Performance of the ATLAS Trigger System

The ATLAS Trigger System reduces the event rate from the bunch-crossing rate of 40MHz to an average recording rate of 200Hz by selecting primarily high-p_T events. The ATLAS Trigger is composed of three levels. The first level (L1) is implemented in custom-built electronics, the two-stage High Level Trigger (HLT) is implemented in software executed on large computing farms. The L1 consists of calorimeter, muon and forward triggers to identify electron, photon, jet and muon candidates, as well as event features such as missing transverse energy. These inputs are used by the L1 Central Trigger to generate an L1 Accept (L1A) decision. L1A and timing information is sent to all sub-detectors and summary information is sent to the subsequent levels of the Trigger System. In this paper the performance of the ATLAS Trigger System in 2010 and 2011 is presented.