Implementation, Operation and Monitoring of the Data Acquisition System of the Belle II Pixel Detector during Physics Data Taking in 2019 – 2022

Abstract

This paper summarizes my contribution to the real-time data taking of the pixel detector of the Belle II experiment at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. The Belle II experiment is located at the SuperKEKB particle accelerator, where electrons and positrons collide with a center-of-mass energy of 7.4 GeV. In this process, short-lived particles decay after a few micrometers. Due to a lifetime of sometimes less than a picosecond, local resolution is crucial for the complete reconstruction of the decay. The pixel detector (PXD), specifically designed for this experiment, consists of two layers, spaced 14 mm and 22 mm apart cylindrically around the interaction point. Due to the small distance and the high luminosity of 6 × 10^35 cm^−2 s^−1, many background signals are expected, which are not suitable for further analysis. Due to the slow readout process of 20 μs, signals are expected in up to 3 % of all pixels. This corresponds to a data rate of over 17 GB/s. The uniform data acquisition system of all other detectors cannot cope with this amount of data, so the ONSEN system was designed at the Justus Liebig University in Giessen. Specially matched hardware components were selected and an FPGA-based hardware platform was developed in cooperation with the IHEP research center in Beijing. Fine tuned hardware components were selected and an FPGA-based hardware platform was developed in cooperation with the IHEP research center in Beijing. The FPGA firmware is designed to reduce data using regions of interest determined by real-time event reconstruction. Thus, before data is stored, a data reduction by a factor of 30 can be performed. During data acquisition, the PXD and all components necessary for its smooth operation are constantly monitored. Software specially developed for physics and accelerator experiments is used here. The Experimental Physics and Industrial Control System (EPICS) supports a large number of devices and allows monitoring and control information to be displayed in a graphical user interface. This document summarizes the successful extension of the ONSEN system and its integration into the Belle II experiment. Monitoring mechanisms for data processing were added to the firmware to ensure smooth data acquisition and maximize efficiency. The continuous calculation of signaling pixels and detection of incomplete data sets turned out to be one of the most important criteria used to determine the detector performance. The integration process included seamless integration of the ONSEN firmware into EPICS to provide real-time information. This has proven invaluable for troubleshooting and operations by enabling rapid identification and resolution of potential problems. The ONSEN system has successfully acquired physics data, demonstrating its reliability and robustness. Over three years, between 2019 and 2022, ONSEN reliably analyzed and processed nearly 300 TB of raw data. In separate studies, test data with data rates of up to 4.8 GB/s were generated and processed by the ONSEN system without interruption. To increase the reliability of the entire PXD and to make the operation of the accelerator and the Belle II experiment as efficient as possible, the integration into EPICS of the entire PXD was reviewed. In addition to troubleshooting, mechanisms were implemented to address constraints that became apparent only after commissioning. By evaluating recurring manual workflows, these were transformed into automated processes to speed up recovery from module failures and sensor calibration.