The Performance of Belle II Data Acquisition System in the First Physics Run

The Belle II experiment is a new generation B-factory experiment at KEK in Japan aiming at the search for New Physics in a huge sample of B-meson dacays. The commissioning of the SuperKEKB accelerator was started in 2017 (Phase 1), and then a pilot run with outer detectors was performed in 2018 (Phase 2). From March this year, an operation with the full detector has been started for the physics data taking (Phase 3). The Belle II data acquisiton system (DAQ) is now fully working in the Phase 3 run. The system consists of the trigger timing distribution system, the common readout modules (COPPER) with the unified optical data link to the detector front end, the readout PCs and the network switch complext for the event building, and the modularized high level trigger (HLT) and storage system. The system features the real time feedback of the track information obtained by HLT to the readout system of the pixel detector for the data reduction. The particle tracks reconstructed using the silicon vertex detector (SVD) and the central drift chamber (CDC) signals are extrapolated to the surface of the pixel detector (PXD) by the real time HLT processing, and only the hits in the sensors associated with the tracks are merged with the data from other detectors after the HLT processing (the 2nd level event building). The first physics data taking has been performed in Phase 3 run for 4 months. The typical level 1 rate is up to 3.5 kHz and the data taking efficiency is more than 90 % when the accelerator operation is stable. The HLT processing is fully implemented and the event reduction rate is about 1/8. However, various troubles occured in both the detector readout and in the HLT operation. The debugging and improvement are now in progress to stabilize the system for the autumn run.