Indirect on-shell constraint on the Higgs boson width via signal–background interference in the diphoton channel at CMS

The Standard Model Higgs boson with a mass of 125 GeV is predicted to have a decay width of 4.1 MeV. Direct measurements of the Higgs width using on-shell Higgs boson production are limited by the experimental resolution, which is about 1 GeV in the diphoton and four-lepton final states. This thesis presents a constraint on the Higgs boson width obtained from the diphoton invariant mass distribution in on-shell Higgs decays, using the interference between the amplitudes of the gluon-gluon fusion to Higgs resonance, with subsequent diphoton decay, and the continuum QCD gluon-gluon to diphoton process. The study was performed using proton-proton collision data at a center-of-mass energy of 13 TeV, collected by the CMS experiment during LHC Run 2, corresponding to an integrated luminosity of 137 inverse femtobarns. The observed (expected) upper limit on the Higgs boson width is 92 MeV (138 MeV) at the 95% confidence level.