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

Light-by-light (LbyL) scattering is a quantum-mechanical process with a small cross-section, but possible to observe in ultra-peripheral high energy heavy-ion collisions. The signature of LbyL scattering consists of two low-$E_{\mathrm{T}}$ photons with 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 tiny compared to hadronic inelastic interactions. Based on 2015 Pb+Pb data, an evidence of LbyL scattering was reported by ATLAS in 2017. The starting point for developing the LbyL trigger for the upcoming 2018 Pb+Pb run is a trigger strategy from the 2015 Pb+Pb data taking. The work presented here is focused on lowering down the photon transverse energy requirement, which can provide almost a factor of two gain in the observed signal rate. In addition, trigger requirements need to be adjusted to the potentially larger instantaneous luminosity delivered by the LHC. This document presents an approach to address the challenges of triggering on LbyL scattering in the 2018 Pb+Pb data taking.