Associated Production of a Top Quark and a Z Boson in <em>pp</em> Collisions at √s = 13 TeV Using the ATLAS Detector

Abstract

The production of a top quark in association with an <i>Z</i> boson is studied using 36.1 fb^-1 of proton-proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 TeV. The search is performed in the trilepton channel, in which both the <i>Z</i> boson and the top quark decay to final states involving charged leptons. Events containing three identified leptons (electron and/or muon) and two jets, one of which is identified as a <i>b</i>-quark jet are selected. The major backgrounds come from diboson, top-pair production and <i>Z</i>+jets production. An artificial neural network is used to improve the background rejection and the <i>tZq</i> production cross-section is extracted. The result is in agreement with the Standard Model prediction and has an observed significance of 4.2 σ, thus yielding the first evidence for <i>tZq</i> associated production.