Making sunshine; new observations focus attention on the sun's missing neutrinos

Science News, Oct 28, 1989 by Ivars Peterson

Making Sunshine

What makes the sun shine? According to the standard solar model, the nuclear engine at the sun's center pumps out a tremendous amount of energy. In this high-temperature, high-pressure environment, protons (the nuclei of hydrogen atoms) fuse to create deuterons (proton-neutron pairs), which then combine with protons or other deuterons to produce heavier atomic nuclei, and so on. Each step in this chain of nuclear fusion reactions releases energy.

But something may be fundamentally wrong with this picture. Theoretical calculations, based on the types of nuclear reactions expected at the sun's core, indicate the sun should emit about 2 percent of its energy in the form of neutrinos--elusive particles that interact only feebly with matter. Yet the number of solar neutrinos actually detected at Earth's surface appears significantly smaller than theory predicts.

The case of the missing solar neutrinos ranks as arguably the most puzzling problem in astrophysics. Is there a basic flaw in the theory of how stars generate energy? Or do neutrinos behave in peculiar ways not yet understood? Solar-neutrino experiments now in progress and others soon to start will probably settle these questions in the next few years.

"The whole subject is coming together experimentally and theoretically," says John N. Bahcall of the Institute for Advanced Study in Princeton, N.J. "We're clearly at a time of very rapid change."

For nearly two decades, starting in 1968, the only indication of something mysterious going on came from a single neutrino detector -- a huge tank of dry-cleaning fluid (perchloroethylene) installed nearly a mile underground in the Homestake gold mine near Lead, S.D. Every two months, Raymond Davis Jr. of the University of Pennsylvania in Philadelphia and his team go down the mine to extract a sample of the fluid. They analyze the material, looking for signs of the telltale reaction between a neutrino and a chlorine-37 nucleus that produces radioactive argon-37 and an electron. The number of neutrinos detected in this long-running experiment is consistently only a quarter to a third of the number expected.

In 1986, Japan's Kamiokande II neutrino detector, which uses ordinary water, began functioning. This year, members of the Kamiokande collaboration reported a neutrino flow for 1987 and half of 1988 amounting to only half that expected theoretically. The new data, published in the July 3 PHYSICAL REVIEW LETTERS, represent the first experimental confirmation that there really is an unexplained solar-neutrino deficit.

Although the Kamiokande measurements have a large uncertainty and cover only a few years, they overlap 450 days of the chlorine detector data. "We have for the first time two different detectors operating, and they give consistent results," says Lincoln Wolfenstein of Carnegie Mellon University in Pittsburgh. "That's very reassuring."

Moreover, because the Kamiokande detector, unlike the chlorine experiment, provides information about the direction in which neutrinos travel, it furnishes clear-cut evidence that the observed neutrinos actually come from the sun. Those observations offer the first experimental confirmation that the sun's energy does originate in nuclear reactions.

One of the more intriguing aspects of the solar-neutrino problem is the possibility that the neutrino flow from the sun varies from time to time. Data from the chlorine detector suggest that the number of neutrinos increases when the solar sunspot cycle is at a minimum.

"We had high results 11 years ago, and we had high results from the end of 1986 until the beginning of 1988," Davis says. Now, with the solar cycle at its peak, the number of neutrinos detected should fall off, he says. But because researchers need nearly a year to analyze a sample, it's too early to see if the trend holds up.

Bahcall, for one, finds Davis' data statistically unconvincing. "I have bet a bottle of the best French wine that the correlation is not physical, that it's a fluke," Bahcall says. The standard model of the sun's inner workings, to which Bahcall has made a considerable contribution, provides no plausible mechanism by which such variations could occur.

Even more controversial is the hint of a correlation between neutrinos and solar flares -- gigantic outpourings of matter and energy from the solar surface. Large solar flares occur most often near the beginning and end of a solar cycle's peak period.

Davis detected a neutrino increase in 1972 that happened to coincide with one of the largest solar flares detected in the last few decades. The present solar cycle has featured several large flares, and Davis will look for any changes in the neutrino signal associated with those outbursts. Searches for a correlation between large solar flares and neutrino events observed at Kamiokande so far show no hint of a link.

Scientists have proposed a variety of solutions for the mystery of the missing neutrinos. Wolfenstein and Eugene W. Beier of the University of Pennsylvania, writing in the July PHYSICS TODAY, put the proposals into two broad categories: those that blame the sun and those that blame the neutrino.