Beam test performance of the ONSEN data reduction system for the Belle II pixel detector

We present an FPGA-based online data reduction system for the pixel detector of the future Belle II experiment. At the SuperKEKB design luminosity of 8 × 10
³⁵
cm
⁻²
s
⁻¹
and a trigger rate of 30 kHz, the occupancy of the pixel detector is conservatively estimated at 3 %. With its almost 8 million channels, this corresponds to a data output rate of more than 20 GB/s after zero suppression, dominated by background. The Online Selection Nodes (ONSEN) system aims at reducing the background data by a factor of 30, in order to reach an output data rate that the Belle II event builder system can handle.