The ATLAS Level-1 Trigger System

The ATLAS Level-1 Trigger is the first stage of event selection for the ATLAS experiment at the LHC. In order to identify the interesting collisions events to be passed on to the next selection stage within a latency of less than 2.5 us, it is based on custom-built electronics. Signals from the Calorimeter and Muon Trigger System are combined in the Central Trigger Processor which processes the overall L1 Accept (L1A) decision. The Level-1 Trigger identifies event features such as missing transverse energy, candidate electrons, photons, jets and muons. This paper presents how the Level-1 Trigger System has performed in 2011 and early 2012, covering techniques used to cope with the increasing LHC luminosity. This paper also covers some of the Level-1 Trigger upgrade plans with respect to managing the increasingly demanding LHC running conditions.