Pentaquark

A pentaquark is a subatomic particle consisting of a group of "five" quarks (compared to three quarks in normal baryons and two in mesons). A pentaquark contains only three countable quarks, since it is composed of four quarks and one anti-quark. It has therefore been assigned a new particle classification, called an exotic baryon. Several experiments have shown that the mass of the pentaquark is 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 proven 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.

References

• BBC News: Behold the Pentaquark (http://news.bbc.co.uk/1/low/sci/tech/3034754.stm)
• New Scientist: Pentaquark discovery confounds sceptics (http://www.newscientist.com/news/news.jsp?id=ns99993903)
• Physicists Find Evidence for an Exotic Baryon (http://www.phy.ohiou.edu/~hicks/thplus.htm)