P5 recognized neutrino oscillation, and the inferred nonzero neutrino masses, as the most unambiguous current evidence of physics beyond the Standard Model. HUNTER will mount an incisive search for sterile neutrinos in the 30–280 keV/c2 mass range by missing-mass reconstruction of K-capture decays from 131-Cs. The experiment combines techniques from disparate physics areas to yield the required missing mass resolution. Atomic physics techniques are key, with isotope selective trapping, laser cooling, magneto-optical trapping, and possibly optical pumping providing the 131-Cs source. Reaction Ion Microscope Spectroscopy [RIMS] gives the necessary momentum resolution for the recoil 131-Xe ions and Auger electrons. The atomic x-ray vector momentum is measured with thin scintillator pixel arrays.

Generating the very small active neutrino masses is a challenge theoretically. The “see-saw mechanism” achieves this by adding additional undiscovered neutrinos, usually heavy, right-handed, and lacking charged and neutral current weak interactions, hence “sterile”, but coupled to known particles only through the neutrino mass mixing matrix. In many theories sterile neutrinos are also able to solve other outstanding problems in physics, notably the baryon-antibaryon asymmetry (via leptogenesis). Theory does not provide any compelling guide as to the mass of a light sterile neutrino, but a mass in the keV range is not ruled out by any existing data, and also would provide an interesting Warm Dark Matter candidate.

Present laboratory limits on the sterile/active-neutrino mixing range from ∼ 10-4 to ∼ 10-2 . Our initial proposed configuration will surpass these limits, and an upgrade path exists to extend the sensitivity by many orders of magnitude. That would allow HUNTER to eventually probe the much more stringent mixing angle constraints from astrophysical x-ray data, which are required if sterile neutrinos are to make up the dark matter in the present universe.

Weak interaction experiments have been performed with optically-trapped sources before, but never with the level of precision for full kinematical reconstruction that is proposed here. Our W. M. Keck Foundation grant and institutional contributions cover most of the base equipment for the initial measurement.