Search for Higgs bosons in the final state ZZ(*)-->llqq with the ATLAS detector at the LHC

This dissertation presents the search for Higgs-like bosons, in the decay mode H->ZZ(*)->llqq, using data collected with the ATLAS detector, at the CERN Large Hadron Collider (LHC). Despite the fact that this channel is mostly sensitive in the Higgs boson mass range above 200 GeV, before the discovery of the Standard Model Higgs boson the search was extended over the mass interval 120-180 GeV. Reasonable sensitivity is observed above 140 GeV. The search uses 4.7 ifb of proton–proton collision data, at a centre-of-mass energy of 7 TeV. After the discovery of the Higgs boson, the search focused towards the detection of additional, heavy Higgs bosons, in the mass range 200-1000 GeV. Proton–proton collision data, at a centre-of-mass energy of 8 TeV, are used, corresponding to an integrated luminosity of 20.3 ifb. Since no significant excess of events is observed over the Standard Model prediction, upper limits are set, at 95% confidence level (CL), on the production cross-section of a heavy Higgs boson times the branching ratio of its decay mode to a pair of Z bosons. Finally, exclusion limits are estimated by the combined statistical interpretation of the results of the H->ZZ: ZZ->llqq, ZZ->llll, ZZ->llvv and ZZ->vvqq search channels.