Gimme five? Try take five, scientists find

Va. team and others must keep looking for subatomic pentaquark

The elusive subatomic particle called the pentaquark is up for grabs again.

Two years ago, scientists led by a team at the Thomas Jefferson National Accelerator Facility in Newport News thought they had captured the pentaquark, whose properties as a new state of matter were conjectured in 1997 by Russians and later glimpsed by Japanese scientists.

The unstable, fleeting pentaquark is thought to consist of five subatomic particles called quarks. Most ordinary matter is built of quarks, the most basic particle. They are usually found as a couple (called mesons) or as a threesome (called baryons) such as protons and neutrons.

The pentaquark offers a new grouping of quarks. If the pentaquark exists, it opens up the hunt for novel particles now barely dreamed of.

Big Bang aftermath

Pentaquarks probably existed in the immediate aftermath of the Big Bang, theorists says, as the universe began to shift independent particles into the now-familiar atoms. Understanding this quark soup could reveal more about how the universe evolved.

This is the world that the Jefferson scientists try to re-create when they go hunting for the dodgy particle by bombarding targets such as hydrogen with photon beams. Quarks can be observed indirectly in the wreckage, for less than a trillionth of a trillionth of a second.

Follow-up experiments conducted by other teams around the world also seemed to spot the Jefferson lab's version of a pentaquark, called theta-plus, but often at differing masses and lifespans. Physicists were waiting for the results of a more sensitive experiment, run in 2004, to learn whether and how the hunt for the pentaquark should continue.

Last month, in a meeting of the American Physical Society, the Jefferson collaborators presented those results, showing that it ain't soup yet.

The experiment called for aiming a beam of electrons into a gold target to produce a beam of high-energy photons. These photons were then directed into a liquid hydrogen target, trying to produce thousands of theta-plus pentaquarks through the interaction of a photon with the proton of a hydrogen nucleus.

But they saw none, Raffaella De Vita, a physicist from Italy's National Institute for Nuclear Physics, told the APS meeting in Tampa, Fla.

The new data, which were statistically more powerful than the Jefferson lab's previous sighting, undermine one line of support for the pentaquark's existence, but that doesn't mean an end to the hunt, according to De Vita.

'May have died'

"The theta may have died," Bob Jaffe, a theoretical physicist at the Massachusetts Institute of Technology, has declared of the particle in talks since the APS presentation.

Killing a theory is hard work, said nuclear physicist Gordon Cates of the University of Virginia. Yet despite the Japanese finding that sparked the pentaquark quest, "it's becoming much harder to believe that it's real," he said.

Jefferson team leader Volker Burkert disagrees. The absence of a statistical signal "doesn't eliminate the possibility" of the theta-plus pentaquark, he said. "The door is not closed on that."

Burkert says he can remain cautiously optimistic because the 2004 experiment's results give his team new parameters, such as a lower mass, to use in continuing their search.

Smaller, not larger, search

In hindsight, Burkert said, they were looking for a sport utility vehicle when they now know to look for a subcompact.

Experiments run since at the Jefferson lab have used beams at different energies and used different targets, such as deuterium, thought to be more conducive to pentaquark manufacturing.

Results from one follow-up experiment may be ready this summer, Burkert said, and another is planned for late next year.

In addition, the Jefferson team is crunching results of an experiment they did duplicating results from the European lab CERN, where physicists claim to have found a type of pentaquark called cascade.

— Contact A.J. Hostetler at (804) 649-6355 or