BERKELEY, CA – First they were seen to go away, now, for the first time, they’ve been seen coming back. An international team of researchers at KamLAND, an underground neutrino detector in central Japan, has shown that not only do anti-neutrinos emanating from nearby nuclear reactors “disappear,” they also “reappear.” This is further evidence that the three known types or “flavors” of neutrinos — electron, muon and tau — all have mass and can oscillate or change from one type to another.

Pictured here is the KamLAND stainless steel container inside of which is the inner detector which reacts to incoming neutrinos. This steel container is now surrounded in ultra-pure water which holds phototubes that serve as an outer detector to tag incoming background radiation.
(Image courtesy of the KamLAND collaboration.)

“In all of the previous neutrino experiments, it was reported that the electron neutrinos were oscillating into the neutrino flavors we can’t detect. Now, with more precise measurements, we’re seeing that the disappearing neutrinos are oscillating back into the electron neutrinos we can detect. This is the most direct evidence yet of neutrino oscillation,” says nuclear physicist Stuart Freedman, co-spokesperson for KamLAND’s U.S. team of researchers. Freedman holds a joint appointment with the Nuclear Science Division of the Lawrence Berkeley National Laboratory (Berkeley Lab) and the Physics Department of the University of California at Berkeley (UC Berkeley).

Neutrinos are subatomic particles which interact so rarely with other particles of matter that a single neutrino could pass untouched through a wall of lead stretching from the earth to the moon. These ghostlike particles, however, are highly prized objects of study in science, for they offer insight into the nature of matter from the smallest to the most cosmological of scales.

Neutrinos are produced during nuclear fusion, the reaction that lights the sun and other stars. Their anti-matter counterparts, anti-neutrinos, are created in fission reactions such as those that drive nuclear power plants. As KamLAND exeperiments previously demonstrated, neutrinos and anti-neutrinos behave in exactly the same way.

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, 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 bath of inert oil which helps suppress interference from background radiation. The steel sphere itself is submerged in water which acts as a cosmic ray veto counter.

Stuart Freedman (left) and Patrick Decowski, are two of the scientists from Berkeley Lab who participated in a KamLAND experiment which showed anti-neutrinos disappearing and re-appearing due to oscillations that caused them to change flavor.

“With the sensitivity and background shielding we have at KamLAND, we can deduce the exact timing, location and energy of anti-neutrino events occurring inside the balloon,” says Freedman. “Furthermore, 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 same oscillations that are measured in the solar neutrino experiments.”

According to the predictions of the Standard Model of Particle Physics, which has been used to explain fundamental physics since the 1970's, neutrinos/anti-neutrinos are without mass. However, experiments at KamLAND, measuring anti-neutrinos, and experiments measuring neutrinos originating from the sun, have indicated that neutrinos do possess mass, which enables them to oscillate and change flavor as they travel across a distance. In all of these previous experiments, neutrino oscillation was inferred from the disappearance of electron neutrinos/anti-neutrinos.

For example, in the case of the KamLAND experiments, there was a deficit in the number of electron anti-neutrinos being detected, versus the number of anti-neutrinos known to be produced in the nearby reactors. That this anti-neutrino deficit matched the deficits being reported from the solar neutrino experiments was taken as evidence that neutrino oscillation was the mechanism behind their disappearance.

KamLAND consists of a weather balloon, 13 meters in diameter, filled with a kiloton of liquid scintillator. The balloon is surrounded by 1,879 photomultipliers attached to a stainless steel sphere which itself is bathed in ultrapure water.
(Image courtesy of the KamLAND collaboration.)

Now, with nearly two years of analyzed data in hand, the KamLAND collaboration of researchers is announcing that, for the first time, they are seeing a statistically significant “distortion of the anti-neutrino energy spectrum” that is consistent with neutrino oscillation. The results are being published in a paper that will appear in Physical Review Letters.

Explains Patrick Decowski, a guest researcher with Berkeley Lab’s Nuclear Science Division and a post-doc with the UC Berkeley Physics Department, who was a major contributor to the PRL paper, “The new KamLAND results constitute further proof that neutrinos have mass and that the Standard Model describing fundamental particles will need to be amended.
There are several theories on how to include neutrino mass in the Standard Model, but nothing comprehensive yet.”

The international research collaboration conducting the KamLAND neutrino experiments is largely comprised of scientists from Japan’s Tohoku University, and more than a dozen institutes in United States. The U.S. team at KamLAND includes researchers from Berkeley Lab, UC Berkeley and Stanford University, 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.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at www.lbl.gov/.

Additional Information

For more information about the KamLAND experiments, please visit the following Websites:

• http://kamland.lbl.gov/
• http://www.awa.tohoku.ac.jp/html/KamLAND/ and
• http://hep.stanford.edu/neutrino/KamLAND/KamLAND.html

Stuart Freedman can be reached by phone at 510-486-7850 or sjfreedman@lbl.gov