TAMPA, FLORIDA--The elusive pentaquark may be about to disappear. A new result presented at a meeting here of the American Physical Society provides the strongest evidence yet that the much-studied θ+ (theta-plus) particle is just a statistical mirage.
The pentaquark saga began 2 years ago when a Japanese experiment, SPring-8, seemed to catch a glimpse of a particle, θ+, that couldn't be made of two- or three-quark ensembles like all the quarky matter scientists have seen. Within months, other experiments had announced nearly a dozen more sightings of the particle (Science , 11 July 2003, p. 153). After data from earlier particle-physics experiments failed to show the θ+ or related exotica (Science , 19 November 2004, p. 1281, physicists awaited the results from several JLab experiments tailor-made to find the pentaquark.
Raffaella De Vita, a physicist from Italy's National Institute for Nuclear Physics in Genova, a revealed the results of the first of those experiments, known as g11, carried out at the Thomas Jefferson National Laboratory (JLab) in Newport News, Virginia. In g11, physicists shined gamma rays at a target full of protons; in theory, a collision between a photon and a proton could create a θ+. In 2003, a German collaboration in Bonn using a similar setup claimed to have produced about 60 pentaquarks, a nearly five-standard-deviation detection. But g11's much more thorough search found nothing. There were huge spikes in the data corresponding to other particles, De Vita says, but none where the θ+ should have been.
Another round of JLab results might seal the pentaquark's fate. Kenneth Hicks, an Ohio University physicist working on another pentaquark experiment at JLab says he and colleagues are "very close" to finishing an analysis of data from an experiment that used targets rich in deuterons, atomic nuclei consisting of a proton bound to a neutron. There are theoretical reasons to think deuteron targets might produce θ+ particles more readily than proton ones do, says Gerald Miller, a physicist at the University of Washington. "If you don't see it in the deuteron, then it's very bad news if you're a pentaquark fan," Miller says.
The new data don't completely rule out the pentaquark, De Vita says. But they do undermine one line of support for the particle's existence and have a much higher statistical significance than the original sightings did.
"I hope the issue will be settled soon," says Curtis Meyer, a physicist at Carnegie Mellon University in Pittsburgh, Pennsylvania. "But I'm not going to buy any pentaquark stock right now."
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