U.S. Focus: Particle Party Just Got Bigger

New state of matter proved Experiments confirm subatomic stuff is real

"Welcome to the club."

In the cocktail party of known matter, physicists at the Jefferson Lab say revelers must make elbow room for a new state of matter, the pentaquark.

For the past few years, small bands of physicists worldwide have sought this subatomic particle state, whose existence would upend accepted theories. A Russian theory put out in 1997 — thought to be a bit harebrained — jump-started the hunt. The elusive quarry may now be in reach.

Experiments at the Thomas Jefferson National Accelerator Facility in Newport News confirm a Japanese report to be published on Friday by Physical Review Letters. The pentaquark is real, the Japanese conclude, and the Jeff Lab folks agree, although they say more research is needed to know for sure whether it's the pentaquark the Russians predicted.

Ordinary matter consists of atoms, whose nuclei are made of combinations of protons and neutrons, which in turn are made of combinations of quarks, the most basic particle. Quarks exist in a kind of symmetry called "colorless," and the new research suggests quarks can achieve that Zen-like state in new ways.

If follow-up experiments agree, it opens up the hunt for novel particles now barely dreamed of.

The particle party just got bigger with the arrival of this very strange beast, says theoretical physicist Winston Roberts of Old Dominion University.

"Welcome to the club," he said yesterday.

"People like me have to go back to our equations, to our drawing boards," Roberts said. "Things that we thought we understood we have to go back to."

The usual crowd of subatomic particles will still be around, Roberts said, but the pentaquark's gatecrashing means there could be a lot more odd states of matter, waiting to be discovered.

"This might be only the first of a family of such states. Not only that but a new classification of matter, like a new limb in the family tree," said the American Institute of Physics in announcing the research.

The Jeff Lab, which is submitting its work to the same publication, confirmed the Japanese work by aiming a beam of electrons at a small chunk of frozen deuterium. In the wreckage, quarks can be observed indirectly, for less than a trillionth of a trillionth of a second.

Quarks are needy and never travel solo, bound together by powerful forces. Moving constantly, they come in six different personalities, called "flavors:" up, down, top, bottom, and, amusingly, strange and charm.

The pentaquark offers a new grouping of quarks, with a twist. Rather than packaged as a duo or trio, pentaquarks come as a quintet: up, up, down, down and anti-strange.

"Nature is showing us that it has more tricks up its sleeve," said Elton Smith of the team that ran the Jeff Lab experiment.

After hearing last October about the search for the pentaquark at the SPring-8 lab in Osaka, Japan, the Jeff Lab decided to dig a little. Scientists re-analyzed data taken during an unrelated experiment in 1999. (Results of a second experiment, targeting liquid hydrogen, have not been fully analyzed.)

The findings are so intriguing that the investigators just won approval to spend 30 days smashing electrons in hopes of spotting the pentaquark. The work could get under way at the busy facility as soon as next year.

The Jeff Lab's work confirms the Japanese conclusions about the pentaquark's average mass and its "width" — the range of the pentaquark's dress size, so to speak. In addition, it has the same positive charge as predicted by the Russians.

Smith said the coming experiment would try to identify other properties of the pentaquark, to help determine if this is the predicted state, or something else.

Two Germans labs are also looking for pentaquarks, but their results have not yet been published.