Abstract
We propose a high statistics experiment to search for invisible decay modes in nuclear gamma cascades. A radioactive source (such as $^{60}$Co or $^{24}$Na) that triggers gamma cascades is placed in the middle of a large, hermetically sealed scintillation detector, enabling photon identification with high accuracy. Invisible modes are identified by establishing the absence of a photon in a well-identified gamma cascade. We propose the use of fast scintillators with nanosecond timing resolution, permitting event rates as high as $10^{7}$ Hz. Our analysis of the feasibility of this setup indicates that branching fractions as small as $10^{-12} - 10^{-14}$ can be probed. This experimental protocol benefits from the fact that a search for invisible modes is penalized for weak coupling only in the production of the new particle. If successfully implemented, this experiment is an exquisite probe of particles with mass below $\sim$4 MeV that lie in the poorly constrained supernova "trapping window" that exists between 100 keV - 30 MeV. Such particles have been invoked as mediators between dark matter and nucleons, explain the proton radius and $(g-2)_{\mu}$ anomalies and potentially power the shock wave in type II supernovae. The hadronic axion could also be probed with modifications to the proposed setup.
Highlights
Coupled particles with mass below the megaelectron-volt scale arise in a number of extensions of the Standard Model
High energy colliders do not have the statistical sensitivity to search for these particles—while they operate well above the energy threshold necessary to produce these particles, colliders do not have the luminosity to overcome the small coupling of these particles
We propose a new method for a high statistics search for such particles
Summary
The scheme would have to distinguish the signal from a variety of systematic backgrounds such as misidentification of the gammas, potential confusion introduced by soft-Compton scattering, and population of other nuclear levels by the decaying source. The feasibility of such an experiment is the focus of this paper. Several dark matter experiments are presently under development to search for interactions between the dark matter and the Standard Model mediated by particles around this mass scale Particles in this parameter space have been invoked to explain the proton radius and ðg − 2Þμ anomalies.
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