UH Manoa Faculty Part of International Team Participating in Experimental Operations

Disappearing neutrinos at KamLAND support the case for neutrino mass

University of Hawaiʻi
John Learned, (808) 956-2964
Department of Physics & Astronomy
Posted: Dec 6, 2002

A University of Hawaiʻi at Mānoa team of scientists and their international colleagues announced results from the first six months of experiments at KamLAND, an underground neutrino detector in central Japan, showing 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 1970s, is in need of updating. The results also point the way to the first direct measurements of the total radioactivity of the earth.

The UH Mānoa scientists from the department of physics and astronomy are among more than 90 scientists from institutions across the United States and Japan. They include Drs. Peter Gorham, Gene Guillian, John Learned, Jelena Maricic, Shigenobu Matsuno, and Sandip Pakvasa. Dr. Stephen Olsen, also a UH scientist, is the principal investigator of a U.S. Department of Energy High Energy Physics grant awarded to UH, from which the project is supported.

"This new result from the KamLAND collaboration is a landmark discovery in the study of one of nature‘s fundamental constituents, neutrinos. It really heralds the close of our initial phase of explorations into uncharted territory, and our findings are unprecedented," said Dr. John Learned, UH physics professor. "We now know the solution to the long running solar neutrino puzzle, and we have evidence for the most peculiar behavior of these particles transmuting into each other as they fly even shorter distances on earth."

UH has a long history of activity in neutrino research, having been involved in experiments at accelerators and in mines and the ocean for more than 25 years. In particular, the UH team played a prominent role in the 1987 discovery of neutrinos from Supernova 1987A, heralding the birth of neutrino astronomy and setting many limits on neutrino properties, such as mass. The discovery was made with the IMB detector, the first massive underground nucleon decay search instrument and neutrino detector, which was built in a 2000-foot deep Morton Salt mine near Cleveland, Ohio in the 1980s.

This specific UH team also played a key role in the analysis and discovery of muon neutrino oscillations, and hence neutrino mass, in the Super-Kamiokande detector in 1998, a discovery that received international coverage and is now heralded as the most important of the 1990s in elementary particle physics.

"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," says Stuart Freedman, a nuclear physicist with a joint appointment at the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley, who is a co-spokesperson for the U.S. team at KamLAND.

Adds Giorgio Gratta, an associate professor of physics at Stanford University and the other co-spokesperson for the U.S. team, "We are seeing the same neutrino deficit, or a deficit that is compatible with the deficit that people have been seeing for years in solar neutrino experiments. 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."

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

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, in addition to Tohuku University participants, also includes researchers from the Institute of High Energy Physics in Beijing.

KamLAND stands for Kamioka Liquid scintillator Anti-Neutrino Detector. Located in a mine cavern beneath the mountains of Japan‘s main island of Honshu, near the city of Toyama, 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 that is 18 meters in diameter and separated from the weather balloon by a buffering bath of inert oil and water that helps suppress interference from background radiation.

"With the sensitivity and background shielding at KamLAND, we can deduce the exact timing, location and energy of anti-neutrino events occurring inside the balloon," says Freedman. 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 abundantly 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.

Since anti-matter is thought to be the mirror image of matter in properties and behavior, to study anti-neutrinos is to study neutrinos. In fact, 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 in Physics, were based on anti-neutrinos produced in nuclear reactors.

"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," says UH physicist John Learned.

According to the predictions from the Standard Model, neutrinos/anti-neutrinos are without mass. Contrary to this, over the past two years, solar neutrino experiments at the SNO and Super-K detectors 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 in an unexpected way with the sun‘s magnetic field enroute to detectors. KamLAND is the first experiment to observe the properties of the neutrino responsible for solar neutrino flavor changes, from a terrestrial source, the reactors in Japan‘s nuclear power plants.

"It‘s an amazing coincidence that KamLAND just happens to be the right distance (an average of about 175 kilometers) from Japan‘s nuclear reactors for us to be sensitive to the anti-neutrino oscillations that are expected from the solar experiments," says Freedman.

In a paper for Physical Review Letters, the 92 physicists who make up the KamLAND collaboration report that over a period of 145 days of operation, 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 collaboration 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, the collaborators deduced that a mixing of the three flavors of anti-neutrinos took place, a phenomenon that will be helpful in pinning down the neutrino mass with better precision than is possible with the solar neutrino experiments as the KamLAND experiments continue their run.

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."

Construction of the KamLAND detector began in 1998 and operations began in January of 2002. More than $20 million of KamLAND‘s construction costs were provided by Japan‘s Ministry of Education, Science, Sports, and Culture. The U.S. Department of Energy‘s Office of Science provided nearly $6 million.

Says Peter Rosen, Associate Director of the Office of Science‘s High Energy and Nuclear Physics programs and a leading theorist on neutrino physics, "The success at KamLAND gives strong support for continued DOE participation in funding international collaborative projects. Science, by its very nature, knows no national boundaries, and so it provides a unique platform for peaceful collaboration among nations."

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. For now, the evidence of neutrino oscillations and flavor has been firmly established.

As collaborator Robert McKeown of Cal Tech explains, "This is really a clear demonstration of neutrino oscillation. 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. KamLAND allows us to study the neutrino in a controlled experiment."

The first local presentation of the scientific results will be made by Maricic, a native of Hawaiʻi, at a Physics Colloquium on Thursday, December 12, at 3:30 p.m. in Watanabe Hall on the UH Mānoa campus.

Downloadable images of the KamLAND detector courtesy of the collaboration are available at www.lbl.gov

More information about the discovery and the UH team involvement can be accessed at http://www.phys.hawaii.edu:80/~jgl/kamland_results_public.html, http://www.phys.hawaii.edu:80/~jgl/kamland_uh_connection.html, and http://www.phys.hawaii.edu:80/~jgl/nu_timeline.html.

For more information, visit: http://www.phys.hawaii.edu/~jgl/kamland.html