Three lepton signatures from tZq interactions in the SM and top-FCNC at the CMS experiment at \sqrt{s} = 8 TeV

Abstract

The search for a top quark in association with a Z boson can be performed in both top pair production as well as single top production. Targeting the three lepton final state, the search can be used to identify the standard model (SM) process of a single top quark associated with a Z boson and to search for flavour changing neutral current (FCNC) interactions. The presented analysis uses a data sample corresponding to an integrated luminosity of $19.7$ fb$^{-1}$ recorded by the CMS experiment at the LHC in proton collisions at $\sqrt{s}=8$ TeV. The cross section for the SM tZq production is measured to be $\sigma(tZq\rightarrow l\nu bl^+ l^- q)= 10^{+8}_{-7}$ fb with a significance of $2.4 \sigma$. Exclusion limits at 95$\%$ confidence level on the branching ratio of a top quark decaying to a Z boson and an up or charm quark are determined to be $BR(t \rightarrow Zu) < 0.022\%$ and $BR(t \rightarrow Zc) < 0.049\%$ respectively.