Experimental Review of Hadron Spectroscopy

Since the invention of the quark model in 1964 hadrons are formed from a quark-antiquark pair called mesons or three quarks called baryons. However, QCD-motivated models for hadrons predict more complex structures on the hadrons components called generically exotics. These include tetraquark, pentaquark, the six-quark H-dibaryon, hybrid, and glueball mesons. Exotic hadrons have been systematically searched for in many experiments and studied in theories. In the past decade, lots of new hadrons that cannot fit into the normal mesons or baryons were discovered, the so-called [Formula: see text] states. Even so, no unambiguous candidates for any of those exotic configurations have been identified. This review presents an overview of the remarkable progress in the field of exotic hadrons over the past few years.