New Experiment Casts Doubt on Elusive Pentaquark

A dedicated hunt for the pentaquark has found nothing, further calling into question previous reports of pentaquark sightings. An experiment at the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility, found no evidence of the five-quark particle, researchers reported at the APS April meeting in Tampa. These new results have greater statistical significance than previous experiments that claimed to have seen the pentaquark.

Quarks normally exist in groups of two (mesons) or three (baryons). But groupings of four quarks and one anti-quark should be possible, according to the theory of quantum chromodynamics.

The first experimental evidence for a pentaquark was reported in 2003 by the LEPS/Spring-8 group in Japan. Several other groups quickly confirmed the pentaquark sightings. But other studies soon produced null results, casting doubt on the original positive sightings and stirring up a controversy over the existence of the five-quark states.

At the April Meeting, Raffaella De Vita of Italy’s National Institute of Nuclear Physics, presented new analysis from the CLAS experiment at Jefferson lab, which was specifically designed to detect pentaquark states.

The experiment, which ran from May to July 2004, fired high energy photons at protons in a target of liquid hydrogen to try to produce the pentaquark.

The particular pentaquark that CLAS searched for, called the theta+, is composed of two up quarks, two down quarks, and an anti-strange quark. It is predicted to have a mass of about 1.5 times the mass of the proton, or about 1540 MeV.

The CLAS experiments found no evidence of a pentaquark state, said De Vita.

This is in contrast to SAPHIR, a similarly designed experiment at the Electron Stretcher Accelerator (ELSA) in Bonn, Germany, that did claim to have seen a pentaquark.

The CLAS experiment has a precision 50 times higher than the SAPHIR result.

The new results don’t entirely rule out the possibility of a pentaquark, but they do provide strong evidence against it. The CLAS collaboration is still analyzing some of the data, and they plan to conduct further studies to look for the pentaquark in a different channel and at higher energies.

"This doesn’t imply that the theta+ doesn’t exist. We are searching other channels," said De Vita, "We really need to complete the analysis." More results from the CLAS analysis are expected later this year.

Also at the April Meeting, Curtis Meyer, a physicist at Carnegie Mellon University and a member of the CLAS collaboration, gave a separate talk reviewing the data from various pentaquark experiments. He concluded that the claimed pentaquark sightings were probably incorrect.

"The data [for the pentaquark] do not look very convincing," said Meyer. "I’m not going to buy any pentaquark stock."