FAMOS, a FAst MOnte-Carlo Simulation for CMS

An object-oriented FAst MOnte-Carlo Simulation (FAMOS) has recently been developed for the CMS (Compact Muon Solenoid) experiment to allow rapid analysis of all final states envisioned at the Large Hadron Collider while keeping a high degree of accuracy for the detector material description and the related particle interactions. After a brief description of the program architecture, the electromagnetic calorimeter fast simulation is emphasized. The material effects in the tracking device, the shower and detector simulation as well as the first results and optimizations are discussed. DETAILED VS. FAST SIMULATION In 2005, CMS will publish its Physics Technical Design Report (TDR). A large amount of simulated data will be required to carry out the analysis. The main tool up to now available to generate these events is the detailed simulation. The CMS detailed simulation The detailed simulation of CMS consists of three programs. The event generator simulates a proton-proton or heavy ions collision with a centre-of-mass energy of 14 TeV, then OSCAR (Object oriented Software for CMS Analysis and Reconstruction), computes the propagation and the interaction of the generated particles in the detector. The OSCAR program is based on Geant4 [1] and contains an accurate description of the detector geometry and of its materials. The digitization, i.e., the simulation of the read-out electronic response, is made by the reconstruction program ORCA (Object Oriented Reconstruction for CMS Analysis). In the process, pile-up events can be superimposed to the hard collision to simulate the multiple collisions occuring at each bunch crossing. The typical timing of this “full” simulation is between four minutes for a Z → ee event and ten minutes for a Z → qq̄ event on a 1GHz computer. With such a timing, it will be impossible to simulate all the samples needed for the physics TDR. As a result, a fast simulation able to generate quickly, large and reliable samples is needed.