Search For Anomalous Gauge Coupling through Vector Boson Scattering and Development of the GEM Detectors at the CMS Experiment

In the Standard Model (SM) of particle physics, masses for the particles are generated by the Higgs mechanism which requires the existence of a spin-0 particle called the Higgs boson. In July 2012, a new Higgs-like particle, with mass ≈125 GeV, was discovered at the Large Hadron Collider (LHC). This might be the long-sought SM Higgs boson predicted in the 1960s, or one of the Higgs bosons predicted by the several beyond the SM scenarios. Several beyond SM scenarios, such as, super-symmetry, little-Higgs models, and others from the extended Higgs sectors such as the Georgi-Machacek model, contain a multitude of neutral as well as charged Higgs bosons. Till now the existing results contain large uncertainties, thus various extensions of the SM cannot be confirmed or ruled out decisively. This necessitates to scrutinize, the ElectroWeak Symmetry Breaking (EWSB) mechanism rigorously by carrying out the precision measurements of the Higgs boson properties and the couplings of the electroweak vector bosons (W and Z) with the Higgs boson via the Vector Boson Scattering (VBS) processes. VBS processes violate the unitarity at an energy scale ≈1 TeV in the absence of the Higgs boson. Thus, it is one of the most important studies that could help us to understand the EWSB mechanism. Due to the statistical constraints, VBS could be probed indirectly by measuring the quartic vertices. This thesis is based on the study of the anomalous Quartic Gauge Coupling (aQGC) processes using the proton-proton collision data at a centre-of-mass energy of 13 TeV, collected using the Compact Muon Solenoid (CMS) detector at the CERN LHC. The aQGC measurement is performed using two channels: WV and ZV (here, V could be either a W or a Z boson) in association with the two jets produced in the forward pseudo-rapidity regions. For the WV (ZV) channel, only leptonic decays ofW (Z) bosons are considered, while the V decays hadronically into jet having large radii (having radius parameter 0.8). The events are selected by requiring two jets with large rapidity separation and di-jet invariant mass, one or two leptons (electrons or muons), a fat jet with large radii and missing transverse momentum. Constraints are imposed on the quartic vector boson interactions in the framework of dimension-eight effective field theory operators at 95% confidence level (CL). Furthermore, a theoretical interpretation of the observed results is given using the Georgi-Machacek model. This model predicts the existence of doubly and singly charged Higgs bosons using the Higgs triplets. The main feature of this model is that it preserves the custodial symmetry and provides neutrino with a Majorana mass. The exclusion limits on the production cross-section for the charged Higgs bosons times the branching fraction at 95% CL as a function of the mass of the charged Higgs boson are reported in this thesis. On the hardware front, work performed for the upgrade studies of the CMS detector’s muon endcap system is reported. For the CMS muon endcap detector system upgrade, the Gas Electron Multiplier (GEM) detectors are proposed to be installed during the Long Shutdown-2 (2019-2020) period. To test the functionality of these GEM detectors, several beam tests were carried out to measure their properties and evaluate their performance in terms of spatial and timing resolution, cluster size and efficiency measurements. I actively participated in these beam test campaigns and also during the data analysis for the GEM detectors. Also, the characterisation studies for the GEM foils developed in India for the CMS upgrade are also described.