Recherche de résonances $W' \rightarrow t\bar{b}$ dans le canal lepton plus jets avec le détecteur ATLAS au LHC

The research work carried out during this Ph.D thesis has been performed in the context of the ATLAS experiment, one of the four major LHC experiments, and was primarily dedicated to the search for a new chaged heavy gauge boson, called $W'$ and predicted by many extensions of the Standard Model of particle physics. \\ This manuscript presents a search for $W'$ boson decaying into a top and a bottom quark through an effective coupling approach, in the lepton plus jets final states. This search is performed with 20.3 $\text{fb}^{-1}$ of proton-proton collision data, produced by the LHC with a center-of-mass energy of 8 TeV and collected by the ATLAS detector in 2012. Several scenarios are considered where the $W'$ boson can couple to left-handed ($W'_{L}$) or right-handed ($W'^{R}$) fermions. A multivariate techniques based on boosted decision trees is used to search for an excess of $W'$ signal process in the recorded data. No excess is observed beyond the experimental uncertainties for the data analysed so far. A statistical analysis is performed in order to extract exclusion limits on the mass and the production cross section of the particle. Masses below 1.92, 1.80 and 1.70 TeV are excluded, respectively for $W'_{R}$ and $W'_{L}$ bosons taking into account or not interference effects. These exclusion limits on the production cross section are also reinterpreted in terms of exclusion limits on the effective coupling $g'/g$ of the particle. The lowest exclusion limits observed on the ratio $g'/g$ are equal to 0.20 and 0.16, respectively, for $W'_{R}$ and $W'_{L}$ searches, and are obtained for a $W'_{R/L}$ mass of 0.75 TeV. \\ A search for charged Higgs boson decaying into a top and a bottom quark is presented in this manuscript. This search is based on an effective coupling approach describing a type II Two Doublet Higgs Model. It reuses the analysis infrastructure developed for the $W'$ search and is completed by phenomenological studies related to the production cross section calculation for the process and the characterisation of the resonance width effects affecting the analysis. Preliminary results on the excluded cross section limits $pp\rightarrow H^{+} \rightarrow t\bar{b}$ show that the analysis is not able to exclude a signal a $H^{+}$ boson for all theoretical scenarios considered, due to low production cross sections predicted. \\ In parallel of these activities, several developments have been performed on the fast simulation of the ATLAS calorimeter system in order to overcome its limitations. In particular, a new parametrisation and fast simulation model for the energy response of the calorimeter is presented in this manuscript. This model, still under development, shows encouraging results for simulated single pion event and allows to reduce considerably the memory footprint of the parametrisation compared to previous versions of FastCaloSim, while enabling future reparametrisations to be faster and automated.