The Celeritas MC particle transport code aims to close the gap between the advanced architectures that now dominate the HPC landscape and the vast computational requirements of the upcoming HEP detector campaigns anticipated for the HL-LHC. To fulfill the goal of accelerating the most computationally intensive MC simulations needed for the HL-LHC, Celeritas must be integrated into established experimental detector simulation workflows built on Geant4 by offloading EM particle showers to GPUs using the new Acceleritas library.

Acceleritas is a planned component of this project that will provide interfaces between Celeritas and Geant4 to enable a hybrid workflow that concurrently executes specialized tasks on device. One example of an Acceleritas application is an end-to-end HEP detector simulation that collects a group of specific particles from either primary collisions or subsequent hadronic interactions in Geant4 and transports them in parallel on the device using Celeritas while processing the remainder of the particles on the host using Geant4. In this two year project the group of “offloaded” particles will be photons, electrons, and positrons, but selected hadronic physics can integrated based on the performance of Acceleritas benchmark problems in future work.

The development of the Acceleritas library and its integration into Geant4 will be done in the first two years of the project. The major steps are (i) collecting candidate particles for offloading from the Geant4 transport loop and converting them to Celeritas primary tracks, (ii) launching the Celeritas on-device transport loop with batches of converted primaries, and (iii) merging sensitive hits and particle track data from the device to Geant4 on the host. Each of these steps is managed by the Geant4 tasking manager, in which the number of tasks in the same group is fully configurable for optimizing IO and load-balancing between device and host.

An important part of this project will be the development of an extensive verification suite that can serve both Celeritas and Acceleritas physics development and performance benchmarking.