The ATLAS Level-1 and Level-2 trigger

The trigger system of the ATLAS experiment is organized as a multilevel architecture where the event selection is performed in three steps. The first level trigger, based on coarse calorimeter and muon detector information, is designed to accept data at the LHC bunch crossing rate of 40 MHz with a maximum output rate of 100 kHz. It also gives pointers to detector regions where to find tracks and energy clusters, thus guiding the Level-2 algorithms. Data from events accepted at Level-1 are stored in digital buffers and held over the Level-2 trigger latency. A second level selection is achieved by processing and combining full granularity calorimeter, muon and inner detector information to reduce the trigger rate to ∼1 kHz. The final selection of events is based on the full data sample to perform a complete event reconstruction. This step reduces the trigger rate by a further factor of 100. This paper describes the Level-1 and Level-2 triggers in terms of hardware implementation, algorithms and trigger strategies with particular emphasis on B physics.