Be Decay with Superconducting Tunnel Junctions*

The search for sterile neutrinos is among the brightest possibilities in our quest for understanding the microscopic nature of dark matter in our universe.  Sterile neutrinos - unlike the active neutrinos in the Standard Model (SM)  - do not interact with normal matter as they move through space, and their existence is best probed via momentum conservation with SM particles in radioactive decay.  One way to observe these momentum recoil effects experimentally is through high-precision measurements of electron-capture (EC) nuclear decay, where the final state only contains the neutrino and a recoiling atom.  This approach is a powerful method for "new physics" searches searches since it relies only on the existence of a heavy neutrino admixture to the active neutrinos - a generic feature of neutrino mass mechanisms - and not on the model-dependent details of their interactions.  The BeEST experiment precisely measures the eV-scale radiation that follows the decay of ⁷Be ions implanted into sensitive superconducting tunnel junction (STJ) quantum sensors, and currently sets the best laboratory limits on the existence of these heavy neutral leptons in the 100 - 860 keV mass region.