Commissioning of the ATLAS Trigger Event Selection with Single‐Beam and Cosmic Rays

ATLAS is one of the two general‐purpose detectors at the Large Hadron Collider (LHC). The trigger system needs to efficiently reject a huge rate of background events and still select potentially interesting ones with good efficiency. After a first processing level using custom electronics, the trigger event selection is made by the High Level Trigger (HLT) system, implemented in software. To reduce the processing time to manageable levels, the HLT uses seeded, step‐wise and fast selection algorithms, aiming at the earliest possible rejection of background events. The ATLAS trigger event selection is based on the reconstruction of potentially interesting physical objects like electrons, muons, jets etc. The recent LHC startup and short single‐beam run provided the first test of the trigger system against real data. Following this period, ATLAS continued to collect cosmic‐ray events for detector alignment and calibration purposes. Both running periods provided very important data to commission the trigger reconstruction and selection algorithms. Several tracking, muon‐finding, and calorimetry algorithms, constituting the first ATLAS trigger menu, were exercised online. The trigger decision was used to stream the events into separate samples. This event streaming facilitated the commissioning of the different ATLAS sub‐detectors. This paper will give an overview of the trigger design and its innovative features. It wil l focus on the valuable experience gained in running the trigger reconstruction and event selection in the fast‐changing environment of the detector commissioning.