Charming decays of the Higgs, Z, and W bosons: Development and deployment of a new calibration method for charm jet identification

This thesis presents searches for the Higgs boson, Z boson, and W boson decaying into charm quark(s) performed with proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment at the LHC, CERN, corresponding to an integrated luminosity of 138 fb$^{-1}$ and recorded between 2016 and 2018. The searches are carried out using events in which the Higgs/Z/W boson is produced in association with a leptonically-decaying Z or W boson. This thesis also discusses a novel calibration algorithm for charm jet identification that enables maximal use of the available information related to charm jets. The new method is used to correct the entire distribution expected as output when jet flavour identification algorithms are applied to jets of different flavours. The calibrated results improve over traditional efficiency measurements and help enhance the sensitivities of the Higgs, Z, and W searches. This thesis primarily reports on the so-called resolved-jet topology of the Higgs/Z/W boson searches, where the boson candidates are reconstructed using two separate small-radius (AK4) jets. Upon statistically combining the results of the resolved-jet search with a complementary merged-jet approach, the observed (expected) upper limit on the Higgs decay to charm quarks corresponds to 14 (7.6) times the Standard Model expectation at the 95% CL which is the most stringent to date. A significance of 5.7$\sigma$ (5.9$\sigma$) over the background-only prediction is observed(expected) in case of the search for charmed Z boson decays. Only the resolved-jet topology is used in the search for charmed W decays and the observed (expected) significance is 5.6$\sigma$ (5.7$\sigma$) over the background-only prediction. These mark the first observations of charmed decays of the Z and W bosons at a hadron collider experiment.