Triggering on light-by-light scattering in the ATLAS experiment

Based on lead-lead (Pb+Pb) collision data from 2015, the first evidence of light-by-light (LbyL) scattering in heavy-ion collisions was reported by the ATLAS Collaboration. The signature of LbyL scattering consists of two low-$p_\mathrm{T}$ photons and the absence of any other activity in the detector. Triggering on such events in the ATLAS experiment is challenging as the rate of LbyL events is small compared to that of central Pb+Pb collisions which are accompanied by a lot of activity in the detector. The LbyL trigger strategy from the 2015 Pb+Pb data taking is a starting point for developing the trigger for the 2018 Pb+Pb run. Work is focused on lowering the photon $p_\mathrm{T}$ requirement which can provide almost a factor of two gain in the accepted rate. In addition, trigger requirements need to be adjusted for the potentially larger instantaneous luminosity delivered by the LHC this year.