ATLAS Trigger: design and commissioning

The ATLAS detector at CERN's Large Hadron Collider (LHC) will be exposed to proton-proton collisions from beams crossing at 40 MHz. A three-level trigger system was designed to select potentially interesting events and reduce the incoming rate to 100-200 Hz. The first trigger level (LVL1) is implemented in custom-built electronics, the second and third trigger levels are realised in software. Based on calorimeter information and hits in dedicated muon-trigger detectors, the LVL1 decision is made by the central-trigger processor yielding an output rate of less than 100 kHz. The allowed latency for the trigger decision at this stage is less than 2.5 micro seconds. The two subsequent levels, called, High-Level Trigger (HLT) further reduce the rate to the offline storage rate while retaining the most interesting physics. The HLT is implemented in software running in commercially available computer farms and consists of Level 2 and Event Filter. To reduce the network data traffic and the processing time to manageable levels, the HLT uses seeded, step-wise reconstruction, aiming at the earliest possible rejection. The fast and simple algorithms at Level 2 trigger operate with data from small region of interest, while more flexible Event Filter allows to more precise algorithms similar to used in offline reconstruction. The trigger system and its design parameters will be described with focus on computing and data aquision challenges. The results from both commissioning cosmic runs and first experiences from the LHC beam in 2008 will be overviewed. These running periods allowed us to exercise the trigger system online, including its configuration and monitoring infrastructure, as well as reconstruction and selection algorithms. The details on the plans for commissioning the ATLAS trigger when the LHC starts operations will be presented.