Search for Dark Sector Particles Using CsI(Tl) Detectors and Radioactive Sources via the Missing-Gamma Approach

Dark matter remains one of the most compelling open questions in modern physics, motivating extensive searches for new light, weakly coupled particles beyond the Standard Model. Numerous experiments have probed a variety of dark matter candidates using diverse detector technologies, yet large regions of parameter space remain unexplored. In this work, we present a novel high-statistics search for invisible decay modes in nuclear gamma cascades, employing approximately 100~kg of CsI(Tl) scintillators at Texas A\&M University. The key feature of this experiment is the use of a high-activity ⁴⁶Sc radioactive source combined with a "missing gamma'' technique, in which the absence of a photon in a well-identified cascade serves as a signature of new physics. Unlike appearance--disappearance experiments, this approach requires only a single photon conversion into a dark-sector particle, thereby enabling sensitivity to weaker couplings. The setup provides simultaneous reach for several dark matter candidates, including axions and axion-like particles (ALPs), dark scalars, dark photons, and milli-charged particles. By carefully controlling detector containment and environmental backgrounds, this experiment aims to search in the  previously inaccessible parameter space and establishes a path toward future ton-scale detectors capable of testing astrophysical hints and closing remaining gaps in laboratory constraints.