New Techniques for Pile-up Simulation in ATLAS

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 the 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 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-simulated set of minimum bias interactions, either from simulation or from real data, is created once and events drawn from this are overlaid with the hard-scatter event being simulated. This leads to significant improvements in CPU time. The 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.