The FTK: A hardware track finder for the ATLAS Trigger

The ATLAS experiment trigger system is designed to reduce the event rate, at the LHC design luminosity of 10³⁴ cm^-2 s^-1, from the nominal bunch crossing rate of 40 MHz to less than 1 kHz for permanent storage. During Run 1, the LHC has performed exceptionally well, routinely exceeding the design luminosity. From 2015 the LHC is due to operate with higher still luminosities. This will place a significant load on the High Level Trigger system, both due to the need for more sophisticated algorithms to reject background, and from the larger data volumes that will need to be processed. The Fast TracKer is a hardware upgrade for Run 2, consisting of a custom electronics system that will operate at the full rate for Level-1 accepted events of 100 kHz and provide high quality tracks at the beginning of processing in the High Level Trigger. This will perform track reconstruction using hardware with massive parallelism using associative memories and FPGAs. The availability of the full tracking information will enable robust trigger selection within the affordable latency available at the High Level Trigger, with only a limited degradation in performance arising from the additional pileup from higher luminosity running.