Neutrino Oscillations on a Roll

摘要

Two 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-neutri-nos 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 neutri- nos 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.