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 1970s, is in need of updating. The results also point the way
to the first direct measurements of the total radioactivity of the
"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 University 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
Adds Stuart Freedman, a nuclear physicist with a joint appointment at the
Lawrence Berkeley National Laboratory (Berkeley Lab) and the University
of California at 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
The KamLAND neutrino experiments are being conducted by an international
collaboration largely comprised of scientists from Japan and the United
States. The U.S. team at KamLAND includes researchers from Stanford,
Berkley Lab and UC Berkeley, 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
research facility funded by the U.S. Department of Energy (DOE), 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 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
which 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 in diameter, 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
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
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 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
"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.
"We haven't done it yet, but in principle I can turn my reactors on
and off and see my neutrinos stop and start again - and I cannot turn off
the sun," says Gratta. "That's the beauty of the measurement,
what makes it very different from all the things that have been done up
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
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
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
Construction of the KamLAND detector began in 1998 and operations began
in January of 2002. Japan's Ministry of Education, Science, Sports, and
Culture provided more than $20 million of KamLAND's construction costs.
The U.S. Department of Energy's Office of Science provided nearly $6
"The success at KamLAND gives strong support for continued DOE
participation in funding international collaborative projects," says
Peter Rosen, associate director of the Office of Science's High Energy
and Nuclear Physics programs and a leading theorist on neutrino physics.
"Science, by its very nature, knows no national boundaries, and so
it provides a unique platform for peaceful collaboration among
The KamLAND experiments will continue for several 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
"The underlying physics and techniques used in KamLAND will allow us
to detect radioactivity in the Earth, which addresses a 150-year-old
problem and lets it be solved quantitatively," says Stanford
geophysics Professor Norman Sleep, whose interests include analysis of
neutrinos from terrestrial sources. Sleep says scientists can use the
KamLAND detector to observe earthly neutrinos, which have different
energies than the reactor-generated neutrinos that Gratta and his
And what Earth scientists learn using neutrinos has implications for
astrophysics. "The elements inside Earth are formed inside
stars," Sleep says. "Contributions from the KamLAND detector
aid in understanding how the Earth and planets formed."
KamLAND, with a more purified liquid scintillator, will also be used to
study solar neutrinos in a new low energy regime.
But 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
Lynn Yarris is a
science writer at Lawrence Berkeley National Laboratory. Stanford science
writer Dawn Levy contributed to this report.