SNO looks for rarely-interacting neutrinos in an immense underground detector. Neutrinos easily penetrate 2000 m of earth to reach a preserve of 1000 tons of heavy water, where the neutrinos can initiate a number of reactions observed by sensitive photodetectors.

Many theorists have come to believe that electron neutrinos coming from boron-8 decays in the solar core should partly turn into muon neutrinos en route to Earth. Of all the neutrino detectors, SNO is the only one that can detect electron and non-electron neutrinos. Thus SNO should observe an electron-neutrino shortfall balanced by a corresponding excess of non-electron neutrinos (although they can't tell muon from tau neutrinos).

SNO is at too early a stage to make this type of demonstration, but the physicists there are able to determine, by comparing present neutrino observation rates (about 8 neutrinos per day) arising from different types of reaction and by using rates from the Super Kamiokande detector in Japan, that some non-electron-type neutrinos are reacting in the detector along with the majority-species electron neutrinos.

The analysis compares the rate of neutrinos seen on Earth from charged-current (CC) reactions, in which an incoming electron neutrino hits a deuteron (a proton-neutron combination constituting a heavy hydrogen nucleus), resulting in two protons and an electron; this reaction proceeds via the weak nuclear force carried by a charged W boson (hence the name "charged current"), with the rate from elastic-scattering (ES) reactions, in which an incoming electron neutrino scatters from an electron in an atom without converting into any other particle.

It is SNO's exclusive rate determined from the CC reaction compared with the ES rate (using data from SNO and Super Kamiokande) that provides the most direct evidence yet for the presence of non-electron neutrinos, (thus affirming neutrino oscillation), and in an amount that would seem to account precisely for the past solar neutrino shortfalls (thus explaining the "solar neutrino problem."

As for the issue of neutrino mass, the current measurements put only a crude limit on the difference of masses for the neutrinos. Owing to the expected large number of neutrinos in the universe, even a small neutrino mass might have provided neutrinos with a considerable role in the original herding of matter and shaping of galaxies in the earlier universe.

Presentations of this material were made at the Canadian Association of Physicists Annual Conference in Victoria, BC, on June 11 and at SNO Institutions in the U.S. and the U.K.

Text of a preprint is available at the Los Alamos server; the SNO website is at this URL.

(Editor's Note: This story is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 544, June 20, 2001, by Phillip F. Schewe, Ben Stein and James Riordon.)

21-Jun-2001