Faster simulated track reconstruction in the ATLAS Fast Chain

Abstract

The production of simulated datasets for use by physics analyses consumes a large fraction of ATLAS computing resources, a problem that will only get worse as increases in the instantaneous luminosity provided by the LHC lead to more collisions per bunch crossing (pile-up). One of the more resource-intensive steps in the Monte Carlo production is reconstructing the tracks in the ATLAS Inner Detector, which takes up about 60% of the total detector reconstruction time. This talk discusses a novel technique called track overlay, which substantially speeds up the Inner Detector reconstruction. In track overlay the pile-up Inner Detector tracks are reconstructed ahead of time and overlaid onto the Inner Detector tracks from the simulated hard-scatter event. We present our implementation of this track overlay approach as part of the ATLAS Fast Chain simulation, as well as a method for deciding in which cases it is possible to use track overlay in the reconstruction of simulated data without performance degradation, and in which it is preferable to use the current approach.