Search for heavy neutral gauge boson Z' using CMS detector at the LHC

The Standard Model of particle physics is a well-established framework which describes the nature of the matter in the universe and the way this matter interacts with each other. Though it is a well-tested framework and acts as a fundamental theory till now, it fails to describe various physics phenomena such as the existence of dark matter and dark energy in the universe, matter-antimatter asymmetry in the universe, prediction of neutrino masses and prediction of force carrier for the fourth fundamental force i.e. the gravitational force. To address these questions, many theories beyond the standard model have evolved with time such as the Grand Unified Theories, Supersymmetry, and String theory. These theories predict new gauge fields and interactions. A common manifestation of these new gauge fields and interactions is particles with TeV scale masses. One such particle is the heavy neutral spin-1 gauge boson, $Z'$, which arises from extensions of the electroweak symmetry of the standard model. $Z'$ is assumed to be a mediator between visible and dark sector solving the mysteries of dark matter and dark energy. Besides this, $Z'$ with non-universal coupling to fermions qualifies as the most suitable candidate to answer the lepton flavour universality violating processes in the B-meson decays observed by LHCb, BaBar and Belle experiments. Additionally, recent results of the muon g-2 experiment at Fermilab strongly motivate the search for $Z'$ to explain the anomalous magnetic moment of the muon. As $Z'$ is predicted to be of the order of TeV mass, it is well within the energy reach (14 TeV) of the LHC experiment at CERN. The ATLAS and the CMS detector at the LHC have a wide range of $Z'$ search programs. The recent results from these $Z'$ searches have eluded the discovery thus far. These searches have assumed the production of $Z'$ via Drell-Yan process at the LHC with sizable coupling to light quarks. One hypothesis is that the $Z'$ coupling to quarks is smaller than that required for sensitivity in the Drell-Yan $Z'$ searches. To complement these searches, a new search for $Z'$ boson produced via vector boson fusion (VBF) processes is presented in this Thesis. The VBF processes consider smaller $Z'$ coupling to light quarks and allow the coupling to standard model vector bosons ($W$ or $Z$). Of particular interest are models with enhanced $Z'$ couplings to third-generation fermions, which may explain the B-meson anomalies observed at LHCb, BaBar, and Belle experiments. Extensions to the standard model, proposed as an explanation for the high mass of the top quark, also predict $Z'$ bosons that typically couple to third-generation fermions, motivating the $Z'\to\tau\tau$ decay states. Additionally, since $Z'$ coupling to the standard model vector bosons is allowed, therefore, the $Z' \to WW$ decay width can be large, and thus motivates using this decay channel to re-interpret results for the $Z'\to WW$ scenario. In this Thesis, we report the first results of the search for $Z'$ through VBF processes at the LHC, using proton-proton ($pp$) collision data at the center-of-mass energy of 13 TeV collected by the CMS detector. The search comprises two decay channels of VBF $Z'\to\tau\tau$ and VBF $Z'\to WW$ production: $Z'\to (WW\to) \tau\tau \to \tau_{e}\tau_{h}$ and $Z'\to (WW\to) \tau\tau \to \tau_{\mu}\tau_{h}$ where $\tau_{e}$ and $\tau_{\mu}$ are the leptonic decays of tau lepton and $\tau_h$ refers to hadronically decaying tau lepton. The potential backgrounds are estimated using data-driven techniques and excess of events above the standard model background predictions are investigated for. In the absence of significant excesses over standard model predictions, 95% CL upper limits on the cross-section times branching ratio are calculated and the estimated value is presented as a limit. For a broad and generic discussion, the upper limits are calculated for various coupling scenarios with different values of branching ratios of VBF $Z'\to\tau\tau$ and VBF $Z'\to WW$ decays. With the present analysis, $Z'$ for masses lower than 1.77 TeV is excluded for $B(Z'\to \tau\tau)$ = 20% with enhanced coupling to third-generation fermions. Similarly, the $Z'$ for masses lower than 1.20 TeV is excluded for $B(Z'\to WW)$ = 80% with enhanced coupling to third-generation fermions. These are the first and the most stringent limits set so far. The present Thesis is also based on the upgradation of the CMS muon detector system with the gaseous electron multiplier (GEM) detectors. These detectors are added in the forward region of the muon detector system to improve the triggering of the forward muons and to increase the reconstruction performance of muons. A total of 144 GEM detector chambers are installed during the long shutdown 2 (LS2) period, out of which 8 of the chambers are assembled and tested at Panjab University, Chandigarh. A brief summary of the assembly procedure and the testing of the fabricated chambers is presented in the Thesis. This Thesis also presents the sensitivity studies for the GEM detector for different background particles such as hadrons, electrons, positrons and neutrons. These studies are performed using FLUKA and \GEANTfour simulation packages. The results are then compared with the $pp$ collision data collected by the GEM chambers installed in the CMS detector during 2017 and 2018 LHC runs. The results obtained from the simulation agree well with the data collected by the slice test of GEM detectors in CMS. The present Thesis also covers radiation damage studies of the barrel hadron calorimeter (HB) and the endcap hadron calorimeter (HE) using isolated muons from the $pp$ collision data. These radiation damage studies measure the depletion in the light yield of the plastic scintillators of the HB and the HE as a function of the LHC delivered luminosity. In the present analysis, it is observed that the towers in the central barrel region show a very small degradation, i.e., of the order of 2%. However, the level of degradation increases in the endcap. It is found that the towers in the forward region of the endcap show 15% degradation in their light output as compared to their observed light output in the beginning of 2018 data-taking.