Measurement of the production cross-section of a single top quark in association with a $Z$ boson in proton--proton collisions at 13 TeV with the ATLAS detector

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 $\sqrt{s}=13\mathrm{TeV}$ are used, corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$. 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, $t\bar{t}$ and $Z+\mathrm{jets}$ production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is $4.2\sigma$ in the data and the expected significance is $5.4\sigma$. The measured cross-section for $tZq$ production is $600\pm 170(stat.)\pm 140(syst.)\mathrm{fb}$.

35 pages in total, author list starting page 19, 4 figures, 4 tables, submitted to Phys. Lett. B., All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2016-14/

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