R. G. Hamish Robertson

The neutrino has challenged physicists for more than 70 years. My interest in these elementary particles began in 1980 when experimental efforts to see if neutrinos had mass reached an impasse set by the atomic excitations of radioactive atoms in sources used for this work. My colleagues and I built a new apparatus at Los Alamos to search for neutrino mass with gaseous molecular tritium, where the excitations could be well understood. We did not find mass then, but were able to show that electron neutrinos were not heavy enough to be the dark matter of the universe. In 1988 our attention turned to the solar neutrino problem, which was generally thought to be a technical problem, but which could have been a signal that neutrinos had mass. In 2001 our Sudbury Neutrino Observatory data showed that indeed all the neutrinos were there, and the "problem" was that they were not all electron neutrinos. That was convincing evidence for neutrino mass. Neutrino mass is beyond our once-trusty Standard Model of particles and fields. Oddly enough, while neutrinos do have mass, they are only a minor component of the dark matter. Other puzzles remain: Are neutrinos and antineutrinos the same particle?