Twenty-eight day cycle
found in ghostly solar neutrinos, team says
BY DAVID F. SALISBURY
Stanford researchers have
found evidence for a 28-day cycle in the number of
neutrinos reaching Earth from the Sun, and they suggest
two controversial mechanisms that might explain their
findings.
Related
Information:
- Elusive neutrino
raises key question in particle physics: 6/4/97
Their preferred hypothesis
postulates a region of intense magnetic fields rotating
deep within the solar interior. Because neutrinos are
created in the nuclear reactions that take place at the
Sun's core, such a region might disrupt the flow of
neutrinos to Earth. For this hypothesis to hold, however,
the basic properties of the neutrino must be redefined in
a way that conflicts with the "standard model,"
the well-tested description of the nature of fundamental
particles and forces. Theories that redefine the neutrino
in this fashion have been advanced to explain the missing
mass in the universe and the unexpectedly low number of
neutrinos that have been detected coming from the Sun.
An alternate explanation
might be regular pulsing in the strength of the nuclear
reactions themselves. There is no outward evidence for
such a cycle. But light created at the Sun's core takes
100,000 years or more to reach the surface, so any
fluctuations in the production of radiation in the core
could be completely smoothed out by the time the light
reaches the Sun's surface, the researchers propose.
The statistical evidence
for this cycle is reported in the Dec. 10 issue of the Astrophysical
Journal by Peter Sturrock, professor of applied
physics; Guenther Walther, assistant professor of
statistics; and physics research associate Michael
Wheatland. Their analysis is based on data collected at
the Homestake neutrino detector in South Dakota over a
24-year period. Using advanced statistical procedures,
they found clear evidence for a 28.4-day cycle. "We
estimate the probability that the cycle is due to chance
to be about three parts in a hundred," Walther said.
Nature's shadow
particles
Neutrinos are nature's
shadow particles. In contrast to light, neutrinos take
less than nine minutes to travel from the center of the
Sun to the Earth. They can make the trip so quickly
because they pass through ordinary matter almost as if it
does not exist. About one million billion solar neutrinos
pass unnoticeably through your body each second.
Despite their ghostly
nature, neutrinos can be detected because they do
interact occasionally with ordinary atoms and molecules.
In one such interaction, neutrinos change chlorine atoms
into argon. Despite the enormous number of neutrinos
passing through the Earth, this interaction is so rare
that just to detect it scientists have been forced to
build tanks holding tens of thousands of gallons of
chlorine-containing liquid and develop methods for
picking individual argon atoms out of such large volumes
of liquid. It wasn't enough to do this on the Earth's
surface, either, because cosmic rays also can convert
chlorine into argon. So the tanks had to be buried deep
underground.
The first successful solar
neutrino detector was located in the Homestake Gold Mine.
Buried a mile underground, the detector contains 100,000
gallons of carbon tetrachloride. For the last 30 years,
the Pennsylvania State University scientists who operate
the detector have filtered out and counted the argon
atoms that have accumulated every few months. Over this
period, the instrument has identified an average of about
one neutrino event every two days.
That rate is about
one-third the number that scientists who study the solar
nuclear reactions had predicted. Two other neutrino
detectors, the Kamiokande experiment in the Japan Alps
and the Gallex experiment in the Gran Sasso Laboratory in
Italy, both have verified the shortfall measured at
Homestake.
One possible explanation
for this deficit was that the neutrino-producing nuclear
reactions were happening more slowly than the scientists
expected, which would be the case if the temperature at
the Sun's core were about one million degrees Celsius
less than predicted. But observations of sound waves
traveling deep into the solar interior have provided a
temperature measurement of 15.6 million degrees Celsius,
too hot to lower neutrino production.
The situation has led some
scientists to invoke "new physics" to explain
the low observed numbers. According to standard nuclear
physics, a neutrino at rest does not have any mass. Now
some theoretical physicists are proposing that these
particles may have an infinitesimal but non-zero mass.
Several major experiments have been built to test this
proposal. Sturrock and his colleagues use these new
theories to explain the variations in neutrino flow that
they have found.
If neutrinos have any mass
at all, they would help account for the "missing
matter" in the universe. Astronomers have found that
galaxies act as if they are swirling around a center of
mass substantially larger than scientists can account for
by summing up the amount of visible matter that they
contain. Neutrinos with mass could account for at least
some of this "dark matter."
Neutrino cycling
The proposed neutrino mass
is far too small to measure directly. So scientists are
trying to detect a predicted side effect. Neutrinos come
in three varieties, each associated with a different
elementary particle (electron, muon and tau). According
to some new theories, if neutrinos have mass, then they
may cycle between the three different neutrino types.
Directly measuring this
effect is the purpose of the Palo Verde Neutrino
Oscillation Project, headed by Stanford Associate Physics
Professor Giorgio Gratta and Professor Emeritus Felix
Boehm from Caltech. They led the design and construction
of a neutrino detector a mile from the Palo Verde
Generating Station in Arizona to determine if the
neutrinos produced by the station's nuclear reactors
undergo this cycling effect.
This effect could explain
the shortfall in solar neutrinos. Only one of the three
types of neutrino, the electron neutrino, is detectable.
If the electron neutrinos produced by the Sun change into
muon and tau neutrinos en route, it would mean that only
one-third of the neutrinos reaching Earth would be
detectable.
To look for regular
variations in the number of neutrinos reaching Earth,
Sturrock and his colleagues analyzed the data collected
at Homestake. Because the data were collected about four
times a year, it normally would be impossible to use this
information to identify cycles as short as 28 days. But
the data were not collected at regular intervals. That
allowed the researchers to piece together evidence for a
shorter cycle by constructing a detailed computer
simulation of the detector, running thousands of
simulations, and comparing the outcomes with the
detector's actual observations.
In an earlier analysis,
conducted in 1995 and 1996, the researchers thought they
had found evidence for a 21.3-day peak. This was reported
in the News and Comment section of Science magazine. When
they submitted this analysis to a scientific journal,
however, one of the reviewers was unable to duplicate
their results. When the researchers reworked the problem
from scratch, they discovered an error in their
transcription of the Homestake data. When this was
corrected the 21.3-day cycle disappeared.
The researchers find the
28.4-day cycle particularly intriguing because it
corresponds almost exactly to the rotation rate of the
Sun's interior, as seen from Earth. The Sun is made up of
three parts: the core, where the nuclear fusion reactions
that power the Sun take place; the radiative zone where
energy is transported outward from the core; and the
outer, convective zone. The radiative zone rotates as if
it were a solid, rather than a gaseous body, at this same
rate. So Sturrock and his collaborators speculate that
the source for this cycle in neutrino flux may originate
in the radiative zone. A region of extra-intense magnetic
fields might modulate the flow of these particles, they
suggest.
Magnetic moment
For magnetic fields to
have such an effect, neutrinos must have a physical
characteristic called a magnetic moment. According to
current particle physics, they don't possess this
quality. But the same theories that assign mass to the
neutrino also give it a magnetic moment, which would make
it respond to magnetic forces. In 1986, a group of
scientists from the [then] Soviet Union, made the case
that the spin of neutrinos with magnetic moments could be
changed by traveling a long distance through a strong
magnetic field.
Sturrock and his
co-authors invoke this effect as their preferred
explanation for the cycle that they have found. Neutrinos
spin in either a left-handed or right-handed direction.
Nuclear reactions produce only left-handed neutrinos, and
only left-handed neutrinos take part in nuclear reactions
such as converting chlorine into argon. If different
parts of the Sun have different strength magnetic fields,
the flux of left-handed neutrinos will vary as they
travel in different directions from the Sun. That would
lead to a detection rate on Earth that varies with the
Sun's rotation period.
As the Earth orbits the
Sun, the neutrinos that are detected on Earth pass
through different solar latitudes, due to a tilt in the
Sun's axis of rotation. This can produce other, weaker
cycles centered on the basic rotation frequency. The
period of 28.4 days corresponds to a frequency of 12.9
cycles per year. Sturrock and his collaborators have also
found evidence of the expected "sidebands" at
10.9, 11.9, 13.9 and 14.9 cycles per year.
At the same time, the new
analysis did not find any evidence for neutrino
variations that correspond to the 11-year solar cycle and
only weak evidence for two other proposed cycles: a
157-day periodicity that Eric Rieger of the Max Planck
Institute in Germany found in the intensity of solar
flares, and a 780-day "quasi-biennial"
periodicity that Kunitomo Sakurai from Kanagawa
University reported finding in the Homestake data.
"I thought [Sturrock
and his colleagues'] analysis was quite convincing,"
said Jeffrey Scargle, a research astrophysicist at NASA's
Ames Research Center, who is an expert in this type of
analysis. "The method they used was very good and
they made a really good case for the signal being in the
data. Of course, what this means for solar and particle
physics is quite problematic." SR
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