The ATLAS Trigger System: Past, Present and Future

The ATLAS trigger has been used very successfully for the online event selection during the first run of the LHC between 2009-2013 at a centre-of-mass energy between 900 GeV and 8 TeV. The trigger system consists of a hardware Level-1 (L1) and a software based high-level trigger (HLT) that reduces the event rate from the design bunch-crossing rate of 40 MHz to an average recording rate of a few hundred Hz. We will briefly review the performance of the ATLAS trigger system during the past data-taking period and point out the challenges for the trigger system during the next LHC run in early 2015 with a smaller bunch spacing, almost twice the centre-of-mass energy and higher peak luminosity. We will show the ongoing improvements and upgrades to the existing system that will ensure an even better performing trigger system despite the harsher machine conditions. This includes changes to the L1 calorimeter trigger, the introduction of a new L1 topological trigger module, improvements in the L1 muon system and the merging of the previously two-level HLT system into a single event filter farm. In addition, we will give an overview of the algorithmic improvements in the various HLT algorithms used to identify leptons, hadrons and global event quantities like missing transverse energy.