Design of the Global Reconstruction Logic in the Belle II Level-1 Trigger system

Abstract

The Belle II experiment is designed to search for physics beyond the Standard Model by investigating rare decays at the SuperKEKB e+e− collider. Owing to the significant beam background at high luminosity, the data acquisition system employs a hardware-based Level-1 Trigger to reduce the readout data throughput by selecting collision events of interest in real time. The Belle II Level-1 Trigger system utilizes FPGAs to reconstruct various detector observables from the raw data for trigger decision-making. The Level-1 trigger rate is limited to 30 kHz at the maximum expected instantaneous luminosity of SuperKEKB. To ensure reliable B physics measurements, the trigger efficiency for hadronic events, specifically, e+e−→BB̄ and e+e−→qq̄ processes, where q=u,d,s,c, must be close to 100%. The Global Reconstruction Logic receives these observables from four sub-trigger systems and provides a global summary for the final trigger decision. Its logic encompasses charged particle tracking, matching between sub-triggers, and the identification of special event topologies associated with low-multiplicity decays. This article discusses the hardware devices, FPGA firmware, integration with peripheral systems, and the design and performance of the trigger algorithms implemented within the Global Reconstruction Logic.