Rare Higgs Processes at CMS and Precision Timing Detector Studies for HL-LHC CMS Upgrade

This thesis describes the search for two rare Higgs processes. The first analysis describes the CMS Run 2 search for $H \to \mu \mu$ decays, with 137.3 fb$^{-1}$ of data at $\sqrt{s}$~=~13~TeV. The analysis targeted four different Higgs production modes: the gluon fusion (ggH), the vector boson fusion (VBF), the Higgs-strahlung process (VH), and the production in association with a pair of top quarks (ttH). Each category used a dedicated machine learning based classifier to separate the signal from the background processes. A combined fit from all these categories saw a slight excess in the data corresponding to 3.0 standard deviations at $M_{H}$ = 125.38 GeV, and gave the first evidence for the Higgs boson decay to second-generation fermions. The best-fit signal strength and the corresponding 68\% CL interval was found to be $\hat{\mu} = 1.19^{+0.41}_{-0.39}$ $\textrm{(stat)}$ $^{+0.17}_{-0.16}$ $\textrm{(syst)}$ at $M_{H}$ = 125.38 GeV. The second analysis describes the CMS Run 2 search for $HH \to$ $b\bar{b}b\bar{b}$ with highly boosted Higgs bosons. This analysis used a dedicated jet identification algorithm based on graph neural networks (ParticleNet) to identify boosted H$\rightarrow$ bb jets. This search targeted the gluon fusion and the vector boson fusion HH production modes, and put constraints on the allowed values of the various Higgs couplings as: $\kappa_{\lambda} \in [-9.9, 16.9]$ when $\kappa_V = 1, \kappa_{2V} = 1; \kappa_V \in [-1.17, -0.79] \cup [0.81, 1.18]$ when $\kappa_{\lambda} = 1, \kappa_{2V} = 1$; $\kappa_{2V} \in [0.62, 1.41]$ when $\kappa_{\lambda} = 1, \kappa_V = 1$. A scenario with $\kappa_{2V} = 0$ was excluded with a significance of 6.3 standard deviations for the first time, when other H couplings are fixed to their SM values. The combined observed (expected) 95\% upper limit on the HH production cross section was found to be 9.9~(5.1)~$\times$~SM. Finally, this thesis also discusses the planned MIP Timing Detector (MTD) upgrade for CMS at the HL-LHC. The MTD will be a time-of-flight (TOF) detector, designed to provide a precision timing information for charged particles using SiPMs + LYSO scintillating crystals, with a time resolution of $\sim$30 ps. This thesis describes several R\&D tests that have been performed for characterizing the sensor properties (time resolution, light yield, etc.) and optimizing the sensor design geometry. This thesis also contains a description of mock test setups for cooling the sensors, since it is known to be an effective way of mitigating the increased dark current rates in the sensors due to radiation damage.