Search for the Standard Model Higgs boson decaying to $b$ quarks with the CMS experiment

Chapter 1 describes the theory of the Higgs boson in the framework of the SM and gives an overview of the physics of the Higgs boson and of its search performed by the CMS and ATLAS collaborations during the LHC Run 1. Chapter 2 introduces the CMS experiment at the LHC and the event reconstruction used by CMS. My contribution to the event reconstruction is an improvement of the tracking in the core of energetic jets, described in Appendix A. The last section of this Chapter will present the trigger system of CMS, with special attention to the High Level Trigger (HLT). The whole Chapter 3 is devoted to the description of the trigger improvements that I developed during my Ph.D. One of the most important achievements is a fast track- ing that allows to identify pile-up jets for any event accepted by the hardware trigger (∼ 100 kHz). This fast tracking exploits a novel algorithm, the Fast Primary Vertex, that localizes the primary vertex without using tracks, but just hits recorded by the pixel detector and jets reconstructed by the calorimeters. The fast pile-up jet cleaning has been used to improve the resolution of the missing transverse energy and of the total transverse energy evaluated using calorimetric jets. The online b-tagging performance of CMS improved taking advantage of this new fast tracking and other developments. All the improvements have been used to develop new triggers dedicated to the search for the H → bb in the LHC Run 2, especially for ZH → ννbb channel. The rates of these triggers were estimated from simulations using a novel method that I developed, described in Appendix B. This method solved the problem of double counting of the rate due to pile-up collisions. Chapter 4 is dedicated to the search for H → bb decay when the Higgs boson is produced in association with a Z boson decaying to neutrinos. The search has been performed using the 2.32 fb−1 of proton-proton collisions at √s = 13 TeV collected by CMS in 2015. This search is a preliminary result and will be updated with the new data delivered by LHC in 2016 (∼ 20fb−1) and by combining it with the WH → lν and ZH → ll channels.