The Three-Level Event Building System for the Belle II Experiment

Belle II, an experiment designed to search for physics beyond the standard model in the decay of B mesons, will soon start operation at the Super KEKB e+ e- collider. The trigger rate is expected to be 30 kHz with a margin of 10 kHz, and the raw data rate will exceed 30 GB/s. Since the system will be unable to record all data on its storage system, online reduction is required. We have developed a three-level event building system using inexpensive switches and software for traffic smoothing, enabling online event processing at a reasonable cost. The data will be recorded on the storage system about 3 GB/s after rate reduction. This paper describes the architecture and the verification result in actual beam tests.