The first level trigger of ATLAS

Due to the huge interaction rates and the tough experimental environment of pp collisions at a centre-of-mass energy s 14 TeV and luminosities of up to 10 34 cm 2 s 1 , one of the experimental challenges at the LHC is the triggering of interesting events. In the ATLAS experiment a three-level tigger system is foreseen for this purpose. The first-level trigger is implemented in custom hardware and has been designed to reduce the data rate from the initial bunch-crossing rate of 40 MHz to around 75 kHz. Its event selection is based on information from the calorimeters and dedicated muon detectors. This article gives an overview over the full first-level trigger system including the Calorimeter Trigger, the Muon Trigger and the Central Trigger Processor.