Polymorphous Particles Solve Solar Mystery
Charles Seife 

For particles with almost no mass, neutrinos are making quite a splash. On Monday, scientists from three countries announced that they had spotted neutrinos that had been missing for 3 decades.

In the late 1960s, physicists calculated the number of relatively energetic neutrinos that should be streaming from the sun--due to the decay of boron-8 cooked up in the solar furnace--but experiments came up short. There were too few neutrinos. This is the mystery that Canada's Sudbury Neutrino Observatory (SNO) has now cleared up. "I'm thrilled by the precision of the result; I'm thrilled it agrees with the solar model calculations; I'm thrilled we have an answer to the problem," says John Bahcall, a physicist at the Institute for Advanced Study in Princeton, New Jersey.

In fact, SNO has confirmed what several experiments, notably Super-Kamiokande in Japan, had already indicated: The missing neutrinos had simply changed flavor. Neutrinos come in three flavors, named after the particles they are linked with. Electron neutrinos are the type produced by the sun; muon and tau neutrinos, which result from various particle interactions, are harder to detect. In the late 1990s, experiments provided fairly strong evidence that electron neutrinos turn into muon and tau neutrinos as they stream away from the sun--something that can happen only if the particles have mass (Science, 4 July 1997, p. 30). The "missing" neutrinos from the sun had merely changed into muon and tau neutrinos and escaped detection.

CREDIT: SNO  Neutrino detector. 
                           At Sudbury, 10,000 photomultipliers on an 18-meter-wide sphere watch for elusive particles.

Buried 2 kilometers underground in a nickel mine in Ontario, SNO has just given a resounding conformation to this picture. The detector measures the neutrinos coming from the sun in two ways. The first method spots the recoil of a neutrino off of an electron. Any of the three flavors of neutrino could potentially cause such a recoil and be detected. The second method detects when an electron neutrino strikes a neutron within a 1000-ton sphere of heavy water. Only an electron neutrino can make the neutron spit out an electron, triggering the detector. The two methods, combined with results from Super-K, reveal just how many neutrinos are coming from the sun and what proportion of them is either muon or tau neutrinos.

"What we find is that there is an appearance of muon and tau neutrinos en route from the sun to the Earth," says SNO project director Art McDonald of Queen's University in Kingston, Ontario. "The electron neutrinos transform into another type." The transformation confirms earlier observations that neutrinos have mass. Better yet, the measurements agree with first-principles calculations of the amount of solar neutrinos created by the sun. "It is in very good agreement," says SNO team member Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory in California. "Basically, we've resolved the solar neutrino problem with a 99% confidence level. It's oscillations."

"This is an absolutely direct measurement," Bahcall says. "Previous results were not so direct." SNO scientists have already added salt to the heavy-water sphere, which will increase the instrument's sensitivity to muon and tau neutrinos and add another level of precision. "That will be a thrill," says Bahcall.

  Volume 292, Number 5525, Issue of 22 Jun 2001, pp. 2227-2229.
  Copyright © 2001 by The American Association for the Advancement of Science.