FullSimLight: ATLAS standalone Geant4 simulation

HEP experiments simulate the detector response by accessing allneeded data and services within their own software frameworks. However de-coupling the simulation process from the experiment infrastructure can be use-ful for a number of tasks, amongst them the debugging of new features, or thevalidation of multithreaded vs sequential simulation code and the optimizationof algorithms for HPCs. The relevant features and data must be extracted fromthe framework to produce a standalone simulation application.As an example, the simulation of the detector response of the ATLAS exper-iment at the LHC is based on the Geant4 toolkit and is fully integrated in theexperiment’s framework "Athena". Recent developments opened the possibilityof accessing a full persistent copy of the ATLAS geometry outside of the Athenaframework. This is a prerequisite for running ATLAS Geant4 simulation stan-dalone. In this paper we present the status of development of FullSimLight, alightweight simulation prototype that is being developed with the goal of run-ning ATLAS standalone Geant4 simulation with the actual ATLAS geometry.The purpose of FullSimLight is to simplify studies of Geant4 tracking andphysics processes, including on novel architectures. We will also address thechallenges related to the complexity of ATLAS’s geometry implementation,which precludes persistifying a complete detector description in a way thatcan be automatically read by standalone Geant4. This lightweight prototype ismeant to ease debugging operations on the Geant4 side and to allow early testingof new Geant4 releases. It will also ease optimization studies and R&D activi-ties related to HPC development: i.e. the possibility to offload partially/totallythe simulation to GPUs/Accelerators without having to port the whole experi-mental infrastructure.