Inclusive-photon production and its dependence on photon isolation in $pp$ collisions at $\sqrt s=13$ TeV} using 139 fb$^{-1}$ of ATLAS data

Measurements of differential cross sections for inclusive isolated-photon production in $pp$ collisions at a centre-of-mass energy of 13 TeV provided by the LHC and using an integrated luminosity of 139 fb$^{-1}$ of ATLAS data are presented. The cross sections are measured as functions of the photon transverse energy in different regions of photon pseudorapidity. The photons are required to be isolated by means of a fixed-cone method with two different cone radii, 0.2 and 0.4. The dependence of the inclusive-photon production on the photon isolation is investigated by measuring the fiducial cross sections as functions of the isolation-cone radius and the ratios of the differential cross sections with different radii in different regions of photon pseudorapidity. The measurements presented here, with a more granular segmentation in photon pseudorapidity leading to a more detailed experimental input to fits of the proton parton distribution functions and with different isolation radii, constitute an improvement with respect to those published by ATLAS earlier. These improvements provide a more in-depth test of the theoretical predictions. Next-to-leading-order QCD predictions from JETPHOX and SHERPA and next-to-next-to-leading-order QCD predictions from NNLOJET are compared to the measurements, using several parameterisations of the proton parton distribution functions. The measured cross sections are well described by the fixed-order QCD predictions within the experimental and theoretical uncertainties in most of the investigated phase-space region.