The BeEST Experiment: A Search for keV-Scale Neutrinos in the EC Decay of 7Be with Superconducting Quantum Sensors

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 SM - do not couple to left-handed currents in the weak interaction, and are thus best observed via their mass-generated effects that result from momentum conservation with SM particles. 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 among the most powerful methods for BSM neutrino mass searches since it relies only on the existence of a heavy neutrino admixture to the active neutrinos, which is a generic feature of neutrino mass mechanisms, and not on the model-dependent details of their interactions. In this talk, we report the first measurements in the Beryllium EC STJ (BeEST) experimental program, which uses the decay-momentum reconstruction technique to precisely measure the $^7$Be$\rightarrow^7$Li recoil spectrum via $^7$Be ions implanted into sensitive superconducting tunnel junction (STJ) radiation detectors.