The ATLAS High Level Trigger Configuration and Steering, Experience with the First 7 TeV Collisions

In March 2010 the four LHC experiments saw the first proton-proton collisions at a center-of-mass energy of 7 TeV. Still within the year a collision rate of nearly 10 MHz was expected. At ATLAS, events of potential physics interest for are selected by a three-level trigger system, with a final recording rate of about 200 Hz. The first level (L1) 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 tests each signature on each L1-accepted event, the test outcome is recorded for later analysis. The HLT-Steering is responsible for this. It foremost ensures the independence of each signature test and an unbiased trigger decisions. Yet, to minimize data readout and execution time, cached detector data and once-calculated trigger objects are reused to form the decision. Some signature tests are performed only on a scaled-down fraction of candidate events, in order to reduce the output rate and further limit the execution time. For some signatures it is important to physics analysts to know the would-be decision of that test when it was scaled out. 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 L1 and HLT. It also archives t he 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.