The BeEST Experiment: Searching for Beyond Standard Model Neutrinos using $^7$Be Decay in STJs

Precision measurements of nuclear $β$ decay are among the most sensitive methods to probe beyond Standard Model (BSM) physics in the neutrino sector. In particular, momentum conservation between the emitted decay products in the final state is sensitive to any massive new physics that couples to the neutrino mass. 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. The BeEST experiment precisely measures the eV-scale radiation that follows the radioactive decay of $^7$Be implanted into sensitive superconducting tunnel junction (STJ) quantum sensors. STJs are ideally suited for measurements of this type due to their high resolution at the low recoil energies in EC decay, and their high-rate counting capabilities. We present the motivation for the BeEST experiment and describe the various phases of the project.