Development of the KAPAE II Detector for New Particle Search in Positronium Decay

he positronium annihilation experiment is important for searching for new particles through invisible decay channels. It allows the study of milli-charged particles, mirror worlds, and extra dimensions via totally invisible decay modes, as well as axion-like particles, dark photons, and dark Z bosons through partially invisible decay modes. The Kyungpook National University Advanced Positronium Annihilation Experiment (KAPAE) aims to study positronium annihilation, focusing on both visible and invisible exotic decay processes. The KAPAE phase II detector enhances sensitivity to the invisible decay of positronium by reducing dead areas and optimizing the channel configuration. It is composed of a 5 x 5 array of bismuth germanate (BGO, Bi4Ge3O12) scintillation crystals, each measuring 30 x 30 x 150 mm3, resulting in an overall size of approximately 150 x 150 x 150 mm3. Simulations suggest an upper limit of sensitivity for invisible decay of approximately 2.7 x 10-9 (90% C.L.). This paper presents the optimization, data acquisition system, construction, and performance testing of the KAPAE II detector.