The apparatus at Los Alamos used to set a limit on the mass of the electron antineutrino. The superconducting solenoid containing the gaseous tritium source is being examined by John Wilkerson (right) and Tom Bowles (center). In the background Hamish Robertson (left) and Mel Anaya are inside the cage of conductors used to cancel the Earth's magnetic field. Behind them is the high-resolution magnetic spectrometer.

Dr. R.G. Hamish Robertson

Neutrinos are created with electrons when the nucleus of an atom disintegrates through beta decay. Scientists know roughly how many neutrinos exist as leftovers from the Big Bang creation of the universe; however, their mass is a mystery. The mass range was narrowed in 1991, when scientists at Los Alamos National Laboratory announced a new top limit (9.3 electron volts, or eV) for the mass of the electron anti-neutrino (one type of neutrino), discovered through measurements of the beta decay of molecular tritium. From these data, the scientists determined that less than 30 percent (and consistent with zero) of the mass of the universe comes from electron anti-neutrinos, and thus, that they cannot constitute the bulk of the "dark matter" that makes up some 90 percent of the mass of the universe. Dark matter is nonluminous material that cannot be detected by observing electromagnetic radiation, but whose existence is inferred from current models of the universe. The electron anti-neutrino would need a mass exceeding 10 eV for it to constitute the primary form of dark matter. R. G. Hamish Robertson won the 1997 Tom W. Bonner Prize from the American Physical Society largely for this work.

Scientific Impact: Determining if dark matter exists, and in what quantity, is among the most challenging problems in modern astrophysics. This work is generally viewed as a textbook example of how to probe the nature of mass in the universe, and its success paved the way for improved experiments that have lowered the electron anti-neutrino mass limit to about 3 eV.

Social Impact: This experiment gives insight into the question of what the universe is made of thus, one possible explanation of what the universe is made of, thus contributing to human understanding of nature and improvements in science education. This finding also helped stimulate a worldwide research effort in neutrino physics that has brought scientists from many countries together to work on the same problem.

Reference: "Limit on Electron Antineutrino Mass from Observation of the b decay of Molecular Tritium," R. G. H. Robertson et al., Phys. Rev. Lett., 67: 957 (1991).

URL: http://cupp.oulu.fi/neutrino/nd-mass.html

Technical Contact: Dr. R.G. Hamish Robertson, rghr@u.washington.edu

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics