Fast Shower Simulation in ATLAS

Wolfgang Ehrenfeld
DESY / ATLAS

The time to simulate $pp$ collisions in the ATLAS detector is
largely dominated by the showering of electromagnetic particles in
the heavy parts of the detector, especially the electromagnetic
barrel and endcap calorimeters.  Two procedures have been developed
to accelerate the processing time of electromagnetic particles in
these regions: (1) a fast shower parameterisation and (2) a frozen
shower library. Both work by generating the response of the
calorimeter to electrons and positrons with Geant 4, and then
reintroduce the response into the simulation at runtime.

In this talk I will review the geometry description and simulation
of the ATLAS electromagnetic calorimeters. Further I will discuss the
two acceleration procedures and their performance in detail.