Subatomic breakthrough announced by physicists

Discovery of 5-quark particle could aid research into matter

Physicists announced Monday that they have evidence for a never-before-seen subatomic particle — one composed of five quarks.

Quarks are the building blocks of matter. This new "pentaquark" is the first particle found to contain more than three quarks, the usual quantity combined to form a proton or a neutron.

Studying the pentaquark, even for the fraction of a second it exists, could help physicists better understand the forces that hold quarks together in everyday life.

"People have looked for these for many years," said Ken Hicks, a physicist at Ohio University in Athens. He is part of a large Japan-based research group, led by Takashi Nakano of Osaka University, that will report the pentaquark findings in Friday's issue of Physical Review Letters.

An independent experiment, at the Thomas Jefferson National Accelerator Facility in Newport News, Va., confirmed the finding, as did researchers at the Institute of Theoretical and Experimental Physics in Moscow.

Together, the three reports strengthen the likelihood that the pentaquark has truly been discovered, other scientists said.

"I'm not generally a betting person, and I'll probably bet that they are right," said Andrew Sandorfi, a physicist at Brookhaven National Laboratory.

Quarks generally cluster in triplets to form protons and neutrons in the nucleus of every atom, from inside your body to the farthest reaches of the universe. There are six types of quarks; each also has an antimatter counterpart known as an antiquark.

The pentaquark is a short-lived mating between two particles — one an ordinary neutron with three quarks and one a combination of a quark and an antiquark. Having an antiquark in the mix seems to help the pentaquark stick together longer, said physicist Peter D. Barnes of the Los Alamos National Laboratory in New Mexico.

"You're trying to understand what are the forces between these building blocks that would make some of them stick together," he said.

Theoretically, there's no reason why particles with four, six or even seven quarks shouldn't also exist, Dr. Barnes added. He, for example, spent years unsuccessfully hunting for a six-quark particle.

But in 1997, Russian theorist Dmitri Diakonov predicted the existence of several kinds of pentaquarks and suggested how scientists could search for them using large atom-smashing machines.

In Japan, Dr. Nakano and colleagues looked at data from a particle accelerator named SPring-8. They hurled energetic gamma rays at the nucleus of a carbon atom, then studied the many particles that splintered off from the collision. Two of those were a neutron and a kaon particle, which combined to briefly form the pentaquark. Dr. Nakano's team first reported its results at a conference in Osaka last October.

"The announcement at that time was greeted with some skepticism, as you might imagine for a new particle," said Dr. Hicks. So he led a second group, at the Virginia lab, to try to confirm the pentaquark's existence. Looking through data from a similar experiment, the scientists also discovered an energy peak that would best be explained by the presence of a pentaquark, Dr. Hicks said.