Twenty-Eight Day Cycle Found In Solar Neutrinos

December 20, 1997

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.

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

Stanford University

Related Neutrinos Articles from Brightsurf:

Big answers from tiny particles
A team of physicists led by Kanazawa University demonstrate a theoretical mechanism that would explain the tiny value for the mass of neutrinos and point out that key operators of the mechanism can be probed by current and future experiments.

Physicists cast doubt on neutrino theory
University of Cincinnati physicists, as part of an international research team, are raising doubts about the existence of an exotic subatomic particle that failed to show up in twin experiments.

Exotic neutrinos will be difficult to ferret out
An international team tracking the 'new physics' neutrinos has checked the data of all the relevant experiments associated with neutrino detections against Standard Model extensions proposed by theorists.

Excess neutrinos and missing gamma rays?
A new model points to the coronoe of supermassive black holes at the cores of active galaxies to help explain the excess neutrinos observed by the IceCube Neutrino Observatory.

Where neutrinos come from
Russian astrophysicists have come close to solving the mystery of where high-energy neutrinos come from in space.

Where did the antimatter go? Neutrinos shed promising new light
We live in a world of matter -- because matter overtook antimatter, though they were both created in equal amounts when our universe began.

Strongest evidence yet that neutrinos explain how the universe exists
New data throws more support behind the theory that neutrinos are the reason the universe is dominated by matter.

Why didn't the universe annihilate itself? Neutrinos may hold the answer
New results from an experiment called T2K suggest that physicists are closer than ever before to answering a major mystery: Why didn't the universe annihilate itself in a humungous burst of energy not long after the Big Bang?

T2K insight into the origin of the universe
Lancaster physicists working on the T2K major international experiment in Japan are closing in on the mystery of why there is so much matter in the universe, and so little antimatter.

Radar and ice could help detect an elusive subatomic particle
A new study published today in the journal Physical Review Letters shows, for the first time, an experiment that could detect a class of ultra-high-energy neutrinos using radar echoes.

Read More: Neutrinos News and Neutrinos Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to