Six years ago, the Super-Kamiokande Collaboration in Japan stunned the physics world with evidence that--contrary to long-standing prediction--neutrinos have mass (ScienceNOW, 5 June 1998). Now the team has filled in the picture, observing the particles as they change from one "flavor" to another and back.
Neutrinos come in three flavors: muon, electron, and tau. The Super-K team made its 1998 breakthrough by looking at muon neutrinos produced when cosmic rays slam into particles in the atmosphere. Neutrinos usually flow through matter the way photons pass through glass. But occasionally a passing muon neutrino collides with a proton or nucleus in Super-K's detector, a 50,000-ton water tank buried in a mine in central Japan, and sensors lining the tank spot charged particles hurtling from the collision. Super-K found more muon neutrinos raining down into the detector than coming up through the ground--evidence that muon neutrinos from the far side of Earth were oscillating into tau neutrinos, which the detector cannot see. By the laws of quantum mechanics, only particles with mass can oscillate.
More evidence was needed, though. The probability that a neutrino will change flavor is a function of the ratio of the distance the neutrino has traveled divided by its energy. The farther a neutrino travels, the more likely it is to oscillate. In theory, the numbers of neutrinos reaching a detector should trace out the peaks and troughs of a classic sine curve as the particles keep switching flavors with distance. Super-K had enough data to show that muon neutrinos were disappearing, but not enough to show that sine curve.
Now, after six more years of neutrino sightings, the team has filled out that curve. Plotting the top 20% of the 14,000 detections, "we can actually see the dip, and then [the curve] comes back up," says Henry Sobel, a physicist at the University of California, Irvine, who is co-spokesperson for the United States side of the multicountry collaboration. The new results, which will be published in an upcoming issue of Physical Review Letters, rule out some alternative theories of why the neutrinos were disappearing and sharpen up the numbers that govern how oscillations take place, which theorists need to construct comprehensive models of neutrino behavior.
"This collaboration has really done a great job," says Eligio Lisi, a physics theorist at Italy's National Institute for Nuclear Physics (INFN) in Bari. "It was unexpected they could see an oscillation pattern." But he cautions that the group is "pushing the data to its limits" and will need better statistics to clinch the case.