The search for sterile neutrinos is among the brightest possibilities in our quest for understanding the microscopic nature of dark matter. Sterile neutrinos, unlike the active neutrinos in the Standard Model (SM), do not couple to left-handed currents in the weak interaction and are best observed via their mass–generated effects that result from momentum conservation with SM particles. This can be done 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, model-independent method in the search for beyond SM scenarios 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. The BeEST (beast) experiment employs the decay–momentum reconstruction technique to precisely measure the ⁷Be ⁷Li recoil energy spectrum in superconducting tunnel junctions (STJs).

• Natural extensions to the SM that can reconcile the small mostly active neutrino masses by adding n RH neutrinos (MSM and Type-I Seesaw)

• Generalizes the PMNS matrix to a (3 + n)×(3 + n) transformation with ν_i ≥ 4 mostly sterile mass eigenstates

Present relic abundance, limits, and regions of interest in the mass-mixing space with ν_e assuming different pre-BBN cosmologies. As shown, cosmological limits are heavily model-dependent. Need: Model-independent search for keV neutrinos in the laboratory

• The weak interaction process of orbital EC produces a two–body final state (rather than three–body in ³H β^– decay)
  • Discrete kinetic energies for the emitted ν_e and daughter recoil
• Heavy neutrino admixtures to ν_e generates less energetic atomic recoil peaks.

• ⁷Be is the ideal system to perform these studies:
  • Covers widest neutrino mass range (m_s ≤ 862 keV)
  • Simple atomic (Z = 4) and nuclear (A = 7) structure

(left) A schematic of the Ta-based STJ detector layers [Ta (165 nm) – Al (50 nm) – Al₂O₃ (1 nm) – Al (50 nm) – Ta (265 nm)].
(right) Operational principle of STJs, where radiation breaks Cooper pairs, creates excess charge-carriers, which then tunnel across the insulating barrier generating the signal current.

• Illumination from a pulsed 355 nm Nd:YVO₄ laser provides simultaneous calibration and characterization of each device
• The response of an STJ pixel to the laser consists of a Poisson distributed set of peaks that correspond to integer numbers of absorbed 3.5 eV photons
• Calibration accuracy of the spectrum is ± 1.6 meV rms in the region of interest at 5 × 10³ counts/s

⁷Be⁺ ions are implanted into STJ through small ( ~ 50 μm diameter) holes in a Si mask to a mean depth of ~ 50 nm at 10⁹ s^-1

Schematic of the TRIUMF-ISAC facility highlighting the ISAC implantation station (IIS) and ⁷Be beam collimation stage.

• First implantation of Phase-II (September 2019)
  • Limited A = 7 to 30 minutes of irradiation due to high ⁷Li beam contamination in this specific source
    • Generated low-rate data set with ≤ 10 counts/s per STJ
  • Data from ~ 20 days of semi-continuous running in the ADR
  • Detector resolution 1.4-2.0 eV in the ROI
  • First evaluation of in-medium effects on peak shapes

• Direct measurement of ⁷Be L/K capture ratio

Preliminary exclusion limits at 95% C.L. from the low-rate Phase-II data, and projections for Phase-III and -IV. The best previous limits from all other decay data are shown by colored exclusion regions.