Measurements of Higgs boson and search for additional Higgs boson in the di-photon final state at CMS.

<!--HTML-->In July 2012, a particle with properties compatible with the Standard Model Higgs boson was discovered at the Large Hadron Collider (LHC) by ATLAS and CMS experiments. The measurements showed that the new Higgs boson so far in accordance with those predictions for the SM Higgs boson. However, this model is not fully able to address several fundamental physics issues such as the dark matter, the dark energy and the asymmetry of matter and anti-matter. So, we can not exclude the possibility of physics Beyond Standard Models (BSM).<br>Precision measurements of the observed Higgs boson with more data collected at the LHC, could lead us to probe deviations from the Standard Model. With more data,the measurements would become even more precise which will push theorists to calculate their anticipations more precisely as well. It will eventually provide a deeper probe to see, if a more-precise prediction diverges from the current Higgs boson experimental measurements which could direct to new physics.<br>Some of the beyond Standard Models, e.g. the Next-to-Minimal Supersymmetric Model (NMSSM), and generalized two-Higgs-doublet models (2HDM) gives rise to an interesting phenomenology including several Higgs bosons, some of which could have masses below the observed mass of SM Higgs boson i.e. 125 GeV. Therefore, searching for the additional Higgs bosons is one of the most direct and effective ways to study the BSM physics.<br>The main part of this dissertation is dedicated to the probe for an additional SM like Higgs boson in the diphoton final state. This analysis searches for an excess in the diphoton invariant mass spectrum over a smoothly falling background in the range 70 < $m$$_{H}$ < 110 GeV with CMS at 13 TeV. The results are presented for 2016 (36.3fb$^{-1}$), 2017 (41.5 fb$^{-1}$, 2018 (54.4 fb$^{-1}$) data and 2016 to 2018 (full Run 2) combination corresponding to an integrated luminosity of 132.2 fb$^{-1}$. No significant excess is observed. One modest excess with approximately 2.9 $\sigma$ local (1.3$\sigma$ global) significance is observed for a mass hypothesis of 95.4 GeV. The expected and observed upper limits with 95% confidence level on the product of cross section times branching ratio into two photons for each of the analysis with 2016 data set, 2017 data set, 2018 data set and the statistical combinations of the full Run 2 data set, are addressed. The observed upper limit for the combined data set ranges from 73 fb to 15 fb. <br>This thesis also includes some of the latest results for the measurements of Higgs boson properties in the diphoton decay channel including the signal strengths, signal strength modifiers, production cross sections, its couplings to other particles, inclusive and differential fiducial production cross sections. Events are selected from a sample of proton-proton collisions at 13 TeV collected by the CMS detector at the LHC from 2016 to 2018, corresponding to an integrated luminosity of 137 fb$^{−1}$. The observed signal strength is 𝜇 = 1.12$^{+0.06}_{-0.06}$(theo)$^{+0.03}_{−0.03}$(syst)$^{+0.07}_{-0.06}$(stat). The measured fiducial cross section is 𝜎 $_{fid}$ = 73.40$^{+5.4}_{-5.3}$(stat)$^{+2.4}_{-2.2}$(syst)fb = 73.40$^{+6.1}_{-5.9}$fb. All results are found to be in agreement with the SM expectations.