Mike Hildreth (University of Notre Dame (US))
The CMS Simulation uses minimum bias events created by a "standard" event generator (e.g., Pythia) to simulate the additional interactions due to peripheral proton-proton collisions in each bunch crossing at the LHC (also known as pileup). Due to the inherent time constants of the CMS front-end electronics, many bunch crossings before and after the central bunch crossing of interest must be included in the simulation, leading to hundreds of minimum bias events being used for each simulated hard-scatter event. We report on the performance gains in I/O load and computational speed made possible by a new framework that allows the combination of pileup events in a "pre-mixing" step. This has been made possible by the development of software that allows single-channel information to be combined at the digitization level in each sub-detector, rather than accumulating simulated hits from Geant. The logistics of large-scale production with pre-fabricated pileup distributions is described.