A pentaquark is a subatomic particle consisting of a group of five quarks (compared to three quarks in normal baryons and two in mesons), or more specifically four quarks and one anti-quark. Hence it has baryon number 1. It has therefore been assigned a new particle classification, called an exotic baryon. Several experiments since 2003 claim to have seen a pentaquark with a mass of about 1540 MeV, presumably composed of two up quarks, two down quarks and an anti-strange quark (<math>{\rm{uudd\bar s}}<math>). This is the minimal quark composition of an object with baryon number 1, and positive strangeness.

These five quarks are not, however, ordinary constituent quarks in the model that predicted the existence of the pentaquark. The `fourth' quark is seen as a higher density of states in the Dirac sea with negative energy, while the antiquark is a lower density of states with positive energy. This does not cost as much energy as the creation of a particle-hole excitation, therefore the pentaquark is lighter than the 2 GeV or so that would be predicted by other constituent quark models.


The existence of pentaquarks was originally hypothesized by Maxim Polyakov, Dmitri Diakonov, and Victor Petrov at the Petersburg Nuclear Physics Institute in Russia in 1997, but their predictions were met with skepticism. Nevertheless, the existence of pentaquarks was first reported in July 2003 from experiments run by Takashi Nakano of Osaka University, Japan, and by Ken Hicks at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia. Their experiments caused a high-energy gamma ray to interact with a neutron, creating a meson and a pentaquark. However, the pentaquark only survived for about 10-20 seconds before decaying into a meson and a neutron.

But the existence of the pentaquark was highly disputed. In order to clear up the issue, the CLAS collaboration set up an experiment at Jefferson Lab with the purpose of searching for pentaquarks. Results were entirely serendipitous.

The CLAS collaboration again hunted in 2005 by accelerating energetic photons into liquid hydrogen. Previously a German team, SAPHIR, produced postive results, but CLAS produced a result 50x more precise than SAPHIR's by collecting 10x the data at the expected energy range of the decay particles. CLAS team member Raffaella De Vita, of Italy's National Institute of Nuclear Physics, presented results on April 17th 2005 at a meeting of the American Physical Society in Tampa, Florida, USA, and showed that CLAS was unable to reproduce the previous results—no evidence for pentaquarks was seen. More results from CLAS at Jefferson Lab are expected later in 2005.

It is worth noting that the experiments that fail to see the pentaquark are at higher energy where the meson exchange production mechanism dies out in favor of flavor neutral gluon exchange; the latter is doubly suppressed by the OZI rule. Therefore the upper limits on the production rate of the pentaquark determined from the null results of some experiments do not necessarily disprove its existence.

See also

External links

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