Disappearing Neutrinos at KamLAND Support the Case for Neutrino Mass

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.