Performance of the advanced gamma-ray trigger system for the High Energy Cosmic Radiation Detection (HERD) facility

The High Energy Cosmic-Radiation Detection (HERD) facility has been proposed as a leading experiment on China's Space Station (CSS). Scheduled for installation around 2027, HERD is expected to operate for at least a decade. The main scientific objectives include indirect detection of dark matter with unprecedented sensitivity, studying the cosmic-ray spectrum and composition up to the knee, and observing all-sky gamma rays with energies above 100 MeV. HERD is designed as a large-acceptance telescope with a unique design aimed at maximizing its efficiency. It comprises a central 3D imaging calorimeter (CALO) made of LYSO crystals, encircled by four complementary subdetectors on its top and four lateral faces: the scintillating fiber tracking detector (FIT), the plastic scintillator detector (PSD), the silicon charge detector (SCD), and a transition radiation detector (TRD) on one lateral side. To fully harness HERD gamma-ray detection capabilities down to 100 MeV, an advanced ultra-low-energy gamma-ray (ULEG) trigger system has been developed. We present an extensive overview of the design, performance, and optimization of the gamma-ray trigger system supported by software simulations and preliminary results from the successful implementation of the HERD prototype at CERN's PS and SPS beam test campaigns in Fall 2023.