From Wired: The mystifying history of neutrino experiments.

Look at the nail on your pinky finger: Every second, about 65 billion neutrinos pass through it. Almost all were produced inside the giant nuclear reactor in our sun's belly.

Astronomers want to detect those neutrinos because they contain important information about processes going on in the sun's center. In 1964, physicist Ray Davis and astronomer John Bacall built an experiment in the Homestake mine in South Dakota to find these neutrinos. The detector needed to be placed deep underground because cosmic rays hitting the Earth's atmosphere would interfere with the results.

After the Homestake experiment was calibrated and run, the researchers noticed an anomaly. According to their calculations, the sun should have been producing three times as many neutrinos as they actually detected. So they went back to the drawing board, looking for mistakes and making more refined estimates. But they still couldn't figure out where they went wrong.

The Homestake experiment ran for more than 30 years, always showing the same result: three times fewer neutrinos than expected. Astronomers feared that their models of the sun might be totally incorrect. The problem persisted into the mid-'90s.

By this point, researchers had discovered that neutrinos come in three different types. The neutrino produced during beta decay or in the sun's center is an electron neutrino, but other processes will create particles known as muon or tau neutrinos.

You might guess why the three-type finding was important to a puzzle in which researchers found one-third the number of neutrinos they expected. Researchers realized that during their flight between the sun and Earth, electron neutrinos -- the type detected at Homestake -- were transforming into the other types. As a result, the experiment missed two-thirds of the neutrinos. When new detectors were built that could catch all three types of neutrinos, the discrepancy vanished.

The finding had profound implications. While some scientists had previously considered the neutrino to be massless, oscillating between different types required the particles to have mass.

It's an excellent article and a partial glimpse into the current state of physics. You'll be more knowledgeable for having read the whole thing.

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