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Stanford Report, January 8, 2003

Vanishing anti-neutrinos at KamLAND support case for neutrino mass

BY LYNN YARRIS

Results from the first six months of experiments at KamLAND, an underground neutrino detector in central Japan, show that anti-neutrinos emanating from nearby nuclear reactors are "disappearing." Their disappearance indicates they have mass and can oscillate, or change from one type to another, report 92 physics collaborators from Japan, the United States and China in a paper for Physical Review Letters.

"We are seeing the same neutrino deficit, or a deficit which is compatible with the deficit that people have been seeing for years in solar neutrino experiments," says Giorgio Gratta, an associate professor of physics at Stanford who is a co-spokesperson for the U.S. team at KamLAND. "Neutrinos from nuclear reactors disappear on the flight from the reactors to our detector. The result almost certainly means that the solar neutrino anomaly is due to neutrino oscillations, which means that neutrino masses are nonzero."

KamLAND workers bolt down one of the 1,879 photomultiplier tubes used to detect anti-neutrinos. Though other experiments have seen the disappearance of subatomic particles generated in the sun, KamLAND is the first experiment to observe the disappearance from a terrestrial source -- reactors in Japan's nuclear power plants. Photo: Courtesy of KamLAND collaboration

That means the Standard Model, which has successfully explained fundamental particle physics since the 1970s and which predicts neutrinos have no mass, may need updating. In addition, since anti-matter is thought to be the mirror-image of matter in properties and behavior, to study anti-neutrinos is to study neutrinos. That means the results may provide independent confirmation of prior results from solar neutrino studies.

Says Stuart Freedman, a nuclear physicist with a joint appointment at the Lawrence Berkeley National Laboratory and the University of California-Berkeley, and the other co-spokesperson for the U.S. team: "While the results from earlier neutrino experiments such as those at SNO (Sudbury Neutrino Observatory) and Super-K (Super-Kamiokande) 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."

KamLAND stands for Kamioka Liquid scintillator Anti-Neutrino Detector. Located in a mine cavern beneath the mountains of Japan's main island of Honshu, it is the largest low-energy anti-neutrino detector ever built. KamLAND consists of a weather balloon, 13 meters (43 feet) in diameter, that is 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 photomultipliers are attached to the inner surface of a stainless steel sphere, 18 meters (59 feet) in diameter, and separated from the weather balloon by a buffering bath of inert oil and water that helps suppress interference from background radiation.

"Due to KamLAND's location deep underground, and its large size, we can reduce backgrounds from other particles -- leaving us able to determine the exact time and energy of anti-neutrino events in the balloon," says Nikolai Tolich, a graduate student in physics at Stanford.

Buried deep within a mountain in Japan (see inset, left) is a giant weather balloon filled with a chemical cocktail that emits a flash when an anti-neutrino hits a proton. Photomultiplier sensors convert flashes into electronic signals for computer analysis. Scientists at the facility, known as KamLAND, see fewer anti-neutrinos than expected. Courtesy of KamLAND collaboration

The anti-neutrino events that were recorded in the KamLAND detector for this study stem from electron anti-neutrinos that originated from the 51 nuclear reactors in Japan plus 18 reactors in South Korea. Anti-neutrinos, like neutrinos, come in three different types, or "flavors," 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. Splitting a single atomic nucleus into two smaller nuclei often yields radioactive nuclei that decay and emit an electron and an anti-neutrino.

Scientists have long used anti-neutrinos to study their anti-matter counterparts, neutrinos. The 1956 experiments of Frederick Reines and Clyde Cowan, which marked the first detection of neutrinos and won for Reines a share of the 1995 Nobel Prize, were based on anti-neutrinos produced in nuclear reactors.

"Nuclear fissions in reactors produce the anti-particles of the neutrinos created in fusion reactions in the sun," explains Stanford physicist Yoshi Uchida. "KamLAND is unique in studying the same oscillation properties of anti-neutrinos as those probed for neutrinos by previous solar experiments. This new, direct observation that they behave consistently with each other will help further the understanding of the physics of neutrinos."

Over the past two years, solar neutrino experiments implied that the ghostlike snippets of matter/anti-matter do possess enough mass to enable them to oscillate and change flavor over a distance. However, some scientists have questioned whether these solar neutrinos might have interacted with the sun's magnetic field en route to detectors. KamLAND is the first experiment to observe the properties responsible for solar neutrino flavor changes from a terrestrial source, the reactors in Japan's nuclear power plants.

The KamLAND scientists report that over a period of 145 days of operation, they recorded 54 electron anti-neutrino events in the energy range of 1 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 collaborators concluded that the likely explanation is anti-neutrinos oscillated on their way from the reactors, which caused some of them to change from electron to muon and tau anti-neutrinos. Furthermore, they deduced that a mixing of the three flavors of anti-neutrinos took place, a phenomenon that will be helpful in pinning down neutrino mass with better precision than is possible with solar neutrino experiments. Japan's Ministry of Education, Science, Sports and Culture and the U.S. Department of Energy fund the experiments, which will continue for several years to enable refinement of measurements.

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 also can be used to measure our planet's internal radioactivity. And what Earth scientists learn using neutrinos has implications for astrophysics. "The elements inside Earth are formed inside stars," says Stanford geophysics Professor Norman Sleep, whose interests include analysis of neutrinos from terrestrial sources. "Contributions from the KamLAND detector aid in understanding how the Earth and planets formed."

With a more purified liquid scintillator, KamLAND also will be used to study solar neutrinos in a new low-energy regime.

Lynn Yarris is a science writer at Lawrence Berkeley National Laboratory. Stanford Report science writer Dawn Levy contributed to this report.