Sun is ok, says latest neutrino experiment

by Ronald Brown - rbrown@lsu.edu of Louisiana State University

The sun is healthy and strong, but physicists will have to change some of the basic assumptions they have made about how the universe works.

These are the results released today from the latest study of neutrinos made with a detector buried a half mile under a Japanese mountain. LSU physicist Bob Svoboda, who worked on an earlier ground-breaking experiment in the same mountain, was construction supervisor on this one.

Basically, scientists were trying to understand why the sun is not producing as many neutrinos as calculations said it should. The possibilities were that the sun doesn't work the way we thought it does, we don't understand nuclear reactions like we thought we did, something may be happening in the heart of the sun that we don't know, or neutrinos violate one or more of the laws of the Standard Model of fundamental particle interactions, which has successfully explained fundamental physics since the 1970s. It turns out neutrinos violate the Standard Model.

"The previous experiment, Super Kamiokonde, showed that neutrinos could change from one type to another – something they are not supposed to be able to do.

"It would be as if you looked at your pet and determined it was a cat at breakfast; but when lunch rolled around you noticed it had become a dog! At dinner it was again a cat. This effect had been seen in neutrinos from astrophysical sources like the sun, but had never been reproduced convincingly under laboratory conditions," Svoboda said.

That, in turn, proved that neutrinos have mass – something else that had been in doubt, and which has implications for the mass of the entire cosmos. Even though the mass of a single neutrino is vanishingly small, they are the most numerous particles in the universe. Billions of neutrinos are passing through every square inch of space every second. The mass of the universe has implications for the cosmological question of whether the universe will expand forever or fall back in on itself.

"We showed that neutrinos are too light to cause the collapse, though the question of the ultimate fate of the universe is still unresolved," Svoboda said.

The reason neutrinos were thought not to change from one type to another – there are three types, or "flavors" – is simply that the other particles in the same family have never been observed to do so. Among those particles are electrons and muons. If neutrinos can change, it brings up the question of why the other particles in the same family cannot, or do not.

Neutrinos are subatomic particles that interact so rarely with other matter that one could pass untouched through a wall of lead stretching from the earth to the moon. They're produced during nuclear fusion, the reaction that lights the sun and other stars. Anti-neutrinos are created in fission reactions such as those that drive nuclear power plants. Splitting a single atomic nucleus into two smaller nuclei often yields radioactive nuclei that decay and emit an electron and an anti-neutrino.

What this project, named KamLAND for Kamioka Liquid scintillator Anti-Neutrino Detector, has done is provide compelling evidence that the wind of neutrinos coming from the sun does indeed change from one flavor to another. The sun is producing the number of neutrinos it is supposed to; it is the Standard Model that will have to be modified. The experiment has been running since January. The computer used to do the modeling for the neutrino interactions is LSU's own Super Mike, which has the speed necessary for the hundreds of thousands of calculations involved. The U.S. Department of Energy contributed $100,000 to LSU Capital, which funded Super Mike, not only to model neutrino interactons, but to read the data from KamLAND in the future. KamLAND was the first experiment to be run on Super Mike. The DOE contributed $6 million overall to the project.

Interestingly, the experiment used neither neutrinos nor the sun for its results, but used anti-neutrinos and the many nuclear reactors in Japan. Nuclear reactors, it turns out, produce huge quantities of anti-neutrinos, which are the same as neutrinos except that they make anti-matter, like anti-electrons, when they interact. Because of this they react in the same way as their neutrino counterparts, but in mirror image. This experiment, Svoboda said, was virtually the same as the experiment performed by Fred Reines in 1956 which was the first to detect neutrinos and which won Reines a Nobel Prize. Svoboda was Reines' postdoctoral student at the time and worked with him on a number of neutrino experiments.

Although based on the same pattern as Reines' experiment, this detector is much larger, more complex and far more sensitive than Reines'. Buried a half-mile underground to reduce interference from other sources of radiation, the detector is a two-story high plastic balloon filled with mineral oil, as Reines' was, suspended inside a steel tank also filled with mineral oil and lined with photomultipliers. The plastic of the balloon is only two hundredths of a centimeter thick and has to hold 1,000 tons of baby oil. The problems this presented were considerable, Svoboda said.

First, the suspension system for the balloon had to be created. It was made of a net of kevlar straps attached to tensiometers at the top of the steel tank. As the balloon was filled with baby oil, the tank it was suspended in was filled also. The oil in the balloon had to have an additive so it was slightly denser than the oil surrounding to keep it from drifting in the tank, but not so dense it would put too much tension on the balloon and rupture it – thus the tensiometers. It also had to have another additive - a chemical that was highly sensitive to any sort of radioactivity and would produce a glow when a radioactive event took place. This massively increased the sensitivity of the detector and was the reason for the balloon. Glass is very radioactive, and had the chemical been put into the oil directly touching the glass photomultiplier bulbs, the glow would have drowned out any reactions anti-neutrinos would have caused.

Another problem confronting the scientists was purifying the oil. The oil had to be so clean there couldn't be more impurities in it than would make up the eraser on a pencil. The reaction they were looking for comes from one flavor of anti-neutrino striking a hydrogen atom, producing an anti-electron and a neutron. This reaction would give an identifiable light signature which could be confused by impurities in the oil. A different flavor of anti-neutrino striking a hydrogen atom will produce a different reaction.

The Japanese government contributed $20 million toward construction of the detector and also aided in another way. The government brokered an agreement between the research team and the 51 nuclear reactor sites in Japan to provide the researchers with information on how much nuclear energy was produced during the test's 145-day run. This gave them an accurate figure on how many anti-neutrinos would be produced and how many events they could expect to see. If neutrinos followed the Standard Model, there would have been 86 events. There were 54, proving almost conclusively that the anti-neutrinos changed from one flavor to another.

Svoboda has made numerous trips to Japan since construction began on the detector in 1998. Postdoctoral students Steven Dazeley and Shuichiro Hatakeyama, graduate students Mitsuko Murakami and Ana Rojas and undergraduates Roger Wendell, Aaron McMorris and Leif Remo have also traveled to Japan and spent time working on the project. Wendell installed about one-third of the 1,879 photomultipliers inside the steel sphere.

The KamLAND neutrino experiments are being conducted by an international collaboration largely comprised of scientists from Japan and the United States. Besides the researchers from LSU, the U.S. team at KamLAND includes researchers from Berkley Lab, UC Berkeley, Stanford, the California Institute of Technology, the University of Alabama, Drexel University, the University of Hawaii, the University of New Mexico, the University of Tennessee, and the Triangle Universities Nuclear Laboratory, a research facility funded by the U.S. Department of Energy, located at Duke University and staffed by researchers with Duke, the University of North Carolina and North Carolina State University.

The Japanese team at KamLAND is led by Atsuto Suzuki, a professor of physics at the Research Center for Neutrino Science at Tohuku University. Suzuki is the overall head of the international collaboration which also includes, in addition to Tohuku University participants, researchers from the Institute of High Energy Physics in Beijing.

The KamLAND experiments will continue for several more years, making refined measurements of reactor neutrinos that should shed more light on neutrino mass and flavor mixing. Since anti-neutrinos are also produced during the decay of radioactive uranium and thorium in the crust and mantle of the earth, the KamLAND detector can also be used to measure our planet's internal radioactivity. KamLAND, with a more purified liquid scintillator, will also be used to study solar neutrinos.

Results of the experiment will be published in an upcoming issue of Physical Review Letters. Downloadable images of the KamLAND detector, courtesy of the collaboration, are available at www.lbl.gov.