PASADENA, Calif.- In the subatomic particle family, the neutrino is a
bit like a wayward red-haired stepson. Neutrinos were long ago
detected-and even longer ago predicted to exist-but everything
physicists know about nuclear processes says there should be a
certain number of neutrinos streaming from the sun, yet there are
nowhere near enough.
This week, an international team has revealed that the sun’s lack of
neutrinos is a real phenomenon, probably explainable by conventional
theories of quantum mechanics, and not merely an observational quirk
or something unknown about the sun’s interior. The team, which
includes experimental particle physicist Robert McKeown of the
California Institute of Technology, bases its observations on
experiments involving nuclear power plants in Japan.
The project is referred to as KamLAND because the neutrino detector
is located at the Kamioka mine in Japan. Properly shielded from
radiation from background and cosmic sources, the detector is
optimized for measuring the neutrinos from all 17 nuclear power
plants in the country.
Neutrinos are produced in the nuclear fusion process, when two
protons fuse together to form deuterium, a positron (in other words,
the positively charged antimatter equivalent of an electron), and a
neutrino. The deuterium nucleus hangs nearby, while the positron
eventually annihilates both itself and an electron. The neutrino,
being very unlikely to interact with matter, streams away into space.
Therefore, physicists would normally expect neutrinos to flow from
the sun in much the same way that photons flow from a light bulb. In
the case of the light bulb, the photons (or bundles of light energy)
are thrown out radially and evenly, as if the surface of a
surrounding sphere were being illuminated. And because the surface
area of a sphere increases by the square of the distance, an observer
standing 20 feet away sees only one-fourth the photons of an observer
standing at 10 feet.
Thus, observers on Earth expect to see a given number of neutrinos
coming from the sun-assuming they know how many nuclear reactions are
going on in the sun-just as they expect to know the luminosity of a
light bulb at a given distance if they know the bulb’s wattage. But
such has not been the case. Carefully constructed experiments for
detecting the elusive neutrinos have shown that there are far fewer
neutrinos than there should be.
A theoretical explanation for this neutrino deficit is that the
neutrino “flavor” oscillates between the detectable “electron”
neutrino type, and the much heavier “muon” neutrino and maybe even
the “tau” neutrino, neither of which can be detected. Utilizing
quantum mechanics, physicists estimate that the number of detectable
electron neutrinos is constantly changing in a steady rhythm from 100
percent down to a small percentage and back again.
Therefore, the theory says that the reason we see only about half as
many neutrinos from the sun as we should be seeing is because,
outside the sun, about half the electron neutrinos are at that moment
one of the undetectable flavors.
The triumph of the KamLAND experiment is that physicists for the
first time can observe neutrino oscillations without making
assumptions about the properties of the source of neutrinos. Because
the nuclear power plants have a very precisely known amount of
material generating the particles, it is much easier to determine
with certainty whether the oscillations are real or not.
Actually, the fission process of the nuclear plants is different from
the process in the sun in that the nuclear material breaks apart to
form two smaller atoms, plus an electron and an antineutrino (the
antimatter equivalent of a neutrino). But matter and antimatter are
thought to be mirror-images of each other, so the study of
antineutrinos from the beta-decays of the nuclear power plants should
be exactly the same as a study of neutrinos.
“This is really a clear demonstration of neutrino disappearance,”
says McKeown. “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.
“Willy Fowler [the late Nobel Prize-winning Caltech physicist] always
said it’s better to know the physics to explain the astrophysics,
rather than vice versa,” McKeown says. “This experiment allows us to
study the neutrino in a controlled experiment.”
The results announced this week are taken from 145 days of data. The
researchers detected 54 events during that time (an event being a
collision of an antineutrino with a proton to form a neutron and
positron, ultimately resulting in a flash of light that could be
measured with photon detectors). Theory predicted that about 87
antineutrinos would have been seen during that time, if no
oscillations occurred, but 54 events at an average distance of 175
kilometers if the oscillation is a real phenomenon.
According to McKeown, the experiment will run about three to five
years, with experimentalists ultimately collecting data for several
hundred events. The additional information should provide very
accurate measurements of the energy spectrum predicted by theory when
the neutrinos oscillate.
The experiment may also catch neutrinos if any supernovae occur in
our galaxy, as well as neutrinos from natural events in Earth’s
interior.
In addition to McKeown’s team at Caltech’s Kellogg Radiation Lab,
other partners in the study include the Research Center for Neutrino
Science at Tohuku University in Japan, the University of Alabama, the
University of California at Berkeley and the Lawrence Berkeley
National Laboratory, Drexel University, the University of Hawaii, the
University of New Mexico, Louisiana State University, Stanford
University, the University of Tennessee, Triangle Universities
Nuclear Laboratory, and the Institute of High Energy Physics in
Beijing.
The project is supported in part by the U.S. Department of Energy.
KamLand Web site:
http://www.krl.caltech.edu/~bmck/kamland/public/
