Search for the Standard Model Higgs decaying into tau--jet final state with the ATLAS experiment at the LHC

The search for the Higgs boson is one of the primary goal of the ATLAS experiment at the LHC. The mass of the Higgs boson is not predicted by the Standard Model theory of elementary particles. Direct searches performed at the LEP experiments have excluded the mass region below 114 GeV. Electroweak fits to the Standard Model parameters prefer a light Higgs boson, with a mass below 185 GeV at 95% C.L. For the low mass region, above 114 GeV, the most promising channel for the discovery is the production of the Higgs via Vector Boson Fusion and decaying into tau leptons. In a previous analysis only the fully leptonic and the semi-leptonic decay modes of the tau pair has been studied. In this thesis work, for the first time, the fully hadronic final state is investigated. Analysis has been performed with Monte Carlo data produced with the full simulation of the ATLAS detector. Being a fully hadronic final state, the biggest challenge is to estimate the background form multijet QCD events. Having a cross section many order of magnitude higher than the signal, its estimate cannot be obtained from Monte Carlo data in a direct way due to the lack of statistics. To tackle this problem, a bigger sample of fast simulated QCD events has been used and a cut factorization procedure has been developed to get the final estimate. Promising results have been obtained which are compatible with the existing ones for the fully leptonic and the semi-leptonic decay modes in terms of signal efficiency and background rejection, the latter with the exclusion of QCD multijet that was not considered for these two final states.