On some models of the exotic hadron states

There are several kinds of exotic hadron states. In this paper, we are concerned with the flavor exotic meson states: the tetraquarks and meson molecules. Two distinct models of tetraquarks are studied: diquark-antidiquark bound states and uncorrelated quarks. Although diquark is not a color singlet, there is a strong theoretical evidence that such states do exist within hadrons. Both tetraquark models as well as methods used to study such states are reviewed. The meson-meson molecules are also mentioned. It is remarked that hadron spectroscopy per se may not always be sufficient to discern between physically distinct models