Single top-quark production cross-section measurement in association with Z boson and performance studies of the RPCs with the CMS detector using 8 TeV pp collisions at the LHC

In this thesis, the measurement of the tqZ cross-section for both the inclusive and exclusive cases is reported. The top quark being the heaviest particle in the Standard Model (SM) has been discovered about two decades ago but many of its properties have still not been fully investigated. The cross-section of single top quark in association with a Z boson is an unmeasured rare SM process and also an irreducible background to many important searches at the Large Hadron Collider (LHC). The process is studied by using the three final state leptons (electrons, muons) for an integrated luminosity of 19.7 fb−1 recorded by the Compact Muon Solenoid (CMS) detector at the LHC using 8 TeV centre-of-mass energy of proton-proton ( pp) collisions. The predicted cross-section times branching ratio for this process is 0.008 pb at the Next-to-Leading Order (NLO) in Quantum Chromodynamics (QCD), which is comparable to the measured cross-section σ_tqZ = 0.018 (stat.)+0.011 −0.009 (Syst.)+0.004−0.004pb. The significance of the tqZ inclusive cross-section as well as the exclusive one for all the leptonic combinations is also presented. The observed significance of inclusive channel is reported to be 1.80, which is consistent to the predicted value. The Resistive Plate Chamber (RPC), a dedicated trigger detector at the CMS experiment. The RPC has a significant contribution in the trigger decisions and reconstruction of muon trajectories in the detector. In this thesis, the stability and performance of the RPC detector is also discussed for the 2012 data-taking period. The detector stability is studied through the high voltage scans to determine the optimal operating working voltage for each individual RPC chamber installed in the CMS experiment. The RPC performance is found to be very stable and almost caused no interruption in the CMS data-taking during 8 TeV pp collisions. The CMS down time due to the RPC was less than 1.5 %, and the fraction of active channels was about 97.5% during the 2012 data-taking period. The background radiation measurements at the RPC remained within the expectation values and no significant issues were observed at the nominal luminosity. Over the last few years of the CMS data-taking, the performance of the RPC detectors both as a triggering and reconstruction system remained well within the CMS specification.