The first of a new breed of experiments on neutrinos has detected an energy pattern consistent with earlier hints that those subatomic particles have mass. However, the prevailing theory of particle physics assumes no mass for the three types of neutrinos—electron, muon, or tau neutrinos.
Since 1998, experimenters have found that neutrinos can change types—a process possible only if neutrinos have some mass (SN: 12/14/02, p. 371: Identity Check: Elusive neutrinos morph on Earth, as in space). The scientists have observed such changes among neutrinos emanating from the sun, from cosmic ray collisions with atmospheric molecules, and from nuclear reactors.
To generate and measure neutrinos under more-controlled circumstances, researchers in Japan working on the experiment known as K2K have been beaming muon neutrinos through the ground to a detector 250 kilometers away. That work has for the first time measured the particles’ initial and final energy distributions.
On June 15 at the Neutrino 2004 meeting in Paris, K2K physicist Tsuyoshi Nakaya of Kyoto University reported a deficit of particles at certain energies. The scientists had predicted there would be such a deficit if muon neutrinos became tau neutrinos. K2K provides the first steps toward further accelerator experiments to more precisely pin down aspects of neutrinos.