A DAQ Upgrade Solution for Belle II Experiment

While the current Belle II data acquisition (DAQ) system has proven to be reliable in the first collisions delivered since May 2018, an upgrade to increase the bandwidth of the readout system is foreseen to be deployed over the next few years for higher target luminosity SuperKEKB running. Currently, the front-end electronics (FEE) of individual subdetectors are connected to the online event building system via a unified optical link called Belle2Link, offering unidirectional data readout as well as bidirectional slow control access. The FEE data are received by high-speed link boards (HSLB) and common pipelined platform for electronics readout (COPPER) boards collecting data from up to four links, forwarding to readout PCs (ROPC), which further aggregate the data and forward the data to the event builder. It is planned to keep the Belle2Link and subsystems FEE as they are, but to replace the HSLB/COPPER (including the back-end side of Belle2link) and ROPC with an upgraded high-density system with increased throughput and processing capabilities, to enable the operation at higher rates. This article presents a solution for this upgrade based on the xTCA-for-Physics (MicroTCA)-compatible concentration preprocessing and fan-out plus (CPPF+) board featuring significant data concentration and processing power, providing up to 40-Gb Ethernet output bandwidth. Belle2Link back-end firmware and subevent building are implemented in the CPPF+ board to deliver equivalent functionality as the COPPER/ROPC. The MicroTCA platform provides scalability, reconfigurability, and reliable operations, even for a future possible increase in bandwidth and potential FEE upgrades. A fully functional demonstration readout system has been constructed and verified with the time-of-propagation (TOP) detector front-end test bench set up at KEK in terms of functionality as well as performance. Results show that this solution fulfills the requirements of the DAQ upgrade and works stably and reliably in long-term tests.