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=13TeV are used, corresponding to an integrated luminosity of 36.1fb−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, tt¯ 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.
Highlights
At hadron colliders, the top quark is typically produced in ttpairs through the strong interaction or as a single top or antitop quark through the electroweak interaction
Studies in the diboson validation regions (VR) indicated that the number of events predicted by the Sherpa Monte Carlo (MC) samples is lower than the number observed
The likelihood is maximised on the neural network (NN) output distribution in the signal region
Summary
The top quark is typically produced in ttpairs through the strong interaction or as a single top or antitop quark through the electroweak interaction. The associated t W production was first observed in 8 TeV proton–proton collisions at the Large Hadron Collider (LHC) [6,7]. These single-top-quark channels allow a direct determination of the dominant t W b vertex and of the magnitude of the CKM matrix element |Vtb| [8] using their measured cross-sections. This channel probes two SM couplings in a single process, whereas the similar final state tt Z only probes the t Z coupling. T Z q, where the Z boson decays into electrons or muons and the W boson from the top quark decays leptonically
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