Application of FPGAs to triggering in high energy physics

Abstract

The High Luminosity upgrade of the LHC will increase the instantaneous luminosity to 5 × 10 34 cm −2 s −1 , resulting in an increase in the number of simultaneous proton-proton collisions per event (pileup) to the range 140-200. The CMS Level 1 Trigger system will be upgraded, and will reduce the 40 MHz event rate to 750 kHz. The system will perform a fast event reconstruction on FPGA devices, and select events for read out within a latency of 12.5 μs. The high level FPGA programming tool MaxCompiler is investigated for use in Level 1 Trigger applications. An existing trigger algorithm, originally developed with a Hardware Description Language, is reimplemented using MaxCompiler and compared to the original. Bitwise agreement between the outputs is observed, with half as many lines of code, at the expense of some extra FPGA resources. A hardware demonstration of a proposed Level 1 track reconstruction is presented, with a Kalman Filter track fit developed with MaxCompiler. The performance of the tracking is investigated, as well as the potential for developing advanced algorithms with low latency using the tool. A high tracking efficiency, and precise parameter resolutions, are achieved with a 3.7 μs latency in high pileup events. A boosted decision tree classifier, implemented with inference latency of a few clock cycles, is presented as a means to reject fake tracks. After the Level 1 Trigger, events are further processed on commodity PCs in the High Level Trigger (HLT). The High Luminosity LHC will also challenge the HLT, which is projected to require twenty times the processing power used during LHC Run II. Part of the HLT tracking is ported to Maxeler Dataflow Engines (DFEs), a hardware acceleration technology. A faster rate of processing is achieved, but with an initial latency of the host-DFE communication that limits the performance. Steps which might yield acceleration are identified.