Associated production of a top quark and a Z boson in pp collisions at $\sqrt s$ = 13 TeV using the ATLAS detector

The production of a top quark in association with a Z 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 Z 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 b-quark jet are selected. The major backgrounds come from diboson, top-pair production and Z +jets production. An artificial neural network is used to improve the background rejection and the tZq production cross-section is extracted. The result is in agreement with the Standard Model prediction and has an observed significance of 4.2 $\sigma$, thus yielding the first evidence for tZq associated production.