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Dec. 9, 2002

Disappearing Neutrinos Support the Case for Neutrino Mass

FOR IMMEDIATE RELEASE

Results from six months of experiments at KamLAND, an underground neutrino detector in central Japan, show that anti-neutrinos emanating from nearby nuclear reactors are "disappearing," which indicates they have mass and can oscillate or change from one type to another.

As anti-neutrinos are the anti-matter counterpart to neutrinos, these results provide independent confirmation of earlier studies involving solar neutrinos and show that the Standard Model of Particle Physics, which has successfully explained fundamental physics since the 1970's, is in need of updating. The results also point the way to the first direct measurements of the total radioactivity of the earth.

Researchers and students at North Carolina State University took part in the study, which was conducted by an international team of scientists from 13 universities and three government research laboratories.

Says Dr. Chris Gould, professor and head of physics at NC State, "This is the first wholly-terrestrial demonstration that neutrinos have mass, and therefore do not move at the speed of light. A succession of nuclear reactor experiments have previously looked for a deficit associated with neutrinos changing from one kind to another, and seen no effect. Now the search is successfully concluded. Neutrinos change form as they travel along, and they definitely are the lightest particles (with non-zero mass) so far discovered in the universe."

"While the results from earlier neutrino experiments…offered compelling evidence for neutrino oscillation, there were some escape clauses. Our results close the door on these clauses and make the case for neutrino oscillation and mass seemingly inescapable," says Stuart Freedman, a nuclear physicist with a joint appointment at the Lawrence Berkeley National Laboratory and the University of California at Berkeley (UCB).

Located in a mine beneath the Japanese island of Honshu, KamLAND is the largest low-energy anti-neutrino detector ever built. It consists of a 43-foot-diameter weather balloon filled with about a kiloton of liquid scintillator, a chemical soup that emits flashes of light when an incoming anti-neutrino collides with a proton. These light flashes are detected by a surrounding array of 1,879 photomultiplier light sensors that convert the flashes into electronic signals that computers can analyze.

The anti-neutrino events recorded at KamLAND for this study stem from electron anti-neutrinos originating at 51 nuclear reactors in Japan and 18 reactors in South Korea. Anti-neutrinos, like neutrinos, come in three different types: electron, muon and tau.

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. Since anti-matter is thought to be the mirror-image of matter in properties and behavior, to study anti-neutrinos is to study neutrinos.

Says John Learned, a physicist at the University of Hawaii, "We're seeing direct evidence that anti-neutrinos and neutrinos have the same structure and behave in exactly the same way. This has never been demonstrated in an experiment before and it is an important contribution towards a better understanding of neutrino physics."

In a paper for Physical Review Letters, the 92 physicists who make up the KamLAND team reported that they recorded 54 electron anti-neutrino events in the energy range of one to 10 million electron volts as opposed to the approximately 86 events predicted by the Standard Model under the assumption that no oscillations occur.

Based on analysis of the events and the energies at which they occurred, the researchers concluded that the anti-neutrinos oscillated on their way from the reactors which caused some of them to change from electron to muon and tau anti-neutrinos.

Says Learned, "The neutrino mixing was surprisingly strong, close to the maximum allowed. This result will be grist for many theoretical papers no doubt, but at the moment we have no understanding of why it is so."

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 in a new low energy regime.

The KamLAND research team includes scientists from: Berkeley Lab; UCB; Stanford; the California Institute of Technology; the University of Alabama; Drexel University; the University of Hawaii; Louisiana State University; the University of New Mexico; the University of Tennessee; Tohuku University in Japan; the Institute of High Energy Physics in China; 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 at NC State, Duke and the University of North Carolina at Chapel Hill.

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Editor's notes: Downloadable images of the KamLAND detector courtesy of the collaboration are available at www.lbl.gov. KamLAND Websites with additional images can be accessed at http://hep.stanford.edu/neutrino/KamLAND/KamLAND.html
and http://kamland.lbl.gov/. The Japanese KamLAND Website can be accessed at
http://www.awa.tohoku.ac.jp/html/KamLAND/.