Measurement of $Z\gamma\to\ell^+\ell^-\gamma$ differential cross-sections in $pp$ collisions at $\sqrt{s} =13$ TeV with the ATLAS detector

The production of a prompt photon in association with a $Z$ boson is studied in proton-proton collisions at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at the LHC between 2015 and 2018. Measurements are made of differential cross-sections for the process $pp\to\ell^+\ell^-\gamma+X$ ($\ell = e, \mu$), within a fiducial phase space region dominated by events with a photon produced in addition to a $Z$ decaying to leptons. Cross-sections are shown as a function of each of four kinematic variables characterising the $\ell^+\ell^-\gamma$ system. The data are compared to theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations.