Commissioning of the ATLAS High Level Trigger with proton collisions at the LHC

ATLAS is one of two general-purpose detectors at the Large Hadron Collider (LHC). The ATLAS trigger system uses fast reconstruction algorithms to efficiently reject a large rate of background events and still select potentially interesting signal events with good efficiency. After a first processing level (Level 1) using custom electronics, the trigger selection is made by software running on two processor farms, containing a total of around two thousand multi-core machines. This system is known as the High Level Trigger (HLT). 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 of background events.