production mechanism of the Z boson in association with b-quarks at the LHC is not yet well understood. A first test consists in the measurement of the corresponding cross section in order to constraint the existing QCD predictions. In addition to this Standard Model motivation, this process is a large background for many searches related to Higgs sectors and predictions beyond the Standard Model and therefore important for investigation. The Z+b-jet cross section with subsequent Z,ightarrow,l^+l^- (where l^+l^-=:e^+e^-,mu^+mu^-) is calculated for events with invariant mass 6025,GeV and |eta| 0.5. The combination of the electron and the muon channels leads to a cross section of sigma_{mathrm{hadron}}(ext{Z}+b,ext{Z}ightarrow ellell) = 5.84 pm 0.08 ext{(stat)} pm 0.72 ext{(syst)} _{-0.55}^{+0.25} ext{(theory),pb}. The shapes of kinematic variables from data are compared with Monte Carlo predictions by the MadGraph event generator, with the parton shower and the hadronisation performed by PYTHIA, and found to be in fair agreement. The absence of the higher order terms in the tree-level MadGraph simulation with massless b-quarks is reflected in some distributions, where small differences are observed. The measured cross section is found to be slightly above the NLO prediction which may indicate that the cross sections of the Higgs boson in association with b-jets could also be higher than the values expected from NLO theory calculations