Flipping Neutrinos

Monday, April 01, 1996
RELATED TAGS: SUBATOMIC PARTICLES
Neutrinos are wispy subatomic particles with no electric charge and little, if any, mass. They’re created during nuclear reactions such as those that power the sun. Given the size of the sun, it should produce lots of neutrinos--and it does. But after 30 years of looking, physicists detect less than a third of the amount they think the sun should be producing. Now Johns Hopkins physicist Ralph McNutt may have an explanation for the shortfall: he has found a link between the number of solar neutrinos counted and the strength of the solar wind near Earth.

Most of the zillions of neutrinos coming out of the sun zip through Earth without stopping; researchers must go to great lengths just to detect a handful. In his study, McNutt used data from a 30-year-old neutrino detector--a 100,000-gallon tank of cleaning fluid located almost a mile underground in the Homestake Gold Mine in South Dakota. Neutrinos from the sun occasionally hit chlorine atoms in the fluid, converting them into a radioactive--and detectable--form of argon. The Homestake measurements are what first made researchers realize that they had a solar neutrino problem.

The most popular solution to that problem lately has been a theory called neutrino oscillation. There are three types of neutrino-- electron, muon, and tau--and the Homestake detector detects only the first. According to the neutrino oscillation theory, some of the sun’s electron neutrinos spontaneously transform into one of the two undetectable types before they leave the sun, thus producing what looks like a shortfall at Homestake.

McNutt’s new observations support an alternative theory. After comparing the Homestake data with 19 years’ worth of satellite measurements of the solar wind--which consists of electrically charged particles like protons and electrons--McNutt has found that the neutrino counts have risen and fallen as the solar wind has. Since the strength of the solar wind is probably controlled by the sun’s magnetic field, McNutt believes his observations suggest that the field is also affecting the neutrinos. As they pass through the convective zone below the sun’s surface, he suggests, the field may flip the magnetic polarity of some of them, making them undetectable.

That would require neutrinos to have magnetism, which hasn’t been observed. (On the other hand, the oscillation theory requires neutrinos to have mass, and that too hasn’t been proved.) McNutt’s theory that neutrino magnetism can be changed will be even harder to verify in the lab. In the sun, we’re dealing with a magnetic field perhaps 10,000 times the strength of Earth’s magnetic field, over a convective zone that is 120,000 miles thick, he says. There is no direct way of re-creating that on Earth.
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