Center for Underground Physics

I will present the design, construction, and first physics results of the Kyungpook National University Advanced Positronium Annihilation Experiment (KAPAE) Phase II detector, a compact setup built to search for invisible decays of positronium. Such decays are predicted in a broad class of dark-sector scenarios—including dark photons, axion-like particles, mirror dark matter, milli-charged particles, and extra dimensions—and provide a clean, table-top way to probe physics beyond the Standard Model.

Compared to KAPAE Phase I (optimized for rare visible decays), Phase II was redesigned for maximum sensitivity to para-positronium (p-Ps) invisible modes. The detector features a 5×5 BGO crystal array surrounding a polyethylene naphthalate (PEN) film positron trigger, with minimal dead area, improved optical coupling, and cryogenic shielding to suppress environmental and thermal backgrounds. A high-rate, low-noise DAQ was implemented with pedestal stabilization, unified charge-integration windows, and pulse-shape discrimination for robust event selection. The experiment operates underground at Yemi Laboratory and is supported by Geant4 simulations tuned to match measured resolutions and response.

Using a combined dataset from ground and underground runs, and a validated simulation–analysis chain, KAPAE-II sets new experimental upper limits (90% C.L.) on both total and partial invisible decays of p-Ps. The total invisible limit improves upon the previous benchmark (ETH Zurich), while the partial invisible limits surpass earlier HPGe-based studies relevant to dark-photon scenarios. These results tighten constraints on dark sectors and demonstrate that laboratory-scale, modular detectors can deliver competitive sensitivity to rare positronium channels. I will conclude with the key systematics, cross-checks, and the upgrade path toward higher statistics, faster timing, and expanded model coverage.