Wild Bunch: First five-quark particle turns up

Physicists on three international teams have recently spotted what's most likely a long-sought subatomic particle known as a pentaquark. It contains five components—four quarks and one antiquark—which are among the most fundamental bits of matter yet known. No subatomic particle detected previously contains more than three of those building blocks.

Gamma rays hitting neutrons in carbon nuclei appear to have created exotic pentaquarks, which quickly shatter into neutrons and kaons.
HIGH FIVE. Gamma rays hitting neutrons in carbon nuclei appear to have created exotic pentaquarks, which quickly shatter into neutrons and kaons.
E. Roell

"After 30 years of failing to find any convincing evidence for something that ought to be there, this recent news is certainly met with excitement," says nuclear physicist Andrew M. Sandorfi of Brookhaven National Laboratory in Upton, N.Y.

Although unusually complex, the newfound particle fits within the prevailing theoretical framework of particle physics, known as the standard model. The newly detected bit may be just the first member of a family of pentaquark particles. The find also underscores the possibility of discovering particles with four or six quarks.

To fathom how five quark components can coexist within one particle, theorists expect to reconsider their models of the interactions among quarks and gluons, the particles that bind quarks together. For instance, it's possible that the fivefold structure is not a spherical lump but rather a moleculelike arrangement in which a so-called kaon, which is made of a quark and an antiquark, orbits a neutron, which is made of three quarks.

"Exactly what form of the theory makes it work now becomes very interesting," says theorist Peter D. Barnes of Los Alamos (N.M.) National Laboratory.

If the pentaquark can exist in labs today, it probably also was present in the very first, fiery moments of the universe, says Takashi Nakano of Osaka University, leader of the team that found the pentaquark at Japan's SPring-8 synchrotron in Hyogo.

The experiments that appear to have bagged the elusive pentaquark weren't designed to look for it. "This is an example of serendipity," says Kenneth H. Hicks of Ohio University in Athens. He's a member of both the SPring-8 team and a group at the Thomas Jefferson National Accelerator Facility in Newport News, Va., that has confirmed the SPring-8 finding.

In 1997, Russian theorist Dmitri Diakonov and his colleagues predicted the existence of a pentaquark with a mass about 50 percent heavier than that of a hydrogen atom. The theorists then urged the SPring-8 team to reexamine data from an experiment in 2001 that might have inadvertently made the particle when it had gamma rays striking a piece of plastic.

When Nakano, Hicks, and their coworkers combed that data, they in fact found signs for about 20 pentaquarks. The team is slated to present its evidence in an upcoming Physical Review Letters.

Inspired by the SPring-8 findings, researchers at the Jefferson lab and the Institute of Theoretical and Experimental Physics in Moscow double-checked old data from different sorts of particle collisions and netted their own pentaquark candidates.

Besides its unprecedented quark count, a pentaquark is unusual in that it includes an exotic antiquark, the so-called strange antiquark. The composite particle also contains two up quarks and two down quarks — the same ones found in ordinary matter.

Nakano says the SPring-8 team is now analyzing data from new collisions that were generated explicitly to make pentaquarks. The aim is to nail down the identity and properties of this new entity. Meanwhile, the Jefferson lab has approved an experiment intended to boost its pentaquark production 20-fold, Hicks says.