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 is composed of 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 shown 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 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 decayinging into a meson and a neutron.