Searching for Invisible Decay Modes of Nuclear Excited States with the BeEST

The BeEST searches for sterile neutrinos using Superconducting Tunnel Junctions (STJs) to measure the energy of the nuclear recoil produced in the electron capture (EC) decay of ⁷Be. The BeEST’s high-resolution energy spectroscopy capabilities enable the detector to search for beyond the Standard Model (BSM) physics through deviations of the observed recoil spectrum from expectations. In approximately 10% of decays, the initial decay will populate an excited state in the ⁷Li daughter, which typically de-excites by emitting a 478 keV ray. It is possible that this decay mode could instead emit a massive particle in the dark sector with a very small branching ratio. This presentation describes the search for missing energy in the de-excitation of ⁷Li using data from the BeEST, which could result from the emission of a BSM massive scalar particle.