Search for the Higgs boson decaying to two photons and produced in association with a pair of top quarks in the CMS experiment

In this thesis, the measurement of the Higgs boson properties in the diphoton decay channel with the CMS experiment at the Large Hadron Collider is presented. The focus of this work is the $t\bar{t}H$ production mode, as it is the only direct access to the top quark Yukawa coupling, a fundamental parameter of the Standard Model. $t\bar{t}H$ is a very rare process, two orders of magnitude smaller than the dominant Higgs boson production by gluon fusion. At 13 TeV, $t\bar{t}H$ production is about 4 times larger than at 8 TeV. This thesis takes over the studies performed at 8 TeV, where the statistics was not enough for an observation of $t\bar{t}H$. Despite a very small branching ratio (only about 0.2\%), the two photons decay channel of the Higgs boson is very promising, because of its excellent mass resolution (about 1\%). Moreover, its signature in the detector is very clear. The diphoton decay channel is also of particular interest as it is the only channel allowing the study of all production modes: gluon fusion, vector boson fusion, associated productions with a W or a Z bosons, or with a top quark pair. The document starts with a theoretical introduction about the Standard Model and Higgs boson physics at LHC, followed by a description of the CMS detector. To achieve an excellent mass resolution in the $H \to \gamma \gamma$ channel, the electromagnetic calorimeter has to be calibrated. The laser monitoring system plays an important role in the calibration chain and it is described in details. On the long term, the laser monitoring system will have to be upgraded as level of radiation influences its electronics. I present my work on the possible upgrade of the laser monitoring system, along with the study of its possible precision. \hgg inclusive analysis had several iterations for conferences in 2016 and 2017. The strategy for 2017 is described in this document. An event classification is used to maximize the signal significance and to study specific Higgs boson production modes. My contributions to the $H \to \gamma \gamma$ analysis are primary vertex identification, photon identification and the study of the $t\bar{t}H$ production mode. Each contribution is described in details in dedicated chapters. The $t\bar{t}H, H \to \gamma \gamma$ analysis is shown for two iterations in 2016 and 2017, with the emphasis on improvements in 2017 analysis. Finally, the results of the inclusive and $t\bar{t}H, H \to \gamma \gamma$ analysis, using the full 2016 dataset corresponding to an integrated luminosity of 35.9 $fb^{-1}$, are shown.