Single top production in association with a Z boson at the LHC

TL;DR

This paper calculates the production rate of a Z boson with a single top quark at the LHC at NLO, highlighting its significance as a background in top quark FCNC searches and its observability in current data.

Contribution

It provides the first NLO calculation of single top plus Z boson production, including decay, and assesses its impact as a background in top FCNC decay searches.

Findings

Rate is comparable to ttZ production.

Single top + Z is a dominant background in FCNC searches.

NLO corrections affect related tH process.

Abstract

We present results for the production of a Z boson in association with single top at next-to-leading order (NLO), including the decay of the top quark and the Z boson. This electroweak process gives rise to the trilepton signature l+l-l'+- + jets + missing energy. We present results for this signature and show that the rate is competitive with the contribution of the mixed strong and electroweak production process, ttZ. As such it should be observable in the full data sample from LHC running at $\sqrt{s}=8$ TeV. The single top + Z process is a hitherto unconsidered irreducible background in searches for flavour changing neutral current decays of the top quark in tt production. For a selection of cuts used at the LHC involving a b-tag it is the dominant background. In an appendix we also briefly discuss the impact of NLO corrections on the related tH process.