The Fast Tracker Real Time Processor and Its Impact on the Muon Isolation, Tau amp; b-Jet Online Selections at ATLAS

As the LHC luminosity is ramped up to the SLHC Phase I level and beyond, the high rates, multiplicities, and energies of particles seen by the detectors will pose a unique challenge. Only a tiny fraction of the produced collisions can be stored on tape and immense real-time data reduction is needed. An effective trigger system must maintain high trigger efficiencies for the physics we are most interested in, and at the same time suppress the enormous QCD backgrounds. This requires massive computing power to minimize the online execution time of complex algorithms. A multi-level trigger is an effective solution for an otherwise impossible problem. The Fast Tracker (FTK)[1], is a proposed upgrade to the current ATLAS trigger system that will operate at full Level-1 output rates and provide high quality tracks reconstructed over the entire detector by the start of processing in Level-2. FTK solves the combinatorial challenge inherent to tracking by exploiting massive parallelism of associative memories [2] that can compare inner detector hits to millions of pre-calculated patterns simultaneously. The tracking problem within matched patterns is further simplified by using pre-computed linearized fitting constants and leveraging fast DSPs in modern commercial FPGAs. Overall, FTK is able to compute the helix parameters for all tracks in an event and apply quality cuts in less than 100 microseconds. The system design is defined and st udied with respect to high-PT Level-2 objects: b-jets, tau-jets, and isolated leptons. We test FTK algorithms using ATLAS full simulation with WH events up to the Phase I luminosity and beyond, comparing FTK results with the offline tracking capability. We present the architecture and the reconstruction performance for the mentioned high-PT Level-2 objects. [1] [1] A. Andreani et al., The FastTracker Real Time Processor and Its Impact on Muon Isolation, Tau and b-Jet Online Selections at ATLAS, Conference Record 17th IEEE NPSS Real Time Conference Record of the 17th Real Time Conference, Lisbon, Portugal, 24 - 28 May 2010. [2] A. Annovi et al., "A VLSI Processor for Fast Track Finding Based on Content Addressable Memories ", IEEE Trans. Nucl. Sci. 53, 2428 (2006)