Search for the associated production of a Z boson with a single top quark using CMS data at 8 TeV and 13 TeV and performance studies of the CMS silicon tracker.

The cross section measurements of a single top quark in association with a Z boson using proton-proton collision data collected by CMS experiment are reported in this thesis. The measurements are performed by analyzing integrated luminosities of 19.7 $fb^{-1}$ and 35.9 $fb^{-1}$ which were collected at 8 TeV and 13 TeV. The measurement are performed within the standard model framework where top quark is produced via the t-channel process. The measurements in three lepton final state, where the W boson from the top quark and the Z boson decay into either electrons or muons, resulting in four possible lepton combinations namely $eee$, $ee\mu$, $\mu\mu e$, $\mu\mu\mu$, are discussed. The final state electrons or muons can also come from from leptonic $\tau$ decays, as they are not specifically excluded. The main sources of background to the tZq process are $t\bar{t}$ production, diboson production (WZ, ZZ), ttV (V = W or Z) and Drell-Yan (DY) production. A simple cut and count technique is used for the cross section measurement. For the $\sqrt {s}$ = 8 TeV measurement, a one bin likelihood fit is performed to event yields in each four lepton channels and to the combined channel. The cross section for the combined channel is measured to be $\sigma (t\ell^{+}\ell^{-}q)$ = 18$^{+11}_{-9}(stat)^{+4}_{-4}(syst)$ fb, where $\ell$ stands for electrons, muons and $\tau$ leptons, with an observed (expected) significance of 1.81 (0.81) standard deviations. The measured value is compatible with the NLO standard model prediction of 8.2~fb with a theoretical uncertainty of less than 10\%. For the second cross section measurement at 13TeV, the signal is extracted by performing a simultaneous binned likelihood fit to yields in the signal and the background enriched control regions. The cross section for the combined channel is measured to be $\sigma (t\ell^{+}\ell^{-}q)$ = 156$^{+47}_{-42}(stat)^{+40}_{-34}(syst)$ fb, which is compatible with the NLO standard model prediction of 94.2 $\pm$ 3.1 fb. The measurement is found to be in agreement with the standard model with an observed (expected) significance of 2.81(1.95) standard deviations.