PERFORMANCE OF TRIPLE-GEM DETECTORS OF THE CMS EXPERIMENT AT THE LHC AND A SEARCH FOR HIGH-MASS RESONANCES DECAYING TO MUON PAIRS IN √s = 13 TeV pp COLLISIONS

This Ph.D. dissertation involves a search for new physics that took place at CERN’s (European Organization for Nuclear Research) one of the two general-purpose particle detectors- Compact Muon Solenoid (CMS) that monitors proton-proton collisions inside the Large Hadron Collider (LHC). Currently, the LHC is undergoing a series of intensive upgrades which will increase the instantaneous luminosity up to 7.5 × 10³⁴ cm^-2 s^-1 in preparation for the High-Luminosity
LHC (HL-LHC). To accommodate HL-LHC conditions, the experiments are also being
upgraded. In particular, CMS is undergoing modifications to its current infrastructure
and putting in three entirely new sub-detector systems; GE1/1, GE2/1 and ME0, based
on the Gas Electron Multiplier (GEM) technology. This has become significant among all
the ongoing CMS upgrades. GEMs are the successors of micro-pattern gaseous detectors
o!ering high-rate capabilities ideal for the forward regions of the CMS muon system. The
addition of these novel detectors will result in a more precise estimation of the muon
bending angle which will improve the muon transverse-momentum (p_T)measurements
and trigger capabilities. Moreover, GEM detectors will eventually improve the muon
reconstruction, which is the cornerstone for future analyses.
The first part of this dissertation introduces the GEM technology and the manufacturing,
quality assurance, quality control, commissioning, and operation of GEM detectors. The
competent outcomes of the rigorous quality control tests done during the production line
have validated the GE1/1 GEM chambers to be installed in the first muon station of
the CMS endcaps and 144 of these chambers are currently participating in the Run III,
meeting the anticipated performance.
The second part of this dissertation presents a search for high-mass resonance decaying
into muon pairs predicted by some benchmark Beyond Standard Models (BSM) using data
from proton-proton collisions at √s = 13 TeV, collected by the CMS detector in 2018
and 2017 corresponding to integrated luminosities of 63.1 fb^-1 and 42.1 fb^-1,respectively.
The results are also presented for the statistical combination of the data sets from 2016,
2017, and 2018, and also for the dielectron channel which corresponds to a total integrated
luminosity of 140 fb^-1. Here, a resonant peak is explored in the dimuon mass spectrum
using an unbinned maximum log-likelihood fit, without any assumptions on the absolute
background level. In the absence of significant evidence for BSM physics, results are
interpreted in terms of the upper limits on the ratio of the production cross-section times
the branching fraction for new resonances to that of the Z boson at 95% confidence level.
The corresponding lower limits on the masses are also evaluated for the considered models.
For the spin-1 model, the lower mass limits are set at 5.15 and 4.56 GeV for the sequential
standard model Z→
SSM and the E6 gauge group inspired Z→
ω respectively. For the spin-2
case, the lower mass limits are set at 2.50, 4.20 and 4.49 GeV for the coupling, k/ ¯
MPI of 0.01, 0.05, and 0.10 respectively. These results provide the most stringent lower limits
up-to-date for the masses of new particles predicted by the benchmark models considered.