Discovery Potential for the Standard Model Higgs $\to ZZ^{(*)} \to 4l$ and Contributions to Muon Detection in ATLAS

The discovery or exclusion of the Higgs boson, the Standard Model's only unobserved particle, is one of the main goals of the LHC. The Higgs boson decay $H \to ZZ^{(∗)} \to 4l$ provides high sensitivity for its discovery. Consequently, efficient lepton identification and precise measurement of their properties is of paramount importance. This thesis is divided in two parts: the first is related to the instrumentation of the ATLAS Muon Spectrometer and the second with the sensitivity to the decay $H \to ZZ^{(∗)} \to 4l$. The installation and commissioning of the ATLAS Cathode Strip Chamber system consisting of 32 chambers with a total of 30720 electronic readout channels is presented. Less than 0.06% of the channels were found to be non-responsive and the in situ noise was brought near the design level. Detailed signal reconstruction studies have been performed and the potential to obtain bi-dimensional position reconstruction has been studied using beam test data with photon background. The first medium-size (0.450 × 0.350 m2) bulk-Micromegas detector - a promising candidate for the upgrade of the Muon Spectrometer for the s-LHC - has been studied in a pion beam test. A position resolution of $(39 ± 5) μm$ was estimated for a strip pitch of 250 $\mu m$, with a reconstruction efficiency of 99%. The beam test measurements have been complemented by a simulation study, while the available simulation tools were validated by c omparing micromesh electron transparency measurements with their predictions. The decay mode $H \to ZZ^{(∗)} \to 4l$, where $l=e,\mu$, is a promising channel for the discovery of the SM Higgs boson in the mass range from 130 to 600 GeV. The experimental signature of this channel is two pairs of opposite charge and same flavor leptons. The main backgrounds are the irreducible $ZZ^{(*)}/\gamma^{(*)} \to 4l$ and the reducibles $Zb\bar{b}\to 4l$ and $t\bar{t}\to W^{+}bW^{-}\bar{b}\to 4l$. The main handles to suppress the reducible backgrounds are lepton isolation and vertexing. For the estimation of the $ZQ\overline{Q}$ and $ZZ$ backgrounds a data-driven measurement has been developed: Initially, the background contribution in a control region is measured and then it is extrapolated to the signal region. For an integrated luminosity of $30 fb^{-1}$ at $\sqrt{s}=14TeV$, a Higgs boson in the mass range from 130 to 500 GeV could be discovered by the $H\to 4l$ channel alone. An exception is the region around 165 GeV, where a discovery significance of 4$\sigma$ is expected. The highest sensitivity for the $H\to ZZ\to 4l$ channel is observed around $M_{H}= 150$ GeV and in the mass region $200 < M_H < 400GeV$, where 5 fb$^{-1}$ would suffice for a discovery. Nevertheless, with $1 fb^{-1}$ at $\sqrt{s}=10 TeV$ it will be possible to exclude the Standard Model Higgs in the mass region between 185 and 235 GeV at 95% confidence level.