The ATLAS trigger

The ATLAS experiment’s physics program requires the trigger to reject a large fraction of background events, primarily QCD processes, while efficiently selecting interesting events. ATLAS employs a three-level trigger to bring the event rate from the 40 MHz collision rate to 75 kHz at Level 1 (L1), 3 kHz at Level 2 (L2) and 200 Hz at the third level, known as the Event Filter (EF), for a total rejection of O(10⁵). L1 is a hardware-based trigger that selects high momentum muons and uses reduced granularity information from the calorimeter to select jets, electrons and taus. L1 passes the position of the triggered object to L2, a software trigger, where the full granularity of the detector is used for reconstruction in a region around the triggered Level1 object. Correlations between objects can also be accessed, allowing for multi-object triggers. At the EF the full event can be reconstructed. The selected events, with a size of ∼1.5 MB, are written to local disk at 300 MB/s and simultaneously copied to the CERN Advanced STORage system (CASTOR) where they are retrieved for full offline reconstruction to be performed. In this paper the design of the ATLAS trigger, the status of the installation, the techniques used to prepare the system for data-taking and the initial performance will be discussed.