Peering inside the sun: the elusive neutrino

Neutrinos are just about as close to nothing as matter gets. As the word "neutrino" suggests, these particles have no charge (neutral) and are very, very tiny (the "ino" ending). Neutrinos have almost no mass, travel through space very near the speed of light, and really don't pay much attention to other matter in the universe at all.

Almost nothing in the universe interacts with neutrinos. A neutrino could sail right through a million miles of solid lead without a single particle-sized hiccup. It stands to reason that if neutrinos can sail through most normal matter, then unlike the photons produced by fusion reactions deep inside the solar core, the neutrinos can speed directly out of the sun. Instead of reaching us after millions of years of random interactions, neutrinos from the sun's fusion reaction would reach the Earth about eight minutes after they were created (eight minutes being the light-speed travel time from the Earth to the sun). Solar neutrinos, therefore, are a simple, direct probe of fusions reactions hidden from us in the superdense core of the sun.

Of course, while it's convenient that neutrinos are almost unstoppable, allowing us to peer into the sun, it's a real challenge to actually catch them. Neutrinos can't be stopped easily; after all, they can fly freely through the sun's core, which is denser than any possible material on Earth.

What they will react with, I kid you not, is dry-cleaning fluid. Perchloroethylene is a common, chlorine-rich fluid used in dry cleaning. When neutrinos pass through this fluid, there is a very small chance that a neutrino will bump up against the nucleus of a chlorine atom and change it into argon. It might only happen one time out of trillion trillion chances, but that problem gets easier if you can get a LOT of dry-cleaning fluid together in one place.

Dr. Ray Davis (one of the recipients of this year's Nobel Prize in physics) did just that. His Nobel Prize-winning idea was to get about 100,000 gallons of perchloroethylene and bury the whole lot deep in an old South Dakota gold mine. The idea behind putting the tank in the mine was to clean up any contamination from other high-energy particles from space, namely cosmic rays. Cosmic rays, which are produced by all kinds of processes in space, might produce false-positive results in the neutrino detector, so the couple-thousand feet of rock above the mine was used to screen them out.

Davis and his team did detect a few chlorine atoms turning into argon (on average, one event was observed every two days or so), and scientists had their first readings directly from the solar core.

Interestingly, the number of events, as statistically small as they seem, was far below what particle physicists had predicted. Over many years, this result became impossible to ignore; for some reason, the detectors were only finding about one third of the number of neutrinos that should be there. This was an important cause for concern, as the neutrino rate should be linked to the energy production inside the sun. Could it be possible that the sun's nuclear furnace was slowing down, maybe even stopping?

In the end, it was discovered that neutrinos come in three "species," of which the dry-cleaning fluid could only detect one. If the neutrinos had mass, no matter how tiny, it would be possible for the neutrinos created in the sun to change on their way to Earth. That would mean we'd only detect about one third of the total neutrinos out there, which matched the results exactly!

Now the search is on for how much mass neutrinos have. There are so many neutrinos, careening freely through space, that even if they have a mass less than a billionth the mass of an electron, neutrinos may make up the majority of the mass of the universe. And that may allow the humble neutrino to go from a convenient tool to understand our sun, to a factor that may change the fate or our universe.

Michelle Thaller is an astronomer at the California Institute of Technology.

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