Nuclear reactors contribute anti-neutrinos for these neutrino mass experiments in Japan.
Randall McArthur
-----Original Message-----
From: Interactions News Wire [mailto:newswire@interactions.org]
Sent: Monday November 15, 2004 4:34 PM
To: interactionsnewswire
Subject: [Interactions News Wire] #66-04 - LBL: New results from
anti-neutrino studies at KamLAND
Interactions News Wire #66-04
15 November 2004
http://www.interactions.org
*******************************************************************
Source: Berkeley Lab
Content: Press Release
Date Issued: November 15, 2004
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For more information contact: Lynn Yarris (510) 486-5375 lcyarris@lbl.gov
An HTML version of this release with images and links to further
information is available at
http://www.lbl.gov/Science-Articles/Archive/NSD-Kamland-results.html
NEW RESULTS FROM ANTI-NEUTRINO STUDIES AT KAMLAND
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.
"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.
"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.
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/.
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