Measurement of the production cross-section of a single top quark in association with a Z boson in proton–proton collisions at 13 TeV with the ATLAS detector

Abstract

The production of a top quark in association with a Z boson is investigated. The proton–proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of √s = 13 TeV are used, corresponding to an integrated luminosity of 36.1 fb−1. Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds are diboson, <math><mi is="true">t</mi><mover accent="true" is="true"><mrow is="true"><mi is="true">t</mi></mrow><mrow is="true"><mo stretchy="false" is="true">¯</mo></mrow></mover></math> and Z + jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2σ in the data and the expected significance is 5.4σ. The measured cross-section for tZq production is 600±170 (stat.)± 140 (syst.) fb.