The First Result on the BeEST keV-scale Sterile Neutrino Search

The BeEST experiment is a direct search for keV-scale sterile neutrinos using $^7$Be atoms that are implanted into superconducting tunnel junction (STJ) detectors. The two-body electron capture decay of $^7$Be produces a neutrino and a $^7$Li nucleus whose momenta and energies are uniquely determined by the mass of the emitted neutrino. We modeled the eV-scale $^7$Be decay spectrum using Voigt and Gaussian functions for $^7$Li recoil peaks, Levinger functions for atomic shaking effects, exponentially modified Gaussian functions for Auger-electron escape, and exponential functions for gamma-ray background events in the substrate. Parameters for nuclear and atomic processes are used to constrain the model shape. We applied a statistical method using the modeled spectral shape and experimental data to find evidence of keV-scale neutrino emission that results in a shift of the $^7$Li recoil peaks in the spectrum. In this talk, we present results of the statistical analysis for the first physics run data obtained with a single-pixel STJ detector, which improves current exclusion limits on keV-scale neutrinos by an order of magnitude. We will also discuss projected sensitivities of next-phase experiments including 10,000-pixel STJ detector arrays with improved energy resolution.