Observation of the associated production of a top quark and a $Z$ boson in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

Single top-quark production 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 ${\text{fb}}^{-1}$. Events containing three isolated charged leptons (electrons or muons) and two or three jets, one of which is identified as containing a $b$-hadron, are selected. The main backgrounds are from $t\bar{t}Z$ and diboson production. Neural networks are used to improve the background rejection and extract the signal. The measured cross-section for $t{\ell }^{+}{\ell }^{-}q$ production, including non-resonant dilepton pairs with $m_{{\ell }^{+}{\ell }^{-}}>30$ GeV, is 97 $\pm$ 13 (stat.) $\pm$ 7 (syst.) fb, consistent with the Standard Model prediction.

45 pages in total, author list starting page 29, 6 figures, 4 tables, submitted to JHEP. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-01/

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