Fast Simulation for ATLAS: Atlfast-II and ISF

Monte Carlo simulations of physics events, including detailed simulation of the detector response, are indispensable for every analysis of high-energy physics experiments. As these simulated data sets must be both large and precise, their production is a CPU-intensive task. Increasing the recorded luminosity at the Large Hadron Collider (LHC), and hence the amount of data to be analyzed, leads to a steadily rising demand for simulated MC statistics for systematics and background studies. These huge MC requirements for more refined physics analyses can only be met through the implementation of fast simulation strategies which enable faster production of large MC samples. ATLAS has developed full and fast detector simulation techniques to achieve this goal within the computing limits of the collaboration. We present Atlfast-II which uses the FastCaloSim package in the calorimeter and reduces the simulation time by one order of magnitude by means of parameterizations of the longitudinal and lateral energy profile, and Atlfast-IIF with the fast track simulation engine Fatras, which achieves a further simulation time reduction of one order of magnitude in the Inner Detector and Muon System. Finally we present the new Integrated Simulation Framework (ISF) which is based on the requirement to allow to run all simulation types in the same job, even within the same sub- detector, for different particles. The ISF is designed to be extensible to new simulation types as well as the application of parallel computing techniques in the future. It can be easily configured by the user to find an optimal balance between precision and execution time, according to the specific physics requirements for their analysis.