Fast Tracking for the ATLAS LVL2 Trigger

We present a set of algorithms for fast track reconstruction at the second level (LVL2) trigger of ATLAS, using three-dimensional space points (= hits) from the silicon trackers. The strategy is to determine the position z_0 of the interesting pp interaction along the beam axis prior to any track reconstruction and then retain only groups of hits which point back to that z_0 and perform combinatorial tracking only inside those groups. We give results and discuss the advantages of this approach, which is generic enough to be applicable to other multi-collision experiments. We also make a qualitative comparison with a complementary approach which is based on Look-Up Tables (LUT) and is also used in ATLAS.