Fast Tracker (FTK): A Hardware Track Finder for the ATLAS Trigger

During the 2010-2012 run of Large Hadron Collider experiment, the ATLAS trigger system was successfully operated and it contributed to several important results such as observation of Higgs boson with a mass of about 125 GeV. From 2015, collision energy will increase to 13-14 TeV and its instantaneous luminosity will reach $1$-$2\times10^{34}$cm$^{-2}$s$^{-1}$ with a 25 ns bunch crossing period. Due to the energy increase, the cross sections for SM processes are expected to get much larger. Additionally, the number of overlapping proton-proton interactions per bunch crossing, which is refereed to as pile-up, is expected to increase significantly up to about 80. Therefore it will be challenging to control trigger rates while keeping good efficiency for interesting physics events. This document summarizes the development of Fast Tracker and its tracking performance for the ATLAS experiment. The Fast Tracker is a custom electronics system that will operate at the full Level 1 accepted rate of 100 kHz and provide high quality track information at the beginning of the high level trigger. This will be performed by hardware based track reconstruction system with massive parallelism using associative memories and FPGAs. The availability of the full tracking information from Fast Tracker will ensure robust selection in high level trigger within the affordable latency available at the high level trigger and mitigation of the negative effects due to high pile-up.