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12/06/2002 08:00:00

Earthbound experiment confirms theory accounting for sun's scarcity of neutrinos

PASADENA, Calif.- In the subatomic particle family, the neutrino is a bit like a wayward red-haired stepson. Neutrinos were long ago detected-and even longer ago predicted to exist-but everything physicists know about nuclear processes says there should be a certain number of neutrinos streaming from the sun, yet there are nowhere near enough.

This week, an international team has revealed that the sun's lack of neutrinos is a real phenomenon, probably explainable by conventional theories of quantum mechanics, and not merely an observational quirk or something unknown about the sun's interior. The team, which includes experimental particle physicist Robert McKeown of the California Institute of Technology, bases its observations on experiments involving nuclear power plants in Japan.

The project is referred to as KamLAND because the neutrino detector is located at the Kamioka mine in Japan. Properly shielded from radiation from background and cosmic sources, the detector is optimized for measuring the neutrinos from all 17 nuclear power plants in the country.

Neutrinos are produced in the nuclear fusion process, when two protons fuse together to form deuterium, a positron (in other words, the positively charged antimatter equivalent of an electron), and a neutrino. The deuterium nucleus hangs nearby, while the positron eventually annihilates both itself and an electron. The neutrino, being very unlikely to interact with matter, streams away into space.

Therefore, physicists would normally expect neutrinos to flow from the sun in much the same way that photons flow from a light bulb. In the case of the light bulb, the photons (or bundles of light energy) are thrown out radially and evenly, as if the surface of a surrounding sphere were being illuminated. And because the surface area of a sphere increases by the square of the distance, an observer standing 20 feet away sees only one-fourth the photons of an observer standing at 10 feet.

Thus, observers on Earth expect to see a given number of neutrinos coming from the sun-assuming they know how many nuclear reactions are going on in the sun-just as they expect to know the luminosity of a light bulb at a given distance if they know the bulb's wattage. But such has not been the case. Carefully constructed experiments for detecting the elusive neutrinos have shown that there are far fewer neutrinos than there should be.

A theoretical explanation for this neutrino deficit is that the neutrino "flavor" oscillates between the detectable "electron" neutrino type, and the much heavier "muon" neutrino and maybe even the "tau" neutrino, neither of which can be detected. Utilizing quantum mechanics, physicists estimate that the number of detectable electron neutrinos is constantly changing in a steady rhythm from 100 percent down to a small percentage and back again.

Therefore, the theory says that the reason we see only about half as many neutrinos from the sun as we should be seeing is because, outside the sun, about half the electron neutrinos are at that moment one of the undetectable flavors.

The triumph of the KamLAND experiment is that physicists for the first time can observe neutrino oscillations without making assumptions about the properties of the source of neutrinos. Because the nuclear power plants have a very precisely known amount of material generating the particles, it is much easier to determine with certainty whether the oscillations are real or not.

Actually, the fission process of the nuclear plants is different from the process in the sun in that the nuclear material breaks apart to form two smaller atoms, plus an electron and an antineutrino (the antimatter equivalent of a neutrino). But matter and antimatter are thought to be mirror-images of each other, so the study of antineutrinos from the beta-decays of the nuclear power plants should be exactly the same as a study of neutrinos.

"This is really a clear demonstration of neutrino disappearance," says McKeown. "Granted, the laboratory is pretty big-it's Japan-but at least the experiment doesn't require the observer to puzzle over the composition of astrophysical sources.

"Willy Fowler [the late Nobel Prize-winning Caltech physicist] always said it's better to know the physics to explain the astrophysics, rather than vice versa," McKeown says. "This experiment allows us to study the neutrino in a controlled experiment."

The results announced this week are taken from 145 days of data. The researchers detected 54 events during that time (an event being a collision of an antineutrino with a proton to form a neutron and positron, ultimately resulting in a flash of light that could be measured with photon detectors). Theory predicted that about 87 antineutrinos would have been seen during that time, if no oscillations occurred, but 54 events at an average distance of 175 kilometers if the oscillation is a real phenomenon.

According to McKeown, the experiment will run about three to five years, with experimentalists ultimately collecting data for several hundred events. The additional information should provide very accurate measurements of the energy spectrum predicted by theory when the neutrinos oscillate.

The experiment may also catch neutrinos if any supernovae occur in our galaxy, as well as neutrinos from natural events in Earth's interior.

In addition to McKeown's team at Caltech's Kellogg Radiation Lab, other partners in the study include the Research Center for Neutrino Science at Tohuku University in Japan, the University of Alabama, the University of California at Berkeley and the Lawrence Berkeley National Laboratory, Drexel University, the University of Hawaii, the University of New Mexico, Louisiana State University, Stanford University, the University of Tennessee, Triangle Universities Nuclear Laboratory, and the Institute of High Energy Physics in Beijing.

The project is supported in part by the U.S. Department of Energy.



Written by Jill Perry