Commissioning the ATLAS Level-1 Central Trigger System

The ATLAS Level-1 central trigger is a critical part of ATLAS operation. It receives the 40 MHz bunch clock from the LHC and distributes it to all sub-detectors. It initiates their read-out by forming the Level-1 Accept decision, which is based on information from the calorimeter and muon trigger processors and a variety of additional trigger inputs from detectors in the forward region. It also provides trigger summary information to the data acquisition system and the Level-2 trigger system. In this paper, we present the completion of the installed central trigger system, its performance during cosmic-ray data taking and the experience gained with triggering on the first LHC beams.