Search for Invisible Decay Modes in Nuclear Gamma Cascades of a ${}^{46}$Sc Source Using a CsI (Tl) Detector

TL;DR

This work tackles the question of whether hidden decay channels exist in nuclear gamma cascades by performing a high-statistics missing-gamma search with a centrally placed $^{46}$Sc source inside a ~100 kg CsI(Tl) detector. The method relies on tagging one gamma at 1120 keV and probing for the accompanying 889 keV photon; invisible decays would manifest as missing photons, enabling sensitivity to light dark-sector particles such as ALPs, dark photons, scalar mediators, and milli-charged particles in the sub-MeV range. In ~1000 hours of data, the measured missing-gamma fraction shows an excess relative to simulation that is largely explained by DAQ pile-up, yielding Z = 1.73 and no evidence for new physics. The results validate the missing-gamma technique and motivate a ton-scale, systematically controlled upgrade to extend sensitivity into unexplored regions of dark-sector parameter space, including reactor-beam-dump and isotope-based environments.

Abstract

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 $γ$ 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 ${}^{46}$Sc 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. With approximately 1000~hours of data collected using the current setup, no statistically significant excess was observed, consistent with existing constraints on dark-sector candidates such as axions and axion-like particles (ALPs), dark scalars, and dark photons in the sub-MeV mass range. This result highlights the sensitivity of the missing-gamma technique and motivates the development of a larger-scale detector with improved systematic control to extend dark-sector searches into previously unexplored regions of parameter space.

Figures (7)

• Figure 1: Experiment design Missing_Gamma2. Schematic diagram of the stacked CsI crystal assembly designed to efficiently contain $\gamma$ rays emitted from the radioactive source positioned at its center.
• Figure 2: Detector Assembly with 100 Kg scale CsI (Tl) with attached PMTs.
• Figure 3: Spectrum of ${}^{46}$Sc co-adding all the channels.
• Figure : (a) ALPs
• Figure : (a) ALPs