The tracking performance of the ATLAS HLT in pp collisions at the LHC

At the design luminosity, the Large Hadron Collider will have a bunch crossing rate of 40 MHz with up to 25 separate pp interactions per bunch crossing. For the ATLAS detector, to reduce this large interaction rate to the 200Hz that can be written offline a three level trigger system is employed. An essential component in achieving this large reduction is the online tracking in the High Level Trigger (HLT). The HLT consists of two components, the Level 2, and the third level, Event Filter, both of which run on farms of commodity CPUs. Level 2 is the earliest that data from the silicon detectors and the transition radiation tracker is available and the entire Level 2 must process each event within around 40 ms to achieve a 50 fold reduction in rate. To help achieve this reduction, custom fast tracking algorithms are used. The Event Filter uses components from the offline tracking to help obtain a further 10 fold reduction and achieve performance comparable to that of the offline reconstruction. Results are presented for the commisioning and performance of both the Level 2 and Event Filter tracking algorithms in pp collisions from the first few months of the LHC operation and show that the track reconstruction efficiency approaches 100% for tracks with high transverse momentum.