Measurement of $tZq$ inclusive and differential cross-sections in $pp$ collisions at $\sqrt{s} = 13~\text{TeV}$ with the ATLAS detector

Abstract

The $t$-channel production of a top quark in association with a $Z$ boson, $tZq$, is measured in the trilepton channel. The measured $pp \rightarrow t\ell^+\ell^-q$ process is defined for $m_{\ell\ell}>30\,\text{GeV}$ and includes both resonant $Z$-boson production and non-resonant dilepton production. The data collected by the ATLAS experiment in 2015 to 2018 in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$ are used, corresponding to an integrated luminosity of $140\,\text{fb}^{-1}$. Events containing three light leptons ($e$ or $\mu$) and two to four jets, one or two of which are identified as $b$-quark-initiated jets, are selected. The inclusive $pp \rightarrow t \ell\ell q$ cross-section with $m_{\ell\ell}>30\,\text{GeV}$ is measured, along with the separate cross-sections for top-quark and top-antiquark production and their ratio. The inclusive cross-section is measured to be $95.2\;^{+8.5}_{-7.9}\;\text{(stat.)}\;^{+5.5}_{-5.3}\;\text{(syst.)}\,\text{fb}$ and found to be in agreement with the most advanced Standard Model predictions. Differential cross-section measurements as a function of many kinematic variables are also presented. Both absolute and normalised differential cross-section measurements are made at particle level and parton level in their respective phase spaces. The top-quark spin asymmetry is measured from the unfolded measurement of the top-quark polarisation angle as $A = 0.34 \pm 0.14 \;\text{(stat.)} \pm 0.06\;\text{(syst.)}$, in good agreement with the Standard Model prediction. The differential cross-section measurement results are interpreted in the framework of Standard Model effective field theory and limits are set on dimension-6 operators involving the top quark.