Using associated production of top quarks and neutral bosons to probe standard model couplings and search for new physics

The unprecedentedly large integrated luminosity accumulated by the ATLAS experiment from proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 13\,\mathrm{TeV}$ at the Large Hadron Collider (LHC) allows the study of rare Standard Model (SM) processes. The associated production of top-quark pairs or single top quarks and neutral bosons is such an example: it directly probes top-quark couplings to photons and $Z$ bosons and tests for deviations from the SM. Three such recent measurements are presented, all of which exploit the full dataset of the most recent Run 2 of the LHC, corresponding to $139\,\mathrm{fb}^{-1}$ of integrated luminosity. The cross-sections for the production of top-quark pairs in association with a photon ($t\bar{t}\gamma$) or with a $Z$ boson ($t\bar{t}Z$) are measured both inclusively and differentially as functions of kinematic observables characterising the $t\bar{t}$+boson system. The measurements are compared to predictions obtained by next-to-leading order (NLO) + parton-shower (PS) Monte Carlo simulations and fixed order NLO calculations, respectively. In a third measurement, the cross-section of single top-quark production in association with a $Z$ boson ($tZq$) probes two SM couplings in the same process, $t$-$Z$ and $W$-$Z$, and it is a background to the rare associated production of a single top quark and a Higgs boson. A neural network is used to improve the background rejection and to extract the signal. The measured cross-section is compared to the NLO SM prediction.