First measurement of the associated production of a Z boson with b jets at the LHC

The start of the LHC in 2009 opened a new era in particle physics. The collisions, produced at an energy and a rate unequalled, allowing to probe energysectors and rare processes so far inaccessible. In summer 2012 the announcement of the discovery a new boson, presenting the properties of the long-soughtHiggs boson was confirming the last prediction of the particle physics theoretical framework called the standard model. This theory, even though highlypredictive and never defeated experimentally, is known to have limitations andlacks explanation for several physics observations.The production of a Z boson in association with b jets is a process for which different theoretical predictions from the standard model differ, depending on thetreatment of the b jets in the calculation (so-called 4- and 5-flavour schemes).The measurement of the Z plus b jets cross section and the study of its kinematics constitute then a strong QCD test. The Z plus at least two b jets processconstitutes the main background for studies of the newly discovered boson, aswell as for searches in theories beyond the standard model, presenting the samefinal state.In this thesis the cross section measurement is done both for Z plus exactlyone b jet and Z plus at least two b jets, together giving the result for Z plusat least one b jet. The data used, corresponding to an integrated luminosityof 5 fb−1 , were recorded by the CMS detector from proton-proton collisionsat a centre-of-mass energy of 7 TeV at the LHC. The event selection is led bythe care to optimise the signal over background ratio and to select robust wellidentified objects. b-tagging techniques are used for the selection of the b jets.A special attention is given to the background estimation, partially data-drivenwhen possible. To yield a cross section at the particle level, all the efficiencies ofreconstruction and selection and the detector effects are estimated taking intoaccount the migrations in terms of b jet multiplicity. The results are comparedwith several theoretical predictions coming from calculations at leading orderor next-to-leading order and in the two schemes available.Important kinematic variables shown at the reconstructed level are also deconvoluted at the generator level. For this, two different unfolding techniquesare explained and used: the Singular Value Decomposition and the Bayesianunfolding. The analysis selection was implemented in the rivet framework, sothat the differential cross sections can be compared to any generator.