The CMS Simulation Software

In this paper we present the features and the expected performance of the re-designed CMS simulation software, as well as the experience from the migration process. Today, the CMS simulation suite is based on the two principal components - Geant4 detector simulation toolkit and the new CMS offline Framework and Event Data Model. The simulation chain includes event generation, detector simulation, and digitization steps. With Geant4, we employ the full set of electromagnetic and hadronic physics processes and detailed particle tracking in the 4 Tesla magnetic field. The Framework provides "action on demand" mechanisms, to allow users to load dynamically the desired modules and to configure and tune the final application at the run time. The simulation suite is used to model the complete central CMS detector (over 1 million of geometrical volumes) and the forward systems, such as Castor calorimeter and Zero Degree Calorimeter, the Totem telescopes, Roman Pots, and the Luminosity Monitor. The designs also previews the use of the electromagnetic and hadronic showers parametrization, instead of full modelling of high energy particles passage through a complex hierarchy of volumes and materials, allowing significant gain in speed while tuning the simulation to test beam and collider data. Physics simulation has been extensively validated by comparison with test beam data and previous simulation results. The redesigned and upgraded simulation software was exercised for performance and robustness tests. It went into Production in July 2006, running in the US and EU grids, and has since delivered about 60 millions of events.