Search for Higgs boson decays to charm quarks at high transverse momenta using deep learning techniques with the CMS experiment

In order to better understand the Standard Model (SM) Higgs boson (H), it is crucial to measure its properties and probe the nature of Yukawa couplings. In particular, the H couplings to second-generation fermions and, specifically, charm quarks are yet to be proven. This thesis reports on a search for the H decaying to a charm quark–antiquark pair (cc) in a boosted topology at high transverse momentum, inclusive in production mode but targeting H produced via the gluon fusion process. The search is performed using proton–proton collision data collected at $\sqrt{s}$ = 13 TeV by the CMS experiment at the LHC, corresponding to an integrated luminosity of $138 fb^{-1}$. Boosted decay products are reconstructed as a single large-radius jet and identified using a deep neural network charm tagging techniques developed in the context of this search and described in detail in this thesis. The analysis approach is validated by measuring the Z → cc decay process, which is observed in association with jets for the first time at the LHC with a signal strength of $1.00^{+0.14}_{−0.12}(exp) ± 0.07(theo) ± 0.06(stat)$, defined as the ratio of the observed process rate to the SM expectation. Subsequently, an observed (expected) upper limit on the product of the H production cross section and branching fraction to charm quarks, σ(H)B(H → cc) of 47 (39) times the SM expectation is set at the 95% confidence level. The work presented in this thesis opens a new avenue for the study of the H boson coupling to charm quarks and will be a key element in reaching the evidence threshold for the H → cc decay at the HL-LHC in the future.