Glimpses of neutrinos reveal more secrets

Billions of the particles pierce human bodies and every other object every second.

July 8, 2004

By Pat Brennan

IRVINE - Scientists at the University of California, Irvine, say they are revealing new secrets about some of nature's most elusive subatomic particles: neutrinos.

Clear evidence detailing how neutrinos change from one type to another as they move through space has emerged from an analysis of thousands of neutrino strikes in a Japanese detector, UCI physics professor Henry Sobel said Wednesday.

The findings further narrow down the possible mass of the neutrino, which interacts so weakly with matter that, if you could somehow line up 10,000 Earth-sized planets in a row, a neutrino could pass through all of them unimpeded.

The findings could also help answer questions about the "fundamental building blocks" of the universe, Sobel said.

Sobel, UCI physicist David Casper and an international team of scientists examined nearly 3,000 neutrino events, or hits, in the Super-Kamiokande detector deep in a Japanese mine.

The detector is a massive tank of pure water surrounded by light detectors. Billions of neutrinos stream through our bodies and every other object every second, but collisions with other particles are exremely rare.

When one hits an atom in Super K's tank, it produces a particle called a muon that gives off light. That tiny burst of light registers on the detectors.

Many kinds of neutrinos exist and are produced in different ways.

In this study, Super K was used to look for neutrinos produced in Earth's atmosphere, created in cosmic-ray collisions.

Super K scientists, including Sobel, achieved a stunning result in 1998: they found that neutrinos oscillate, or change form, as they pass through Earth.

That told the scientists that neutrinos had mass; a massless particle cannot oscillate. But the scientists only saw the neutrinos disappear, which showed they were changing form.

In the new study, the scientists describe observing the neutrinos reappear, bringing a much more precise understanding of how the oscillations happen. The oscillations turned out to match theoretical predictions, Sobel said.

"This is giving much more detail," Sobel said. "And we're actually seeing it oscillate."

Among other things, the result, to be published in the science journal Physical Review Letters, helps set a lower limit on the possible mass of the neutrino. That, in turn, suggests that neutrinos could make up about 1 percent of the total mass of the universe.

Some 70 percent is believed to be made up of mysterious "dark energy," a repulsive force that appears to counteract the effects of gravity. About 25 percent is thought to be dark matter, made up mostly of so-far undetected particles.

Only 1 to 5 percent is the visible matter that lights up stars and galaxies and makes up our bodies, our planet and everything on it.