Data acquisition system of the DEPFET detector for the Belle II experiment

The upgrade of the Belle experiment and the KEKB accelerator aims to increase the experimental data set of the experiment by the factor 50. This will be achieved by increasing the luminosity of the accelerator which requires also a significant detector upgrade. A new pixel detector based on the DEPFET¹ technology is one of the detector upgrade requirements to handle the increased reaction rate and provide better vertex resolution. One of the features of the DEPFET detector is a long integration time of 20 μs. With the expected detector occupancy of about 2%, the detector will generate about 22 GB/s of data. A custom two-level read-out system consisting of Data Handling Hybrid and Online Selection Node is built to control the detector, process data and eventually reduce amount of data by one factor of magnitude. The DHH system synchronizes all detector modules to an external clock, provided by the clock and trigger distribution system of the experiment. It performs initial data processing that includes pixel remapping, sub-event building and cluster reconstruction. Then the data are further processed in the Online Selection Node and reduced by accepting only those hits which are associated to regions of interest pointed by other tracking detectors with better time resolution. The functionality, architecture and current state of the Belle II pixel detector readout system are discussed.