Usage of vertex detectors in the ATLAS trigger software

The challenging environment in which the Large Hadron Collider (LHC) experiments are going to operate implies a sophisticated trigger system, capable of real-time track and vertex reconstruction. In the ATLAS experiment, the first selection stage where these ingredients are available is the software-based High-Level Trigger (HLT), which reduces its 75 kHz input to ~200 Hz in two subsequent steps: the LVL2 and the Event Filter (EF) triggers. In this contribution we present algorithms for fast reconstruction of charged tracks and vertexing in the HLT framework, including common extrapolation and fitting tools. Their application to different trigger selections and in particular to b-jet selections, used to improve the flexibility of the trigger scheme and extend its physics performance, is also discussed. Examples of performance of the presented algorithms on simulation and cosmic-ray data are given. Efficient and robust tracking capabilities are demonstrated to be achievable with average execution times well within the trigger requirements.