To select the potentially interesting events, reduce the read-out rate and store these events on computer disks Belle II relies on high performance triggering and data acqusition.
Belle II detector keeps outputting data corresponding to several hundred kilo channels typically every 8 nano second, which is too huge to record them all, regardless whether the data contains physics data or not -- each subdetector component does not know when a physics event occurs. Any data is once kept in data buffer for 5 micro seconds only and oldest one in the buffer is discarded every 8 nano second. Belle II Level 1 (L1) Trigger system is electric circuits implemented on FPGA-based readout boards to extract physics data only from continuous data flow before they are lost, and is designed to have background separation power to achieve maximum allowed rate of 30 kHz, keeping 100% detection efficiency for physics events.
Analyzing CDC, ECL, TOP and KLM data available between around 2 micro second and 4 micro second obtained after beam crossing the L1 trigger system detects a physics event and let DAQ system know that the present data in buffer is what we want to record.
Data acquisition system handles a flood of data from detectors.
Due to the luminosity increase with the SuperKEKB accelerator, handling data flow from detectors is challenging. With state-of-the-art information technology, the data acquisition system processes and analyzes the data for online data reduction and records them in the storage for further physics analysis.