The ATLAS Trigger System

ATLAS is one of two general-purpose detectors at the next generation proton-proton collider, the LHC. The high rate of interactions and the large number of read-out channels make the trigger system for ATLAS a challenging task. The initial bunch crossing rate of 40~MHz has to be reduced to about 200 Hz while preserving the physics signals against a large background. ATLAS uses a three-level trigger system, with the first level implemented in custom hardware, while the high level trigger systems are implemented in software on commodity hardware. This note describes the physics motivation, the various selection strategies for different channels as well as the physical implementation of the trigger system.