A pentaquark is an hypothetical 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) and is represented by Θ. It has therefore been assigned a new particle classification, called an exotic baryon. Several experiments since 2003 have been claimed to reveal a pentaquark with a mass of about 1540 MeV, presumably composed of two up quarks, two down quarks and an anti-strange quark (uudds). 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.

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 at LEPS by Takashi Nakano of Osaka University, Japan, and by Stepan Stepanyan (for CLAS) 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, apparently creating a meson and a pentaquark. However, the putative pentaquark only survived for about 10-20 seconds before decaying into a meson and a neutron.

Subsequently, a number of other experimental groups examined their own data in the appropriate energy ranges and channels. In total, 12 groups reported positive signals for a pentaquark state. For example, two HERA experiments, ZEUS and HERMES, and SVD experiment (in Protvino) have claimed the observation of the pentaquark candidate with statistical significances ranged from 4 to 8 sigma.

However, 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.The experiment involved firing a high-energy photon beam into liquid deuterium. Previously a German team, SAPHIR, produced positive results, but CLAS produced a result much more precise than SAPHIR's by collecting hundreds of times as much data at the expected energy range of the decay particles. CLAS was unable to reproduce the previous results; no evidence for pentaquarks was seen.

In addition, a variety of high-energy experiments, such as BaBar and Belle yielded null results. Despite these null results, LEPS results as of 2005 continue to show the existence of a narrow pentaquark state. Experiments continue to study this controversy.

See also

* List of particles
* Exotic hadron
* Hadron
* Quark model
* Tetraquark

Links

* Behold the Pentaquark (BBC News)
* Pentaquark discovery confounds sceptics (New Scientist)
* Pentaquark hunt draws blanks, Is It or Isn't It?, (mirror)
* 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
* Relativistic Mean Field Approximation to Baryons
* News article published in Nature (April 2005)
* "The Rise and Fall of the Pentaquark" in symmetry magazine (Sept 2006)