# Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at sqrt(s) = 8 TeV

## TL;DR

This study searches for the production of a single top quark with a Z boson and possible flavor-changing neutral currents at 8 TeV, measuring the standard model process and setting limits on rare decays.

## Contribution

It provides the first measurement of tZq production cross section at 8 TeV and sets new upper limits on top quark flavor-changing decays to Z bosons.

## Key findings

- Measured tZq production cross section as 10 +8/-7 fb.
- No evidence found for flavor-changing neutral current tZq production.
- Set upper limits on top quark decays to Z u and Z c at 0.022% and 0.049%.

## Abstract

A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model process and to search for flavour-changing neutral current interactions. The data sample corresponds to an integrated luminosity of 19.7 inverse femtobarns recorded by the CMS experiment at the LHC in proton-proton collisions at sqrt(s) = 8 TeV. Final states with three leptons (electrons or muons) and at least one jet are investigated. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be sigma( p p -> t Z q -> l nu b l+ l- q ) = 10 +8/-7 fb with a significance of 2.4 standard deviations. No presence of flavour-changing neutral current production of tZq is observed. Exclusion limits at 95% confidence level on the branching fractions of a top quark decaying to a Z boson and an up or a charm quark are found to be B( t -> Z u ) < 0.022% and B( t -> Z c ) < 0.049%.