Search for exotic decays of the Higgs boson using photons with the Compact Muon Solenoid experiment

The search for answers to the fundamental questions of nature are being explored by the experiments at the Large Hadron Collider at CERN. The discovery of the Higgs Boson at the LHC marked the culmination of several decades of experimental search. However, the experimental investigation of the Higgs Boson is just ramping up. With its mass near 125 GeV, the Higgs Boson poses a naturalness question and hints at the possibility of fine tuning or the hierarchy problem in the Standard Model. The Higgs particle can thus be an excellent tool to probe the unknown. A search for the exotic decays of the 125 GeV Higgs Boson is presented in this thesis. The analysis selects event with at least four photons in the final state, and uses data sample of $\sqrt{s}=13 TeV$ proton-proton collisions collected by the CMS detector between 2016 and 2018, and corresponds to 132 $fb^{-1}.$ No significant excess of events is observed over the background expected from the Standard Model, and upper limits are set on $\sigma(pp\to h) \times BR(h\to a a\to \gamma\gamma\gamma\gamma$). The performance of the CMS Electromagnetic Calorimeter, which plays a crucial role in these results, is also presented in this document.