Common Readout Subsystem for the Belle II Experiment and Its Performance Measurement

The Belle II experiment is the major upgrade of the Belle experiment, which verified Kobayashi-Maskawa mechanism to explain the violation of charge and parity symmetry in the Standard Model. The Belle II experiment will focus on a search for physics beyond the standard model with the upgraded high-luminosity electron-positron collider, SuperKEKB at KEK. The readout subsystem of the Belle II data-acquisition system needs to receive a large amount of data from different front-end electronics (FEEs) of several subdetectors and process and send the data to a high-level trigger system which performs event-rate reduction. The common readout subsystem for the Belle II experiment was developed for all subdetectors except for the innermost pixel detector. The subsystem consists of readout boards with high-speed serial links and readout PC servers. In the Belle II experiment, once event building is performed, events are distributed over processor nodes, which can be scaled up rather easily during the experimental period. For the readout subsystem before the event builder, however, that event-based parallel processing is impossible. Therefore, the performance of the readout subsystem should be checked carefully before the experiment starts. To check the performance of the readout subsystem, we performed a stress test with dummy data produced by field-programmable gate array on FEEs and readout boards. We used one unit of the readout subsystem for the test, which consists of one PC server and several readout boards. In this test, the system achieved to handle the target event rate of 30 kHz with loss of very few events. The readout test with some of the Belle II subdetectors was also performed in cosmic-ray tests measurements. The readout subsystem worked stably and the basic functionality was tested with real data from the subdetectors.