December 6, 2002 |
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,” 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.
From the Lawrence Berkeley National Laboratory:
Disappearing Neutrinos at KamLAND Support the Case for Neutrino Mass
BERKELEY, CA — 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,” 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.
“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 which 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. The U.S. effort was primarily funded by the Office of Science in the U.S. Department of Energy (DOE). Said Raymond Orbach, Director of the Office of Science, “This brilliantly-conceived experiment uses terrestrial anti-neutrinos to confirm the knowledge that we have gleaned from solar neutrinos. The results will have a major impact on our understanding of neutrino physics.”
The U.S. team at KamLAND includes researchers from Berkeley Lab, UC Berkeley and Stanford, plus 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, and the Triangle Universities Nuclear Laboratory, a DOE-funded research facility located at Duke University, and staffed by researchers with 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 also includes, in addition to Tohuku University participants, 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 13 meters (43 feet) in 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 which convert the flashes into electronic signals that computers can analyze. The photomultipliers are attached to the inner surface of an 18 meters in diameter stainless steel sphere and separated from the weather balloon by a buffering bath of inert oil and water which 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 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.
Says John Learned, a physicist who leads the University of Hawaii’s participation in the KamLAND collaboration, “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.”
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 neutrino properties 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. DOE’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.”
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