Speaker
Description
In addition to the main physics program based on proton-proton collisions, the ATLAS experiment also collects heavy-ion data with lead nuclei. Among these data sets, ultra-peripheral collisions provide a highly interesting class of events. They give an opportunity to study very rare processes involving two-photon exchange, such as light-by-light (LbyL) scattering - a phenomenon which was theoretically postulated more than 80 years ago but due to its small cross section it had not been measured directly until 2017. Based on lead-lead (Pb+Pb) collision data from 2015, the first evidence of this process was reported by the ATLAS Collaboration. The signature of LbyL scattering consists of two low-pT 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 pT 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. This poster presents an approach to address the challenges of triggering on LbyL scattering in Pb+Pb collisions during 2018.
