Towards tuning electromagnetic shower properties to data with AtlFast3

A precise simulation of electromagnetic shower properties in the ATLAS calorimeter is crucial for the identification of particle showers originating from electrons and photons. AtlFast3, the next-generation of fast simulation in ATLAS, precisely simulates the properties of electromagnetic showers, but is limited by the accuracy of the full Geant4 simulation, upon which its parametrization is based. Differences between the Geant4 simulation and data collected by the ATLAS detector are well known but insufficiently understood. This note shows that AtlFast3 can be tuned to accurately describe the shower shapes observed in data and describes the underlying models that are employed to outperform Geant4.