Athena Pile-Up Requirements

Requirements for the pile-up algorithm in Athena are derived from experiences of simulating pile-up for the Physics TDR and from user requirements. Simulating pile-up is one of the most demanding jobs that the Athena framework will be required to do. Care must be taken to make sure that the implemented algorithm will meet the diverse requirements of the various subdetectors and still be able to generate events efficiently. This document looks at how ATLSIM and a special algorithm implemented by LAr subdetector were used during the Physics TDR studies to generate pile-up and also at how ATLSIM has been modified since then to be more robust while doing pile-up. Based on these experiences and detailed requirements put forth by the subdetector groups, a list of requirements on the Athena pile-up algorithm is developed.