Absolute neutrino mass measurements are of central importance to both particle physics and astrophysics. The search has taken on a new urgency since the discovery through neutrino oscillation measurements that these particles indeed have non-zero masses, providing the first solid evidence of physics beyond the standard model. The best and most direct upper limits to date for the electron antineutrino mass come from electrostatic beta spectrometers measuring Tritium decay endpoints, with limits around 2.5 eV. A much larger successor experiment could reduce this limit by another factor of five if it is built and successful. However, it is inherent in this type of measurement that confidence in the results can be obtained only through confirmation by independent experiments that have different systematics.
A quite different approach to beta decay endpoint measurements is to embed the source in the detector and make a calorimetric measurement of the decay energy. Only the neutrino escapes from the detector, so this gives a direct measurement of the desired quantity, which is the total decay energy minus that carried away by the neutrino. The advent of very sensitive cryogenic thermal calorimeters has made this a practical method. These devices currently have energy resolutions of a few eV, and with some additional development it should be possible to implement an experiment that reaches mass limits below 0.5 eV.
With a modest amount of funding to cover travel and some undergraduate lab assistants, Wisconsin will participate in planning and optimizing how the detectors are installed and used, and will do appropriate parts of the small-scale assembly in their lab where they are already equipped for such work. Later on, with increased funding to add a graduate student, they will participate significantly in the operation and data analysis. This will create a real intellectual involvement for the U.S. in an important experiment and its possible successors. It will also allow some Wisconsin undergraduates to be involved in an exciting scientific endeavor, and will create a partnership that will help share some fruits of the considerable investment that the astrophysics community has made in large microcalorimeter arrays with another part of physics. In return, it is expected that efforts to improve these detectors for neutrino investigations will benefit both astrophysical and industrial applications.
