Japanese physicists' 'pentaquark' hints at answers to makeup of matter

Physicists have discovered a new class of subatomic particles, offering unexpected insights into the building blocks of matter. The discovery involves tiny particles called "quarks," the bricks and mortar of protons and neutrons in the atomic nucleus.

Until now, physicists had only seen quarks packed into two- or three-quark combinations inside the larger subatomic particles.

These combinations have always been something of a mystery. In their efforts to unravel the secrets of matter, scientists have tried for three decades to come up with different combinations.

And now a Japanese team led by Takashi Nakano of Osaka University says it has created a five-quark particle — "pentaquark" — in an experiment at the SPring-8 physics lab. Testing a theory from Russian scientists, the team blasted carbon atoms with high-energy X-rays to make the pentaquarks.

Determining why the pentaquark appeared in the experiment should offer great insight into the nature and stability of the essential building blocks of all matter, says physicist Ken Hicks of Ohio University in Athens, who took part in both the experiment and a confirmatory effort at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility.

"It took me two months to convince myself this was real," Hicks says. "It has been a real roller coaster."

Quarks come in six types, or "colors." The type of quarks inside protons and neutrons determines the mass, energy and magnetism of those particles. The pentaquark's stability likely comes from a unique combination of quarks, says physicist Peter Barnes of Los Alamos (N.M.) National Lab.

The findings appear in next Friday's Physical Review Letters.