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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 have indicated that a pentaquark exists with a mass of about 1540 MeV.
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 theory was met with skepticism.
The existence of pentaquarks was first reported in July 2003 by 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. Since then, several other experiments have also reported evidence for the pentaquark. However, other experiments which would also be expected to produce the pentaquark have not found it. Also, its half-life is unexpectedly large and difficult to explain theoretically. Because of this the existence of the pentaquark is disputed. The issue should be cleared up by an experiment at Jefferson Lab dedicated to searching for the pentaquark (all experiments to date which have reported evidence of the pentaquark were planned for other purposes, and their observation of the pentaquark was serendipitous). As of January 2005, the experiment is complete and is due to report its results within the next few months.
The first pentaquark discovered, with the symbol Θ+, is composed of two up quarks, two down quarks and an anti-strange quark (<math>{\rm{uudd\bar s}}<math>).
See also
List of particles
External links
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