Neutrino Oscillation

Has now been seen in the long-distance travel of a beam of muon neutrinos (ν_µ) created at an accelerator. Since 1998, the international K2K collaboration has been directing a ν_µ beam from the KEK 12-GeV proton synchrotron in Tsukuba, Japan, to the Super-Kamiokande detector 250 km away. The study of neutrinos created in the atmosphere by cosmic rays has already yielded strong evidence that a ν_µ can metamorphose into a tau neutrino (ν_τ). But the new K2K result, reported in June by Tsuyoshi Nakaya (Kyoto University) at a meeting in Paris, is a reassuring confirmation of the oscillation mechanism. With an accelerator beam, one can directly compare the neutrino spectrum at birth with what’s left over after the journey. In five years of running, 108 ν_µ particles from KEK were seen in Super-Kamiokande. In the absence of oscillation, one would have expected 151 ± 11. Both that shortfall and the measured energy spectrum agree well with the oscillation parameters from the atmospheric data. For example, the difference between the squared masses of the relevant neutrino eigenstates comes to about 2.7 × 10⁻³ eV² from K2K and, when combined with the atmospheric data, yields a joint result of 2.5 × 10⁻³ eV². (See http://neutrino.kek.jp .)