Where have all the neutrinos gone?

In the subatomic particle family, the neutrino is a bit like a wayward stepson. Neutrinos were long ago detected, and everything physicists know says there should be a certain number streaming from the sun—yet there are nowhere near enough.

An international team, including Caltech experimental particle physicist Robert McKeown, has revealed that the sun’s lack of neutrinos is a real phenomenon, probably explainable by conventional quantum mechanics theory. Observations were based on experiments involving nuclear power plants in Japan.

The project is called KamLAND, after Japan’s Kamioka mine, where the neutrino detector is located. Properly shielded from background and cosmic radiation, the detector is optimized for measuring neutrinos from the country’s 17 nuclear plants.

Neutrinos are produced in nuclear fusion, when two protons fuse together to form deuterium, a positron (the positively charged antimatter equivalent of an electron), and a neutrino. The deuterium nucleus remains near where it formed, while the positron eventually annihilates both itself and an electron. The neutrino, being unlikely to interact with matter, streams away into space.

Therefore, physicists would expect neutrinos to flow from the sun in much the same way photons flow from a light bulb. The bulb throws out photons (bundles of light energy) radially and evenly, as if illuminating the surface of a surrounding sphere. Because a sphere’s surface area increases by the square of the distance, an observer 20 feet away sees only one-fourth the photons as an observer at 10 feet.

Thus, observers on Earth expect to see a given number of neutrinos from the sun—assuming they know how many nuclear reactions are going on in the sun—just as they expect to know a light bulb’s luminosity at a given distance if they know the bulb’s wattage. But this hasn’t been the case, and experiments have found far fewer neutrinos than predicted.

A theoretical explanation for this lack is that the neutrino “flavor” oscillates between the detectable “electron” neutrino type and the heavier “muon” neutrino and possibly the “tau” neutrino, neither of which is detected by the experiments. Utilizing quantum mechanics, physicists estimate that the number of detected electron neutrinos is constantly changing from 100 percent down to a small percentage and back again.

Therefore, the theory says, the reason we see only about half as many neutrinos from the sun as we should is because, outside the sun, about half the electron neutrinos are at that moment one of the undetected flavors.

The KamLAND experiment’s triumph is that, for the first time, physicists can observe neutrino oscillations without making assumptions about the properties of the neutrinos’ source. Because the nuclear plants have a precisely known amount of material generating the particles, it’s much easier to determine with certainty whether the oscillations are real or not.

Actually, the plants’ fission process differs from the sun’s in that the nuclear byproduct includes antineutrinos—neutrinos’ antimatter equivalent. But antimatter and matter are thought to be mirror images, so a study of nuclear antineutrinos should be exactly the same as a study of neutrinos.

“This is really a clear demonstration of neutrino disappearance,” says McKeown. “Granted, the laboratory is pretty big—it’s Japan—but at least the experiment doesn’t require the observer to puzzle over the composition of astrophysical sources. This experiment allows us to study the neutrino in a controlled experiment.”

McKeown’s Caltech team includes senior researchers Petr Vogel and Glenn Horton-Smith. Other collaborators include Japan’s Tohuku University; the University of Alabama; UC Berkeley and the Lawrence Berkeley National Laboratory; Drexel University; the University of Hawaii; the University of New Mexico; Louisiana State University; Stanford University; the University of Tennessee; Triangle Universities Nuclear Laboratory; and the Institute of High Energy Physics in Beijing. The project is supported in part by the U.S. Department of Energy.
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