Search for a new light boson in exotic Higgs boson decays and the CMS muon spectrometer upgrade using GEM technology

The work presented in this thesis consists of a search for a new light boson in the exoticdecays of Higgs boson into the final state with a pair of bottom quarks (b¯b) and a pair ofmuons (µ− µ+ ), and muon endcap upgrade of the Compact Muon Solenoid (CMS) experimentusing Gas Electron Multiplier (GEM) technology.The Brout-Englert-Higgs mechanism of the Standard Model (SM) explains that all massive particles acquire their mass through interaction with the Higgs field. The existence ofsuch a field could be proven through the experimental detection of the Higgs boson. A similar particle with a mass of 125 GeV and featuring properties as that of the Higgs boson wasdetected by the ATLAS and CMS experiments at the CERN Large Hadron Collider (LHC)in 2012. The measurements of the couplings and properties of this newly discovered particleshow compatibility with SM within the experimental uncertainties. However, the existence ofBeyond the Standard Model (BSM) exotic decays of the Higgs boson into light pseudoscalarparticles which might have eluded our detection so far are not yet fully excluded. Therefore, the current work is carried out to explore the possibilities of such decays. The searchconsiders exotic decay of newly discovered boson (h) into a new light pseudoscalar boson(a) via two different decay channels h → Za → b¯bµ− µ+ and h → aa → b¯bµ− µ+ . The twotwo-Higgs-doublet model extended with extra singlet (2HDM+S) and the Next-to-MinimalSupersymmetric Model (NMSSM) are used for benchmarking the performance of the search.The search uses the proton-proton collision data collected by the CMS experiment duringthe year 2017 at the center-of-mass energy equal to 13 TeV corresponding to an integratedluminosity of 41.5 f b−1 . The analysis is performed by simulating private signal samples atthe leading order in gluon-gluon and vector boson fusion processes. The event categorizationtechnique is exercised to enhance search sensitivity. The dimuon mass distributions for thesimulated signal events are parameterized with the combination of Voigtion and double-sidedCrystal Ball functions. The background contribution from various processes is directly extracted from data and its shape is modeled with a set of analytical functions by employingdiscrete profiling. Various experimental and theoretical factors contributing to the uncertainties are taken into account and the search results are extracted from the unbinned fit tothe reconstructed dimuon mass distribution.Since a successful physics program can only be achieved with sufficient understandingand optimization of the detector performance, therefore part of the work is dedicated tothe detector R&D and muon endcap upgrade of the CMS experiment. After the secondLong Shutdown (LS2) of the LHC, the center-of-mass energy is expected to reach 14 TeVwith the steady increase in the beam luminosity over the years to a peak value 7 × 1034cm−2 s−1 . The corresponding increase in particle rates pushes the present CMS detectionand trigger capabilities to their limits and strongly affect the muon system. Therefore, CMScollaboration has proposed to upgrade the muon system in the endcap using GEM detectors.These detectors are known to operate under high rates with excellent performance and arePage vii