Speaker
Description
The BeEST (Beryllium Electron capture in Superconducting Tunnel junctions) experiment searches for physics beyond the standard model (BSM) in the neutrino sector through momentum conservation in electron capture decay (EC) of Be-7 [1]. Be-7 atoms are directly implanted into Ta-based superconducting tunnel junction (STJ) sensors that can measure the energy of the recoiling Li-7 daughters with a few-eV precision. Sterile neutrinos in the keV mass range would reduce the recoil energy add a spectrum at lower energy whose shift depends on the neutrino mass and whose amplitude depends on |Ue4|2. We have measured a high-statistics Li-7 recoil spectrum with a single STJ pixel for 30 days at a low rate of 10 counts/s and excluded sterile neutrino mixing for |Ue4|^2 > 0.0002 at 90% confidence level in the 100 keV mass region [1]. This is an order of magnitude better than results from previous experiments. Surprisingly, the Li recoil spectra are broadened beyond the STJ detector resolution of ~2 eV FWHM. We have therefore started an effort to precisely model the electron energies for Li in different sites of the bcc Ta lattice of the STJ detector. We have also started to develop Al-based STJ detectors to distinguish BSM physics from known effects. In the full-scale BeEST experiment, the statistical precision to the sterile neutrino mixing |Ue4|^2 will approach O(1e-7) levels with increased number of pixels and a higher dose of 7Be. We will present our experimental approach, analytical methods, recent progress and projected experimental sensitivities.
