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 ().

History

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. That surprise discovery led other researchers around the world to scour their archived data for evidence of the pentaquark, and within months, a dozen teams announced seeing the particle, with a couple of groups even claiming to have discovered two more types of pentaquark.

But just as quickly, cracks began to appear in both the theory and observations of pentaquarks. About a dozen other teams failed to find the particles where they should have been. When experiments detected it, different groups reported slightly different values for its mass, and alarmingly, all of the measurements suggested the particle took about 100 times longer to decay than other particles of its mass.

Because of all this, 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. Results were also published in Nature (April 2005). More results from CLAS at Jefferson Lab are expected later in 2005.

See also

List of particles

External links

• Behold the Pentaquark (BBC News)
• Pentaquark discovery confounds sceptics (New Scientist)
• Pentaquark hunt draws blanks
• Physicists Find Evidence for an Exotic Baryon (Ohio University)
• hep-ex/0412048: An Experimental Review of the Θ₊ Pentaquark
• hep-ph/0401115: Prospects for Pentaquark Production at Meson Factories
• hep-ph/0404019: An Attempt to Study Pentaquark Baryons in String Theory