Neutrinos' Identity Crisis

Change is in the air--and in the Earth, at least as far as neutrinos are concerned. The latest results from a Japanese experiment show, beyond any reasonable doubt, that neutrinos created in the laboratory change their identities as they travel through Earth. The find firms up suspicions about the fickle nature of these lightweight particles.

For the past decade or so, scientists have become increasingly certain that the almost-massless neutrino changes its identity periodically. They suspect, for example, that one variety of neutrino, such as the relatively easy-to-detect muon neutrino, can change spontaneously into another type, such as a hard-to-detect tau neutrino. Scientists have seen powerful evidence for this "oscillation" behavior in neutrinos coming from the sun (Science, 12 June 1998, p. 1689) and in those created in the atmosphere by cosmic rays (Science, 26 April 2002, 632b p. 632).

Now there's another extremely strong, independent piece of evidence that muon neutrinos can change their identity. At the KEK high-energy physics lab in Tsukuba, Japan, physicists created a beam of muon neutrinos and shot them through the ground toward the Super-Kamiokande neutrino detector 250 kilometers away. When the scientists analyzed 5 years' worth of neutrino data from the K2K experiment, they discovered that nearly one-third of the muon neutrinos somehow disappeared en route to the detector. Of about 150 muon neutrinos expected to be detected at the Super-K detector, only about 110 showed up. The most likely explanation is that the missing muon neutrinos had oscillated--changed--into the hard-to-detect tau variety.

"This proves that these neutrinos oscillate," says Jacques Bouchez, a physicist at the French Center for Atomic Energy in Saclay, France, and K2K collaborator. Furthermore, the K2K data, which were released 11 June, also contain evidence that the oscillation effect depends on the energy of the neutrinos. The more energy a neutrino has, the longer it travels, on average, before it changes its identity. Therefore, theorists predicted, the K2K experiment should see low-energy neutrinos changing identity proportionally more often than high-energy neutrinos do. And this is precisely what K2K has found.

Although K2K is scheduled to be shut down in 2005, a U.S.-based experiment, MINOS, and a European-based one, OPERA, are going to turn on in 2006 and 2007, respectively, and will greatly improve on K2K's results, says Bouchez. Until then, though, scientists will have to be content knowing that they are well on the way to understanding the neutrino--no matter which identity it chooses.

Related sites
K2K home page
Introduction to the K2K experiment