Search for Long-Lived Particles Decaying to Photon Pairs in Rare Higgs Boson Decays using 13 TeV Proton-Proton Collisions at the Compact Muon Solenoid Detector

This thesis describes the search for rare Standard Model (SM) Higgs boson decays to long-lived scalar bosons ($\Phi$) decaying to photon pairs using data collected by the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider. The data were collected from 2016, 2017, and 2018 proton-proton collisions at center-of-mass energy of $\sqrt{s}=13\,TeV$ and correspond to an integrated luminosity of $138\,{fb^{-1}}$. Events are triggered using the leptonic decays of the Z boson produced in association with the SM Higgs boson. Although the CMS electromagnetic calorimeter has limited vertex reconstruction capabilities for photons, this analysis utilizes a novel reconstruction technique using kinematic constraints to calculate the displaced vertices of the diphotons. Signal is extracted using a counting-based maximum-likelihood approach in bins of the diphoton vertex $l_{xy}$ and the invariant photon mass. Results are interpreted in terms of upper limits on branching ratios $\mathcal{BR}(H\to\Phi\Phi)\times\mathcal{BR}(\Phi\to\gamma\gamma)$ and in terms of the model-independent production cross section $\sigma(Z+\Phi\Phi)\times\mathcal{BR}(\Phi\to\gamma\gamma)$. No significant excess above SM expectation is observed.