New techniques for pile-up simulation in ATLAS

Tadej Novak

doi:10.1051/epjconf/201921402044

Abstract

The high-luminosity data produced by the LHC leads to many proton-proton interactions per beam crossing in ATLAS, known as pile-up. In order to understand the ATLAS data and extract physics results it is important to model these effects accurately in the simulation. As the pile-up rate continues to grow towards an eventual rate of 200 for the HL-LHC, this puts increasing demands on the computing resources required for the simulation and the current approach of simulating the pile-up interactions along with the hard-scatter for each Monte Carlo production is no longer feasible. The new ATLAS “overlay” approach to pile-up simulation is presented. Here a pre-combined set of minimum bias interactions, either from simulation or from real data, is created once and a single event drawn from this set is overlaid with the hard-scatter event being simulated. This leads to significant improvements in CPU time. This contribution will discuss the technical aspects of the implementation in the ATLAS simulation and production infrastructure and compare the performance, both in terms of computing and physics, to the previous approach.

Highlights

In addition to the hard-scatter pp interaction which causes the event to be triggered, the ATLAS detector [1] is sensitive to proton-proton collisions in the same or surrounding bunch crossings
The Transition Radiation Tracker (TRT) energy deposits in drift tubes from pile-up and the hard-scatter event cannot be directly added together, because the information is stored with coarser granularity in the pre-mixed Raw Data Objects (RDOs)
Monte Carlo (MC)+MC overlay is a promising method of pile-up simulation which will help ATLAS computing cope with the ever increasing number of simulated proton-proton collisions

Summary

Introduction

In addition to the hard-scatter pp interaction which causes the event to be triggered, the ATLAS detector [1] is sensitive to proton-proton collisions in the same or surrounding bunch crossings. Compared to the current digitisation, simulated pile-up events are pre-mixed in an independent step and hard-scatter events are overlaid on the merged background. For any given hard scattering interaction the additional pile-up interactions must be included in a realistic model of detector response They are simulated separately at the event generation and simulation stages. Each simulated hard-scatter event is digitised and overlaid on pre-mixed pile-up digits at the overlay stage. This will not present a large increase in needed storage since the current pileup simulation requires 80 copies of the minimum bias dataset, which at 40 TB each give rise to 3.2 PB in total

Physics performance

Conclusions and outlook