wo papers on neutrino oscillations (#4, #10) from the Super-Kamiokande Collaboration provide the main excitement in our latest snapshot of what’s hot in physics. These Super-Kamiokande papers on the properties of solar and atmospheric neutrinos pre-date the horrendous accident of November 12, 2001, when thousands of photomultiplier tubes imploded catastrophically. The detector is currently being reconstructed with half the original number of tubes, and it is expected to be operational by the end of 2002.

One of the deepest mysteries in physics is the origin of mass. Neutrinos, which exist as electron-, muon-, and tau-neutrinos were long thought to be the only massless particles. The Standard Model of particle physics is silent on this issue, unable to decide whether or not neutrinos have mass. In 1998, the Super-Kamiokande found the first clear evidence that neutrinos produced in the atmosphere, by cosmic-ray collisions, were oscillating between the three flavors. This remarkable discovery immediately implies that neutrinos have mass, and each type has a different mass. That’s because the nature of the oscillation can be explained quantum mechanically as a mixing of the matter waves associated with each particle. Neutrinos are not the only particles capable of changing their stripes in mid-flight: the same phenomenon has been experimentally observed for kaons.

Super-Kamiokande’s track record is impressive: in five separate investigations they have found that solar or atmospheric neutrinos are "disappearing." In effect the electron- and muon-neutrinos are turning into undetectable tau-neutrinos. But that deduction stimulated physicists to suggest there might be a fourth particle, called the sterile neutrino, a possibility that grew in importance as theorists struggled to explain all the experimental results using just three neutrinos. A "sterile" neutrino, with a much weaker interaction with matter or, more exotic still, with large extra dimensions, offered an escape route. However, paper #4 finds no compelling evidence for sterile neutrinos. It describes a detailed analysis of atmospheric muon events recorded in 1,144 live days of the detector. Yoichiro Suzuki, spokesman for the Super-Kamiokande Collaboration, tells Science Watch why #4 is grabbing a lot of attention. "We’ve shown that the dominant mode of the atmospheric neutrino oscillations is to tau-neutrinos. We find no evidence for sterile neutrinos, and can reject that hypothesis at the 99% confidence level."

In newcomer #10, a broadly similar analysis is carried through for solar neutrinos, using 1,258 days of data. These have traveled a far greater distance than atmospheric neutrinos, with a much longer time frame for the oscillations. For more than 30 years astrophysicists have known that the solar neutrino flux at Earth is way below the prediction of standard solar models. Oscillations are a natural explanation of this discrepancy. However, the results in #10 place quite strong constraints on neutrino mixing and the mass differences between the three flavors. The experimental method included contributions from all three flavors, with muon- and tau-neutrinos present in the Super-Kamiokande data, clear evidence indeed for oscillations. These clear results will be seized upon by solar astrophysicists, as well as the neutrino oscillation industry. "This work is getting high citations because it gives the first experimental evidence for the oscillation of solar neutrinos, and it is the first paper to present large mixing angle solutions as the favored solution," comments Suzuki.