A multi-Hough-based displaced vertex track trigger for the Belle II experiment

Abstract The Belle II experiment at the asymmetric-energy electron positron collider SuperKEKB aims to explore physics beyond the standard model (BSM). One of the widely discussed signatures for BSM are new, long-lived neutral particles, which decay into charged mesons or lepton pairs originating from a vertex usually far from the interaction point (IP) of the colliding beams. The current level-1 (L1) track trigger system is optimized for particles created at the IP and will thus reject, with high probability, such interesting events. This makes it necessary to develop a special L1 track trigger for events with a vertex displaced from IP, the Displaced Vertex Trigger (DVT). The pipelined and deadtime-free L1 trigger system of Belle II utilizes a set of FPGA boards to make rapid decisions within 5 microseconds. The new DVT will be operating in parallel with the existing track trigger systems. The DVT identifies events, where two tracks with opposite charge originate from a common vertex away from the IP. The track finding is done by a set of Hough transformations, each one assuming a certain track origin from a grid spanning the tracking volume. The correct vertex is then determined via a shape analysis of the Hough clusters, using neural networks. To manage the large number of track origin hypotheses, a pre-selection of candidates based on the properties of the Hough map is employed, which significantly reduces the required FPGA resources.