Impact of different primary particles on Geant4 simulation execution time - study on protons and pions

Abstract: The High Luminosity Large Hadron Collider (HL-LHC) at CERN, an improvement to the LHC, scheduled to be operating in 2027, will further the potential for scientific breakthroughs in High Energy Physics. The project’s demand in computational resources, however, is predicted to exceed what will be available then. Thus, efficiency optimization of Monte Carlo simulations of the collisions that occur in the LHC experiments, which today occupy 40% of those resources and are pivotal for the correct interpretation of the collected data, is the aim of recent research and development efforts. This study focuses on the ATLAS experiment, specifically, and makes use of the Geant4 simulation software and its extensive libraries, used at LHC to simulate the passage of particles through matter. It aims at providing useful data for future full scale studies on software time response improvement. Parameters such as the type of build method - static or dynamic - and version of the GCC compiler, from recent studies, have been shown to have a considerable impact in reducing the execution time of the simulations. This research carries forward this analysis and studies the impact of different primary particles, which are created in the pp collisions, on the simulation time, specifically the pions, π+/−, and protons, p. The software’s virtual particles, geantino and charged geantino, were also studied. A simulation benchmark was used, with a simplified version of the ATLAS detector, and was run through the Aurora cluster at Lund University. This was carried out for 10 and 20 GeV particles, using both static and dynamically compiled libraries. The statically compiled simulations were confirmed to decrease time by 10%, as was foreseeable. In addition, all considered particles exhibit simulation time distributions which agree with what would be expected from theory. The virtual particles confirm the large contribution that the simulation of interactions in the detector has on the execution time. Moreover, both the negative and positive pions registered a mean execution time about 4% smaller than the proton’s, in agreement with the pions’ smaller probability of interaction.