CERN Accelerating science

ATLAS Note
Report number	ATL-SOFT-PROC-2024-010
Title	The ATLAS Monte Carlo detector simulation for Run 3 at the LHC
Author(s)	Chapman, John Derek (University of Cambridge (GB))
Corporate Author(s)	The ATLAS collaboration
Collaboration	ATLAS Collaboration
Publication	2024
Imprint	11 Oct 2024
Number of pages	7
In:	42nd International Conference on High Energy Physics (ICHEP 2024), Prague, Czech Republic, 18 - 24 Jul 2024
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Free keywords	Simulation ; ATLAS ; Geant4 ; Optimization
Abstract	In preparation for Run 3 at the Large Hadron Collider (LHC), the Monte Carlo Detector Simulation performed with Geant4 (Full Simulation) within the ATLAS experiment has undergone signif- icant improvements to enhance its computational performance and overall efficiency. This talk offers a comprehensive overview of the optimizations implemented in the ATLAS Full Simulation for Run 3. Notable developments include the application of EM range cuts, the implementation of Neutron and Photon Russian roulette and the recent development of the Woodcock tracking in the Electromagnetic Endcap Calorimeter (EMEC), a critical sub detector that for its peculiar structure represents a significant consumer of CPU resources in the ATLAS simulation. Beyond optimizations targeting specific detector components, the improvements include the tuning of simulation parameters (including the magnetic field), smarter and more efficient geometry de- scriptions, the implementation of new Geant4 core features and improvements that target the way Geant4 is linked and used within the ATLAS simulation framework. These enhancements collectively resulted in a remarkable achievement, with a speedup in CPU time of a factor of 2 compared to the baseline simulation configuration used in Run 2. In addition to showcasing the Run 3 ATLAS simulation, this contribution provides an overview of forthcoming optimizations, emphasizing both immediate and longer-term enhancements.