Tetraquarks

Tetraquark states are composed of two quarks and two antiquarks and represent a type of exotic hadronic state that extends beyond the conventional quark model of one-quark-and-one-antiquark mesons and three-quark baryons. In this review, we summarize the experimental advances in the study of tetraquark states, highlighting various configurations of quark flavors. We show the lineshape measurement of the X(3872), observation of charged quarkonium states consisting of a heavy quark-antiquark pair alongside a light quark-antiquark pair, discoveries of tetraquark states with four distinct flavors, with a pair of heavy quarks and a pair of light antiquarks, or with two-pairs of charm-anticharm quarks.