Search for resonant $WZ\rightarrow \ell\nu \ell^{\prime}\ell^{\prime}$ production in Proton-Proton Collisions at $\mathbf{\sqrt{s} = 13}~TeV$ with the ATLAS Detector, and Performance studies of the sFCal granularity as a proposed replacement for the ATLAS Forward Calorimeter at HL-LHC.

A search for $WZ\rightarrow \ell\nu \ell^{\prime}\ell^{\prime}$ resonance has been performed with the ATLAS experiment at the CERN Large Hadron Collider. The search uses a data sample corresponding to an integrated luminosity of 36.1 fb$^{-1}$, which was recorded in 2015-2016 at the LHC from proton-proton collisions at a center-of-mass energy of 13 $TeV$. No evidence for a resonance is found and upper limits are set on the production cross section times branching ratio of a heavy particle produced either in quark–anti-quark fusion ($q\bar{q}$) or via vector-boson fusion (VBF). Limits are derived as function of the mass of a charged member of a Heavy Vector Triplet or of the singly charged scalar in the Georgi-Machacek model. This dissertation reports also on the performances, in term of energy and $\phi$ pointing resolutions of the finer readout granularity super forward calorimeter proposed as a replacement of the ATLAS forward calorimeter at the High luminosity LHC (HL-LHC).