Emulating the impact of additional proton-proton interactions in the ATLAS simulation by pre-sampling sets of inelastic Monte Carlo events

TL;DR

This paper introduces a pre-sampling method for simulating additional proton-proton interactions in ATLAS, significantly reducing computational costs while maintaining accuracy in physics analysis.

Contribution

A novel pre-sampling approach for inelastic interactions in Monte Carlo simulations that enhances efficiency without compromising accuracy.

Findings

20% reduction in CPU time for MC production

Accurate reproduction of reconstructed physics quantities

Efficient handling of multiple interactions per bunch crossing

Abstract

The accurate simulation of additional interactions at the ATLAS experiment for the analysis of proton-proton collisions delivered by the Large Hadron Collider presents a significant challenge to the computing resources. During the LHC Run (2015-2018) there were up to 70 inelastic interactions per bunch crossing, which need to be accounted for in Monte Carlo (MC) production. In this document, a new method to account for these additional interactions in the simulation chain is described. Instead of sampling the inelastic interactions and adding their energy deposits to a hard-scatter interaction one-by-one, the inelastic interactions are presampled, independent of the hard scatter, and stored as combined events. Consequently, for each hard-scatter interaction only one such presampled event needs to be added as part of the simulation chain. For the Run 2 simulation chain, with an average of 35 interactions per bunch crossing, this new method provides a substantial reduction in MC production CPU needs of around 20%, while reproducing the properties of the reconstructed quantities relevant for physics analyses with good accuracy.