Search for single production of vector-like quarks decaying to a Z boson and a top or a bottom quark in proton-proton collisions at 13 TeV

A search for the single production of a vector-like quark, T, decaying into a Z boson and a top quark is presented. The Z boson decays leptonically while the top quark decays hadronically. The search is performed using data collected by the CMS experiment at the LHC in proton-proton collisions at $\sqrt{s}=13$ TeV in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The presence of jets produced in the forward region of the detector is a particular characteristic of the single production of vector-like quarks and is utilized in the analysis. Limits are set for different hypotheses, from negligible width to a width equal to $30\%$ of the resonance mass. An additional production mode is also studied for the T quark, through the decay of a heavy Z$^{\prime}$ resonance that decays to Tt. The product of cross section and branching fraction above values in the range 0.27-0.04 pb is excluded at $95\%$ confidence level for the range of resonance mass considered, which is between 0.7 and 1.7 TeV in the case of a T quark with negligible width. A similar sensitivity is observed for widths of up to $30\%$ of the resonance mass. In the case that the T comes from the decay of a Z$^{\prime}$, limits on the product of cross section and branching fraction are set between 0.13 and 0.06 pb for Z$^{\prime}$ boson masses in the range from 1.5 to 2.5 TeV.