Study of associated production of W-Higgs boson with the ATLAS detector

On the 4$^{th}$ of July 2012 both the ATLAS and CMS experiments announced the observation of a new particle in the search for the Standard Model Higgs boson. This observation contributes to the assignment of the Nobel Prize in Physics 2013 to P. Higgs and F. Englert ``for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider'' (http://www.nobelprize.org/nobel\_prizes/physics/). After the discovery, the effort was focused on the study of the properties of the observed particle with data acquired in 2011 and 2012 years. The main goal of this thesis is to present a study of one of the Standard Model (SM) Higgs boson production mechanisms allowed at the Large Hadron Collider (LHC). The presented analysis is focused on the Higgs boson produced in association with a vector boson W and a $H \rightarrow WW^{(*)}$ Higgs boson decay. This study gives direct access to the coupling between the Higgs and the W boson. Due to the more clear signature only a fully leptonic final state is analyzed. An introduction to the SM is presented in chapter 1 while a description of the Higgs boson production at the LHC is reported in chapter 2. In chapter 3 the LHC collider and the ATLAS apparatus are described. Chapter 4 presents the physical object definitions in the ATLAS experiment while in chapters 5 and 6 a description of the analysis, from event selection to statistical treatment, and final results are reported. The thesis focuses on the data acquired by the ATLAS detector in the 2012 but to have a more complete study of the $WH$~$\rightarrow$~$WWW^{(*)}$~$\rightarrow l \nu l \nu l\nu$ channel the analysis described is combined with the analysis performed using the data acquired by the ATLAS detector in the 2011. An excess, although still compatible with a statistical fluctuation, is observed in data, and is carefully investigated. A detailed discussion on the statistical analysis is presented focusing on a Higgs boson mass interval between 110 and 200~ eV. The resulting expected, computed in the absence of a SM Higgs boson, and observed upper limits at 95\% CL are presented together with a study of the significance of the excess. The latter was quantified by computing the local probability $p_0$ for a background fluctuation to produce, in the absence of any signal, a number of events at least as large as the observed one. Also the signal strength and the probability of obtaining the observed number of events from the expected background plus the SM Higgs boson signal (local probability $p_1$) are computed. To complete the study of VH associated production the result of the analysis involving the three leptons final state has been combined with a search for the ZH production with the $H \rightarrow WW^{(*)}$ Higgs boson decay and a four leptons final state. Finally a combination of the main Higgs boson production mechanisms (vector-boson fusion, gluon-gluon fusion and associated production with a W or Z bosons) at the LHC considering only the $H \rightarrow WW^{(*)}$ decay is reported to give to the reader a complete description of the $H \rightarrow WW^{(*)}$ channel. Although the associate production for this decay channel is not yet sensitive enough to the SM Higgs boson production this result combined with other production mechanisms has contributed to a small increase in the significance of the Higgs boson observation, but above all it contributes in setting an upper bound to the Higgs boson coupling to the vector bosons strengthening the SM-like nature of the observed particle. The associate production will play a key role in the Higgs boson properties studies in the next LHC run.