R&D status of the Zero Degree Calorimeter for the BESIII Experiment

The Beijing Spectrometer III (BESIII) experiment at the Beijing Electron-Positron Collider II (BEPCII) carries out precision measurements in tau-charm physics, hadron spectroscopy, nucleon form factors, and corrections to the muon anomalous magnetic moment. Initial State Radiation (ISR) return method provides a unique path to access lower center-of-mass energies without interrupting high-energy data taking, which will greatly support the BESIII energy-scan programs. However, the standard BESIII electromagnetic calorimeter (EMC) has low efficiency for forward ISR photons (∣ cos θ∣ > 0.99), and untagged ISR analyses suffer from large backgrounds and poor precision. A dedicated Zero Degree Calorimeter (ZDC) in the very forward region will strongly enhance ISR photons detection. This paper reports the full R&D status including detector design, simulation, readout electronics, prototyping, laboratory tests, beam-test trials, and future timeline of the BESIII ZDC project. The ZDC uses a radiation-hard LYSO+SiPM array with modular design to profile electromagnetic showers and achieve good energy and position resolution. Extensive tests have validated the design. Joint test, mass production and final commissioning are expected to be done by the end of 2026. The ZDC also serves as an R&D platform for future collider projects in China, including the Circular Electron-Positron Collider (CEPC) and the Super Tau-Charm Facility (STCF).