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 sqrt(s)=14TeV 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 trigger 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. In addition, recent results are reported that have been obtained from test-beam studies performed at CERN where the full first-level trigger chain was established successfully for the first time and used to trigger the read-out of up to nine ATLAS sub-detector systems. 
Johannes Haller (CERN) (for the ATLAS TDAQ LVL1 group)