Search of a graviscalar particle of the Randall-Sundrum model with the CMS experiment at LHC

From the introduction: In the past ten years the four LEP experiments provided the best way to prove theoretical predictions in the framework of the Standard Model. All the collected data have been well described by theory and no room has been left for signal leading to new physics. However physicists never give up and they started to think about new models involving new concepts that can reproduce the experimental data and enlarge our understanding of the ultimate nature of fundamental interactions. Among these theoretical models, one of the most amazing new concept is, in my opinion, the introduction of extra spatial dimensions. These extra dimensions could explain some theoretical black points not yet fully understood, and provide a set of clear signatures that may be investigated at the greatest particle accelerator under construction: the Large Hadron Collider (LHC). LHC will provide the best possibilities to study any kind of signals that may lead to the discovery of new physiscs. In this thesis the candidate considers one of the most interesting consequences of the extra dimensions models: the existence of a new scalar particle. This particle (φ) interacts with all the other Standard Model particles and can mix with the Higgs boson, making much more interesting the study of the Higgs sector at LHC. In order to exploit in the best way the performance of the Large Hadron Collider, a detector, with extremely good reconstruction capabilities of the final states of hadron interactions, is needed. The Compact Muon Solenoid (CMS) has been designed for this purpose. The aim of this thesis is to understand if CMS will be sensitive to the decay of the new scalar particle into a pair of Higgs boson (φ →hh). In the first chapter of this thesis the theoretical framework of the extra dimensions is presented. After a short review of the principal ideas of the various models, the Randall-Sundrum one is presented with more details. The search for the new scalar particle has been studied, indeed, in this specific model. In the second chapter a schematic view of the Large Hadron collider is presented, with the main CMS subdetector: the tracker, the calorimeters, the magnet and the muon chambers. The use of the tracker to tag b and τ jets is very important for the analysis presented in this thesis. For this reason the subdetector is described in detail in the third chapter. At the end of the chapter a measurement of the dead time of the electronics of the tracker modules is presented. The candidate has performed this measurements at the CERN laboratories in collaboration with Jacopo Bernardini, Laurent Mirabito and Roger Grabit. The fourth chapter describes the CMS trigger strategies. The candidate has developped and implemented the isolation algorithm for the hadronic decays of the τ lepton, used as a filter in the High Level Trigger chain. This selection is of great importance in the study of the Higgs sector in CMS. In the last chapter the analysis of the process: gg → φ → hh is described with three different final states: • hh → ττ bb • hh → γγ bb • hh → bb bb. The candidate has studied in detail the final state with two τ leptons. The effects of possible systematical uncertainties on the knowledge of the background rate has been properly considered.