Real-time digital hardware trigger system for the Central Detector of the Taishan Antineutrino Observatory

The Taishan Antineutrino Observatory (TAO) is a high-energy resolution reactor antineutrino experiment designed to measure the fine structure of the reactor antineutrino energy spectrum. It employs silicon photomultipliers (SiPMs) to detect photons produced by secondary particles from antineutrino interactions in a gadolinium-doped liquid scintillator. The physics event rate of the TAO is ∼520 Hz. However, the use of 4024 SiPM arrays results in a high dark noise event rate, leading to a total event rate of up to 1 GHz. This presents a significant challenge in the trigger system design: how to accurately and efficiently select rare effective physics events in real-time amidst a vast amount of noise. This paper introduces a fully digital hardware trigger system. The system features a flexible, reconfigurable two-level processing architecture, combined with a real-time triggering algorithm based on the multiplicity trigger criterion. The trigger system has been tested with the simulation data, and a preliminary joint test with the detector system has been completed. The results of the simulation test with a single module suggest that the trigger system can accurately extract the 1 kHz simulation physics events from the substantial amount of dark noise and upload the triggered data to the DAQ system. Besides, in the preliminary joint test, the trigger system accurately extract the given effective physics event data while compressing the hit rate of dark noise from 2 MHz to 500 Hz. The trigger system has been successfully installed and deployed at the TAO experimental site. It has undergone integrated debugging with the full-scale detector and Front-End Electronics (FEC), and preliminary data acquisition tests have been completed. The design objectives of the triggering system have been fulfilled, demonstrating its correctness and reliability in practical application scenarios.