Elusive pentaquark captured

Four decades ago, physicists proposed that subatomic particles were made up of different combinations of even smaller units known as quarks — a word that was lifted from James Joyce's difficult-to-comprehend novel "Finnegans Wake."

Quarks are similarly difficult to comprehend: As shown in this complex chart, it takes three quarks to make a proton or neutron, and two quarks to make a pion or kaon.

But could there be a combination of, say, five quarks? No one could say for sure — until now. For the first time, two teams of particle physicists are reporting solid evidence for a new class of particles known as "pentaquarks."

"People have looked for this for over 30 years," said Ohio University's Ken Hicks, who led a team at the CLAS Collaboration at the Jefferson Lab. "So now we've found the first unambiguous evidence."

Hicks' team is just now submitting its results for publication. Meanwhile, research from the LEPS Collaboration, headed by Osaka University's Takashi Nakano, is being published in the July 4 issue of Physical Review Letters.

Both groups use high-powered beams to read the entrails of particle collisions — but neither group expected to find the five-quark combination, Hicks said. It took a specific prediction by theoretical physicist Dmitri Diakonov to spark a closer examination of data that were collected for other purposes.

"We thought, 'Well, we already have the data, so it's easy to just filter the data and we'll come up with a no-result and publish that his theory was wrong.' But the tables were turned in this case, which doesn't happen very often in this field. ... He was almost bang-on," Hicks said.

Hicks was reluctant to call the newly discovered particle a five-quark combination, because one of the constituent particles is actually an antistrange quark. "A four-quark object where they're all regular quarks could not exist by the rules, and five quarks alone couldn't exist. But the four quarks and one antiquark could," he said.

Physics News Update says the pentaquark discovery is of compelling interest to physicists because it represents "a new classification of matter, like a new limb in the family tree of strongly interacting particles."

Hicks put it in similar terms: "In biology, if you look at all living things, they fit into two categories, plants and animals. This would be like finding a new strain of animal. It's not going to upset the apple cart, it's not going to change the Standard Model. In a sense it is filling in the blanks, but it's one of those things where people have searched for it for a long period of time."

Hicks said he already has proposed a new series of Jefferson Lab experiments that would delve more deeply into pentaquarks, to determine more precisely how long such particles live and how the constituent quarks hang together.

Among Hicks' key collaborators on the research is Stepan Stepanyan of Jefferson Lab, who reported the results at a particle physics conference last month. Hicks' research is funded by the National Science Foundation; Jefferson Lab is managed and operated by Southeastern Universities Research Association for the U.S. Department of Energy. You can learn much more about the pentaquark findings at Hicks' Ohio University Web site.

The federal government is also funding other lines of quark research, including studies on quark-gluon plasma at Brookhaven National Laboratory's Relativistic Heavy Ion Collider. Such research may not result in immediate practical benefits, but it should shed new light on the mysterious forces that bind quarks to each other and make matter possible, Hicks said.

"Looking at new phases of matter is interesting," he said, "and we learn more about the universe, and what could have happened in the early universe."