An object-oriented simulation program for CMS

The CMS detector simulation package, OSCAR, is based on the Geant4 simulation toolkit and the CMS object-oriented framework for simulation and reconstruction. Geant4 provides a rich set of physics processes describing in detail electromagnetic and hadronic interactions. It also provides the tools for the implementation of the full CMS detector geometry and the interfaces required for recovering information from the particle tracking in the detectors. This functionality is interfaced to the CMS framework, which, via its "action on demand" mechanisms, allows the user to selectively load desired modules and to configure and tune the final application. The complete CMS detector is rather complex with more than 1 million geometrical volumes. OSCAR has been validated by comparing its results with test beam data and with results from simulation with a GEANT3-based program. It has been successfully deployed in the 2004 data challenge for CMS, where more than 35 million events for various LHC physics channels were simulated and analysed.