The ATLAS Level-1 Central Trigger System in operation

The ATLAS Level-1 Central Trigger (L1CT) system is a central part of ATLAS data-taking. It receives the 40 MHz bunch clock from the LHC machine and distributes it to all sub-detectors. It initiates the detector read-out by forming the Level-1 Accept decision, which is based on information from the calorimeter and muon trigger processors, plus a variety of additional trigger inputs from detectors in the forward regions. The L1CT also provides trigger-summary information to the data acquisition and the Level-2 trigger systems for use in higher levels of the selection process, in offline analysis, and for monitoring.

In this paper we give an overview of the operational framework of the L1CT with particular emphasis on cross-system aspects. The software framework allows a consistent configuration with respect to the LHC machine, upstream and downstream trigger processors, and the data acquisition. Trigger and dead-time rates are monitored coherently on all stages of processing and are logged by the online computing system for physics analysis, data quality assurance and operational debugging. In addition, the synchronisation of trigger inputs is watched based on bunch-by-bunch trigger information. Several software tools allow to efficiently display the relevant information in the control room in a way useful for shifters and experts. We present the overall performance during cosmic-ray data taking with the full ATLAS detector and the experience with first beam in the LHC.