Measurement of missing transverse momentum and search for $H\to \tau\tau$ in $\mathit{pp}$ collisions at $\sqrt{s}=8$ TeV with ATLAS

This thesis discusses two main subjects studied in the context of the ATLAS collaboration: the missing transverse momentum ($E_T^{miss}$) reconstruction and the search for the $H\rightarrow\tau\tau$ process. ATLAS is one of the four main experiments operating at the proton-proton ($\mathit{pp}$) Large Hadron Collider (LHC) at CERN. The high energy and luminosity provided by LHC make possible the observation of rare events like the Higgs production, but also introduce large pile-up effects on detector signals. Substantial part of this thesis is devoted to pile-up suppression in the $E_T^{miss}$ reconstruction. Techniques using either tracks or calorimeter information or a combination of both can improve the $E_T^{miss}$ performance. In particular the resolution, which is the most affected by pile-up, can be restored to values closer to those observed in the absence of pile-up. The Higgs boson is a particle predicted by the mechanism which gives mass to elementary particles in the framework of the Standard Model. It received experimental confirmation in July 2012 when it was observed in the diboson channels $\gamma\gamma$, $ZZ$ and $WW$. The $H\rightarrow\tau\tau$ analysis reported in this thesis is performed with $\mathit{pp}$ collisions at $\sqrt{s} = 8$ TeV corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The cut-based analysis in the semileptonic τlepτhad final state provides an expected signal significance of 1.76$\sigma$. The more sensitive multivariate analysis provides an observed significance of 4.1$\sigma$ combining all $\tau\tau$ decay modes. This is the first direct evidence that the Higgs boson couples to fermions.