Modelling of isolated multi-photon production in Monte Carlo generators in ATLAS

Predictions for three- and four-photon processes are compared for a number of Monte Carlo generators and fixed-order calculations in $pp$ collisions at $\sqrt{s}=8$ and $\sqrt{s}=13$~TeV using the ATLAS simulation infrastructure. Several kinematic distributions are obtained using the Sherpa and MadGraph$+$Pythia8 simulations and the MATRIX and MCFM programmes. In the case of the three-photon production, two different setups of the Sherpa event generator differing in the accuracy and the inclusion of electroweak corrections are studied.