Overview
Exotic hadrons are subatomic particles composed of quarks other than those particles composed of a quark and an anti-quark or of three quarks. While "ordinary" or "well-known" hadrons such as protons and neutrons contain three quarks and mesons pair a quark with an antiquark, exotic hadrons like pentaquarks contain four quarks and an antiquark. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.