The ATLAS Level-1 Central Trigger

The ATLAS Level-1 trigger system is responsible for reducing the anticipated LHC collision rate from 40 MHz to less than 100 kHz. The custom-built electronics of the ATLAS Level-1 Central Trigger receives inputs from the ATLAS Level-1 Triggers and the LHC. The Level-1 calorimeter triggers are based on coarse detector information to identify high-ET jets, electrons/photons and hadrons, along with missing and total energy. In addition there are dedicated muon and forward detectors, providing triggers for different energy thresholds. The Level-1 Central Trigger combines these trigger inputs to form a Level-1 accept. This, along with trigger summary information, is then passed onto the higher levels of the trigger. From the LHC itself the Level-1 Central Trigger passes the bunch clock to all ATLAS sub-detectors. We present how the rigger information, along with dead-time rates, are monitored and logged by the online system for physics analysis, data quality assurance and operational debugging. Also presented are the software tools used to efficiently display the relevant information in the control room in a way useful for shifters and experts. In preparation for proton beams, the ATLAS detector was commissioned by recording cosmic ray data. For proton beams, electrostatic button pick-up detectors were used to clock the arriving proton bunches, allowing fast timing in of the ATLAS detector. The first proton collision events were the n triggered using signals from forward scintillator trigger detectors. The performance of the level-1 trigger during the commissioning of the ATLAS detector with and without beams will be presented.