Observation of W associated single top (tW) production and search for FCNC in tZ events in proton-proton collisions

The first observation of the associated production of a single top quark and a $W$ boson in proton-proton collisions at $\sqrt{s}$ = 8 TeV with the CMS experiment at the LHC is presented. The data correspond to an integrated luminosity of 12.2 $\mathrm{fb^{-1}}$. The measurement is performed using events with two leptons and a jet originated from a b quark. A multivariate analysis based on kinematic properties is used to separate the signal from the $t\bar{t}$ background. The signal is observed with a 6.0 standard deviation excess above a background only hypothesis.A production cross section of $23.4^{+5.5}_{-5.4}$ pb is measured, in agreement with the standard model expectation of $22.2\pm1.5$ pb.\\A study of top-quark anomalous couplings is performed through the search for a single top-quark produced in association with a $Z$ boson. The event selection requires the presence of three isolated leptons, electrons or muons, and of at least one jet. The signal extraction is done using kinematic variables and information related to b-tagging, combined using a Boosted Decision Tree. The search is performed in a data sample corresponding to about 5 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=7$ TeV recorded with the CMS detector. No evidence of flavor-changing neutral currents is observed and upper limits at 95\% confidence level are determined. The corresponding upper limits on the coupling strengths of an effective model are found to be $\kappa_{gut}/\Lambda < 0.10$ TeV$^{-1}$, $\kappa_{gct}/\Lambda < 0.35$ TeV$^{-1}$, $\kappa_{Zut}/\Lambda < 0.45$ TeV$^{-1}$ and $\kappa_{Zct}/\Lambda < 2.27$ TeV$^{-1}$, where $\Lambda$ is the expected scale at which new physics could appear. The equivalent top-quark-decay branching fractions are found to be $\mathcal{B}(t \to gu)\leq 0.56\pct$, $\mathcal{B}(t \to gc)\leq 7.12\pct$, $\mathcal{B}(t \to Zu)\leq 0.51\pct$ and $\mathcal{B}(t \to Zc)\leq 11.40\pct$.