ATLFAST 2.0 a fast simulation package for ATLAS

This paper documents the update of the ATLFAST program [6-1] for fast detector simulation and physics analysis. The algorithm is implemented as an independent package written in FORTRAN77 but exists also in an OO/C++ implementation. It can be used for fast event-simulation including the most crucial detector aspects: jet reconstruction in the calorimeters, momentum/energy smearing for leptons and pho-tons, magnetic field effects and missing transverse energy. It provides, starting from the list of particles in the event, a list of reconstructed jets, isolated leptons and photons and the expected missing transverse energy. Optionally, the package provides also a list of reconstructed charged tracks. The package is com-pleted by ATLFAST-B, a set of useful routines which simulate efficiencies/rejections for tagging of b-jets, c-jets and τ-jets. In most cases, the detector-dependent parameters are tuned to what is expected for the performance of the ATLAS detector from full simulation.