Search for the Higgs boson decaying to four leptons in the ATLAS detector at LHC and studies of muon isolation and energy loss

The central subject of this thesis is the evaluation of the discovery potential of the Higgs boson through its decay into four leptons (electrons and muons) in the ATLAS experiment installed at the Large Hadron Collider (LHC). The LHC was designed to accelerate proton beams at a center of mass energy of 14~TeV and started its physics program with 7~TeV collisions in the beginning of 2010. An inclusive analysis involving all the production modes and an exclusive one aiming at production through vector boson fusion (VBF), studied for the first time in the collaboration, are presented. Both are capable of discovering the Higgs boson after a few years of LHC operation, with integrated luminosities of 30~fb$^{-1}$. The first one covers most part of a Higgs mass window from 130 to 500~GeV. The second one concentrates on masses around 180~GeV and above, exploiting the presence of high energy jets with large separations in pseudo-rapidity to increase the signal over background ratio. An important part of the document is devoted to the reconstruction of muon isolation and energy loss in the ATLAS calorimeters. A software package that optimized the way of treating the energy deposits was developed and tested on simulated data and cosmic-ray events, leading to improvements in the muon momentum resolution and the distinction between muons from heavy quark and vector boson decays. As a consequence of the last result, one of the dominant bac kgrounds to the $H o 4mu$ channel, $Zbar$, is expected to be reduced by almost a factor of two.