Search for an exotic decay of the Higgs boson to a pair of light pseudoscalars in proton-proton collisions at 13 TeV and Luminosity measurement in Pb-Pb collisions at 5.02 TeV

In the first part of the thesis, we report the results of a search for exotic decay of a Higgs boson with m$_{H}$ = 125 GeV to a pair of new light bosons, a$_{1}$, where one of the light bosons decays to a pair of muons and the other to a pair of b quarks. Such signatures are predicted in a number of well-motivated extensions of the standard model, including the next-to-minimal supernumerary and generic two-Higgs-doublet models with an additional scalar singlet. A data sample corresponding to an integrated luminosity of 138 fb$^{-1}$ recorded with the CMS detector during 2016-2018 is exploited in $\mu$$\mu$bb final state where no statistically significant excess is observed with respect to the standard model backgrounds for different m$_{a1}$ hypotheses above 15 GeV and below m$_{H}$/2. Upper limits on $B$($H$ → $a$1$a$1 → $\mu$$\mu$bb) are obtained to be (0.17 − 3.3) × 10$^{−4}$ depending on the m$_{a1}$ values. The second part of the thesis includes a study of luminosity measurement using lead-lead (PbPb) collisions at 5.02 TeV in 2015.