Dylan Cooper Frizzell - Top2019 YSF - tZq Observation

Abstract

The production of a top quark in association with a $Z$ boson is measured in the trilepton channel. The proton–proton collision data collected by the ATLAS experiment from 2015 to 2018 at a centre-of-mass energy of 13 TeV are used, corresponding to an integrated luminosity of 139 fb$^{-1}$. Events containing three isolated charged leptons (electrons and muons) and two or three jets, one of which is identified as containing a b-hadron, are selected. The main backgrounds are from $t\bar{t}Z$, diboson, $t\bar{t}$ and $Z$ + jets production. A neural network is used to improve the background rejection and extract the signal. The measured cross-section for $tl^{+}l^{-}q$ production, including non-resonant dilepton pairs with $m_{ll}$ > 30 GeV, is presented