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).

Sterile Neutrinos on the keV Scale
  • 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 νi ≥ 4 mostly sterile mass eigenstates


Model-Dependent Cosmological Limits on keV Neutrinos

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

Decay Momentum Reconstruction as a Neutrino-Mass Probe
  • 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 νe and daughter recoil
  • Heavy neutrino admixtures to νe generates less energetic atomic recoil peaks.


Orbital Electron Capture Decay of Beryllium-7

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


The BeEST Experimental Concept

Operational Principle of Superconducting Tunnel Junctions (STJs)

(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.

In-Situ Calibration and Characterization
  • 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

Harvesting 7Be Sample at the TRIUMF-ISAC Facility

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.

Phases of the BeEST Experiment

7Li Recoil Energy Spectra from Calibrated Phase-II Data
  • 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

(left) 7Li recoil energy spectrum (red) and laser calibration signal (blue) for a single 22 hour run in the low-rate part of Phase-II.
(right) Sum of all individually calibrated spectra from a single (138 μm)2 STJ detector with a fit to all known and assumed effects in the decay (χ2/ν = 0.95).
Current and Projected Limits for the BeEST

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.