ghost particles
by Angela Spivey

see accompanying article TUNL vision

ou can't see them, but they are here. These so-called ghost particles have no charge, so they rarely interact with others; they can pass through lead. Their name — neutrino, Italian for "little neutral one" — explains it well.

Though neutrinos are tiny, a recent study concludes that, yes, they do have mass. Hugon Karwowski, professor of physics and astronomy, collaborated with an international team of ninety-two researchers to study neutrinos using KamLAND, a huge detector built underground in a mine in Japan. While other studies have suggested that neutrinos have mass, the KamLAND study "puts the nail in the coffin," Karwowski says.

.: Artist's conception of the main detector at KamLAND. Courtesy RCNS, Tohoku University. Click to enlarge. :.

KamLAND stands for Kamioka Liquid Scintillator Anti-Neutrino Detector. Wait a second, anti-neutrino? This machine actually detects anti-neutrinos, which are created in the decay of products of fission reactions such as those that drive nuclear power plants. Neutrinos are created during fusion reactions, which occur inside the sun. Sun travel not being practical, researchers built a detector in Japan near numerous nuclear reactors. And in the world of physics, as far as scientists know, anti-neutrinos behave like neutrinos. Anti-matter is the mirror image of matter. So, you study one, you study the other.

So let's just call everything neutrinos. Because the nuclear reactors surrounding KamLAND are a relatively well-controlled source, researchers could predict how many neutrinos they'd expect to find. "The physics of nuclear reactors is very well known," Karwowski says. "So if you know the power output of the reactor and you know the fuel, then you can predict to very good accuracy which fission products are going to be produced, and therefore you know exactly the neutrino flux." Taking into account the distance from the detector to each reactor, the scientists expected to find eighty-six neutrinos in six months. But the detector found significantly fewer — fifty-four. The neutrinos seemed to disappear.

ow does that disappearance tell them anything about mass? Neutrinos, it seems, come in three different "flavors" — electron, muon, and tau — determined by the way in which they're made. The KamLAND detector records only electron neutrinos. The researchers conclude that the only way the neutrinos could have vanished is if they were oscillating — changing from electron flavor to some other flavor. The only way the neutrinos could change flavor is if they have a nonzero mass.

What? If your mass is zero, Karwowski says, your mass cannot change. Zero is always zero. "It cannot change flavor unless it has a mass," Karwowski says. "If it's a massless particle, then it will always remain whatever it is." This explanation makes common sense as long as you don't think about it too much. To truly understand, Karwowski says, you have to delve into quantum mechanics.

The detector consists of a steel casing surrounding a forty-three-foot diameter balloon filled with 1,000 tons of liquid scintillator, a chemical mixture that converts energy lost by ionizing radiation into pulses of light. At KamLAND the scintillator emits flashes of light in sequence when certain "neutrino events" happen. The flashes can't be seen by the naked eye but are detected by almost 2,000 photomultiplier light sensors.

n the outside of the balloon, a tank of ultrapure water detects and filters out nonneutrino events. Karwowski, graduate student Doug Leonard, and other TUNL scientists traveled to Japan to help build this outer water detector. Leonard helped fill it, which had to be done at a controlled rate. He describes his work there as "valves and gauges and running around from the top of the detector to the bottom to make sure nothing went wrong." Data collection began in January 2002, and Karwowski has taken his share of shifts in the control room.

The confirmation that neutrinos have mass gives them the title of "lightest particle in the universe with nonzero mass." It also means that neutrinos could have been involved in density fluctuations that helped create galaxies. Karwowski says, "There are a lot of processes that are dependent on the presence of neutrinos — stellar evolution, super nova explosions." Even more so, for him, neutrinos are another tiny piece of "a puzzle worth solving." Because they are here.

This study was published in the December 2002 issue of Physical Review Letters and was funded by the U.S. Department of Energy and the Japanese Ministry of Education and Science. Adjunct associate professor Ryan Rohm also participated in KamLAND.

see accompanying article TUNL vision

Angela Spivey is the associate editor of Endeavors magazine.
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