Calibration of b-jets and search for higgs boson in the channel ttH -> lnub jjb bb in the ATLAS experiment at LHC

B-tagging calibration and search for the Higgs boson in the ttH -> lnub jjb bb channel with ATLAS at LHC. The ATLAS experiment, based at CERN, will use the LHC proton collisions to deepen the Standard Model measurements, and look for possible signs of new physics. One of its main tasks will be the search for the Higgs boson. This thesis first describes the latest b-tagging algorithms, shows the importance of its calibration, and proposes a b-tagging calibration method based on the first data recorded by ATLAS. Then, considering that the Higgs boson may be light (mH < 135 GeV/c2), the ttH channel is studied, with H decaying in a b quark pair. Full simulation and the latest realistic b-tagging methods have been combined for the first time. For an integrated luminosity of 30 fb−1 (3 years of data taking), the S/pB ratio, determining the sensitivity of this analysis, is raised to 4.9. This should allow a non-ambiguous observation of the Higgs boson in this channel.