Probing top quark and Higgs boson production in multi-jet events at the LHC with the ATLAS detector

The ATLAS detector is a general-purpose detector located at the Large Hadron Collider (LHC). It aims at the discovery of new physics phenomena and improving our knowledge of the Standard Model (SM). The LHC is an hadron collider designed to provide proton proton collisions at 14 TeV center-of-mass energy and 1034cm-2s-1 peak luminosity. The ATLAS experiment collected 4.7 fb-1 pp collisions delivered by the LHC at a centre-of-mass energy of 7 TeV in 2011 and about 20 fb-1pp collisions in 2012 at a centre-of-mass energy of 8 TeV. The thesis reports the measurement of top-anti top cross section performed using the 4.7 fb-1 data recorded by the ATLAS detector during the 2011 data-taking campaign with a center-of-mass energy of 7 TeV. The top-anti top pair decay mode investigated is the all-hadronic, where both the W bosons, produced in the top quark disintegration, decay in a quarks pair. The all-hadronic decay mode has the advantage of a high branching fraction, about 46%, but on the other hand it suffers from an irreducible QCD multi-jet background. The main background is coming from the QCD processes with at least six quaks or gluons in the final state. This processes is estimated using dedicated data-driven techniques. The measured total cross section is 168±12 (stat.) +60/-57(syst.) ± 7 (lum.) pb. The thesis presents also the first search for the Higgs boson produced in the association with a top quark pair, where the Higgs boson decays in a bottom quark pair and the top-anti top pair in the fully hadronic mode. The analysis is performed on the same data sample used in the top-anti top cross section measurement. The main backgrounds of the ttH signal come from the tt + jets production and from the QCD multi-jet process. The former is estimated using the Monte-Carlo simulation, instead the latter using a dedicated data-driven technique. The aim of this study it is to show the power of fully hadronic channel looking at the signal-over-square root of background achieved in the signal regions and making a comparison with the public results presented by the ATLAS in the semileptonic channel.