The ATLAS High Level Trigger Configuration and Steering: Experience with the First 7 TeV Collision Data

In March 2010 ATLAS saw the first proton-proton collisions at a center-of-mass energy of 7 TeV. Within the year, a collision rate of nearly 10 MHz was achieved. At ATLAS, events of potential physics interest are selected by a three-level trigger system, with a final recording rate of about 200 Hz. The first level (LVL1) is implemented in customized hardware, the two levels of the high level trigger (HLT) are software triggers. For the ATLAS physics program more than 500 trigger signatures are defined. The HLT Steering is responsible for testing each signature on each LVLl-accepted event, the test outcome is recorded with the event for later analysis. The steering code also ensures the independence of each signature test and an unbiased trigger decision. To minimize data readout and execution time, cached detector data and once-calculated trigger objects are reused to form the decision. In order to reduce the output rate and further limit the execution time, some signature tests are performed only on an already down scaled fraction of candidate events. For some of these signatures it is important for physics analysts to know the would-be decision on the event fraction that was not analysed due to the down-scaling. For this the HLT-Steering is equipped with a test-after-accept feature. The HLT-Steering receives the setup of the signatures from the trigger configuration system. This system dynamically provides the online setup for the LVL1 and HLT. It also archives the trigger configuration for analysis, which is crucial for understanding trigger efficiencies. We present the performance of the steering and configuration system during the first collisions and the expectations for the LHC phase 1.