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Nuclear beta and electron capture (EC) decay serve as sensitive probes of the structure and symmetries at the microscopic scale of our Universe. As such, precision measurements of the final-state products in these processes can be used as powerful laboratories to search for new physics, as well as addressing fundamental questions of quantum mechanics at the subatomic scale.

 

For the past few years, the BeEST (Beryllium Electron capture in Superconducting Tunnel junctions) collaboration has taken the approach of embedding EC decaying radioisotopes in thin-film superconducting tunnel junctions (STJs) to precisely measure the recoiling atom that gets an eV-scale “kick” from the neutrino following EC decay. Since these recoils must conserve energy and momentum with the neutrino, they carry unique and potentially “hidden” signatures of weakly coupled beyond standard model (BSM) physics; including neutrino mass, exotic weak currents, and

potential “dark” particles created within the Q-value window of the decay. Such measurements provide a complimentary and (crucially) model-independent portal to the dark sector with sensitivities that push towards synergy between laboratory and cosmological probes. 

 

Recent advances in rare isotope beam production, combined with decades of development in sensing technology have opened exciting new experimental opportunities for conducting fundamental tests of nature involving unstable nuclei. In this presentation, I will explore the experimental concept of utilizing rare-isotope-doped STJs to investigate beyond the Standard Model physics through weak nuclear decay and showcase notable recent experimental achievements.

 

The BeEST experiment is funded by the Gordon and Betty Moore Foundation, the DOE-SC Office of Nuclear Physics, TRIUMF (Canada), EMPIR (Europe), and Lawrence Livermore National Laboratory.