We tested the feasibility of an experiment to search for keV sterile neutrino dark matter with superconducting radiation detectors. We implanted radioactive 7Be nuclei into the detectors, which decay by electron capture with a half-life of 53 days. The electron neutrino produced in this decay escapes from the detector enabling recoil energy measurement of the 7Li daughter nucleus with high accuracy. The recoil spectrum consisted of four peaks: two due to capture of a 1s electron ("K-capture") and two due to capture of a 2s electron ("L-capture") for decay into the 7Li ground state and the 7Li excited state. If the electron neutrino contains some admixture of a keV sterile neutrino, the 7Li recoil energy will shift and produce an additional peak in the spectrum for which energy depends on the sterile neutrino mass and amplitude depends on the neutrino admixture.
Our measurements demonstrated the feasibility of this experimental approach for use as a sterile neutrino dark matter search. We measured a recoil spectrum with the expected shape, although the detector resolution had degraded after 7Be implantation, most likely due to detector damage by large amounts of 7Li in the beam. Despite this outcome, this feasibility study already exceeds the sensitivity of all other nuclear physics sterile neutrino searches. We have also shown the path to further increase the sensitivity of sterile neutrino coupling searches by several orders of magnitude in the ~100 keV scale.
Our project leveraged Lawrence Livermore National Laboratory's core competencies in nuclear, chemical, and isotopic science and technology, and the results have wide ramifications for future particle physics and cosmology studies. In addition, our detector technology supports accurate measurements of nuclear materials applicable to Laboratory mission areas such as counterterrorism.
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