Physics Bedrock Cracks, Sun Shines In.

By solving a decades-long mystery about the sun, researchers have set off a scientific ripple that could alter conceptions of the universe as a whole.

In the first scientific data to emerge from the Sudbury (Ontario) Neutrino Observatory (SNO) in Canada, physicists have found evidence that their most fundamental theory of the universe--the so-called standard model (SN: 7/1/95, p. 10)--contains a major flaw. They've also uncovered a new clue to the composition of the dark matter thought to make up 95 percent of the mass of the cosmos (see p. 394, this issue).

The revelations arise from a new study of the neutrino, an elusive form of matter. Neutrinos come in three types--which scientists call flavors--known as the electron neutrino, muon neutrino, and tau neutrino. These neutrinos span a large range of energies.

Since the 1960s, scientists have measured the sun's copious neutrino output as an indicator of its internal, nuclear-fusion reactions. However, solar-neutrino observatories examining different energy ranges have consistently detected only about half or fewer of the neutrinos that theorists predict they should find. Finally, scientists can now say they know why.

"Our measurements provide an answer to what has been a major puzzle in science for over 30 years," says SNO Director Arthur B. McDonald of Queen's University in Kingston, Ontario.

Solar-fusion reactions unleash only enough energy to make electron neutrinos, so that's the flavor that scientists were expecting to detect. But the new data indicate that electron neutrinos oscillate with other flavors as they make their way to Earth. Because detectors have been less sensitive to muon and tau than to electron neutrinos, the result was a systematic undercounting of solar neutrinos.

"Neutrinos are very confused, very schizophrenic," explains John N. Bahcall of the Institute for Advanced Studies in Princeton, N.J. By pinpointing the source of the neutrino shortfall, the Sudbury team has made "a beautiful measurement" that "confirms our calculations of how the sun shines," he says.

Sudbury scientists unveiled their data on Monday in Victoria, British Columbia, at the annual congress of the Canadian Association of Physicists.

Evidence of neutrinos' schizophrenia first surfaced in 1998. In Japan, physicists were studying muon neutrinos produced by cosmic rays hitting Earth's atmosphere. The researchers at the Super-Kamiokande detector in Kamioka observed that more muon neutrinos arrived at their instrument from directly above the underground device than along other paths through the Earth (SN: 6/13/98, p. 374). The scientists concluded that muon neutrinos taking the longer routes had time to oscillate into other flavors that were more difficult for their detector to see.

These findings shook up particle physics (SN: 1/30/99, p. 76) because the standard model rules out such oscillations. What's more, according to quantum mechanics, neutrinos could oscillate only if they had mass--another contradiction to the standard model.

Although the standard model has been a remarkably successful theory, the crack in it that appeared at Super-Kamiokande has now been widened by the SNO finding, researchers say.

Besides studying atmospheric neutrinos, researchers at Super-Kamiokande have measured solar neutrinos. They find 45 percent of the predicted particle flow. The new Sudbury data follow up directly on that measurement by tallying neutrinos in the same energy range. But unique features of the Canadian instrument enable it to reveal details about those neutrinos unattainable with Super-Kamiokande.

The Japanese detector, filled with ordinary water, can't distinguish among the solar-neutrino flavors. However, using 1,000 tons of heavy water as a detector, the Sudbury researchers can measure how many electron neutrinos reach their device, which sits in a working nickel mine.

This measurement has enabled scientists to infer how many electron neutrinos reach Super-Kamiokande. Discerning electron neutrinos from other neutrinos is the key capability that had been needed to solve the solar puzzle, scientists say.

The Sudbury device reveals that the electron neutrinos arriving at an earthly detector total only 35 percent of the predicted solar-neutrino output. Therefore, the additional neutrino flow detected at Super-Kamiokande must be made of up muon and tau neutrinos. Moreover, they must have left the sun as electron neutrinos--the only kind it produces--and changed flavor along the way.

By knowing the sensitivity of Super-Kamiokande to muon and tau neutrinos, the Sudbury team has been able to extrapolate that the total solar output is almost precisely what theorists have called for.

Although the SNO team is still collecting data, the evidence for oscillating solar neutrinos "is pretty definitive at this point," comments Stephen J. Parke of Fermi National Accelerator Laboratory in Batavia, Ill.

Moreover, these new data refine scientists' view of dark matter, says SNO's Kevin T. Lesko of Lawrence Berkeley (Calif.) National Laboratory.

For decades, scientists have known that the mass of the visible objects in the cosmos is far too little to account for features of the universe such as the gravitational tug that keeps galaxy clusters together. Researchers have hypothesized that some combination of familiar, nonluminous matter and some unknown matter--together dubbed dark matter--makes up the difference.

Researchers had suspected that the total neutrino mass was probably not the overwhelming contributor to dark matter, Lesko notes. Now, the Sudbury findings permit researchers for the first time to set a firm limit: The universe's neutrinos make up no more than about half the dark matter.