The Higgs Boson in the $H\to ZZ^{(*)}\to4l$ decay channel with the ATLAS detector at the LHC

This thesis is aiming to present the work contributed by the author in the context of the ATLAS experiment and the search for the Standard Model Higgs boson in the $H\to ZZ^{(∗)}\to 4l$ decay channel using the proton-proton collision data collected during 2011 and 2012. This decay channel is one of the main channels where the discovery of the Higgs boson has been made. It provides a very clean final state signature and the possibility to fully reconstruct the Higgs mass with excellent detector resolution. Moreover, it is the most suitable instrument to study the spin and parity state of the Higgs boson, since one can reconstruct the full decay chain and derive the intrinsic properties of the $H\to ZZ$ decay amplitude from angular and invariant mass distribution of the final state. This thesis is organized as follow. An introduction of the theoretical aspects of the Standard Model and the spontaneous symmetry breaking are presented. The LHC and the ATLAS detector are introduced. For the search for the Higgs boson in the four lepton decay channel, electrons and muons are used. A description of the lepton and event identification and reconstruction techniques is presented. Due to the better identification and reconstruction of the muons with respect to the electrons, the $4\mu$ final state is the most promising one for measuring the Higgs-boson mass. A detailed description of the determination of the muon momentum resolution of the ATLAS detector is provided, reporting results of the two muon reconstruction algorithms performance for both the Muon Spectrometer and the Inner Detector. The observation of a new particle in the search for the Higgs boson in the $H\to ZZ^{(∗)}\to 4l$ decay channel is presented with emphasis on the optimization of the analysis selection and the background estimation. The combination with all other Higgs boson decay channel analyses in ATLAS is also presented. An update of the analysis in the $H\to ZZ^{(∗)}\to 4l$ channel using the full data sample collected in 2011 and 2012 is presented. The description of the spin and parity measurement of the newly observed particle is provided, taking into account various spin and parity hypotheses. The multivariate technique approach based on the Boosted Decision Tree has been presented. Results show that the Standard Model hypothesis, $0^{+}$, is clearly preferred.