Scientists Report Discovery of a New Kind of Subatomic Particle

In a discovery that has physicists relearning how to count, three teams around the globe report that they have identified a subatomic particle consisting of five quarks -- a number never before seen in the zoo of fundamental particles.

"It's extremely important," says Stepan Stepanyan, a staff scientist at the Jefferson National Accelerator Laboratory, in Newport News, Va., one of the labs that made the announcement. He describes it as the most important physics finding of the decade. It suggests that a whole new class of pentaquark particles may be awaiting discovery.

The everyday world is filled with protons and neutrons, each harboring three tightly bound quarks. More-exotic particles, such as kaons and pions, contain a pair of quarks. Physicists have reported seeing hints of a pentaquark particle in the past, but none of those results have stood up to subsequent scrutiny.

The first group to report finding the new pentaquark particle was from the Japan Synchrotron Radiation Research Institute near Osaka. The researchers who led that effort are publishing a paper on their result in the July 4 issue of the journal Physical Review Letters. The team at the Jefferson lab — which is operated by a consortium of 53 colleges and universities — quickly confirmed the Japanese result, measuring a particle with essentially the same mass, of 1.543 billion electron volts.

A third group, from the Institute of Theoretical and Experimental Physics, in Moscow, has also detected a pentaquark particle, says Mr. Stepanyan.

The detection at the Jefferson lab was statistically strong, but Mr. Stepanyan cautions that physicists cannot yet rule out the possibility that they have actually detected something far more ordinary. "To claim it as a final discovery," he says, "we still need more statistics."

The pentaquark particle does not upend the Standard Model of particle physics because such arrangements are allowed under the prevailing theoretical foundation. But the discovery — if it holds — would open the window to a previously unexplored type of matter.