Test of the Standard Model in the $H \to ZZ^{*} \to 4\ell$ decay channel with the Effective Lagrangian framework and construction and performance of the Micromegas chambers for the ATLAS New Small Wheel upgrade

The first measurements performed during the Run1 at the Large Hadron Collider (LHC) allowed for the discovery of the Higgs boson in July 2012. Since that moment, one of the main goal of the ATLAS experiment has been to study the Higgs boson properties. An ambitious upgrade programme of the LHC is planned and one of the most important upgrade in the forward region of the muon spectrometer of the ATLAS experiment is the New Small Wheel project. It consists of the replacement of the innermost muon station in the endcaps with a completely new detectors technology: the Micromegas (MM) and sTGC. The first part of this thesis describes the Micromegas chambers and my work on the construction of the SM1 module type. The assembly procedure together with the validation of the detector performance are presented, with a focus on the high voltage stability studies. The second part of this thesis is focused on the analysis work performed on the $H\to ZZ^* \to 4\ell$ Higgs boson decay channel. It is one of the best channel to perform precision measurements of the Higgs boson properties. This thesis describes my work on the Higgs boson fiducial cross section measurements, both inclusively and differentially, performed with full Run 2 dataset, together with their interpretation in different theoretical frameworks: Pseudo-Observables and $\kappa$-framework to constrain the Yukawa coupling with the charm quark. The Higgs boson CP properties have been also tested, in particular in the VBF production mechanism, looking for possible CP-violation effects in the Higgs sector in an Effective Field Theory (EFT) framework.