Tetraquarks: OPE and Quark-Hadron Duality

This lecture explores the concept of tetraquark states in QCD, focusing on interpolating currents, large-Nc QCD, molecular vs compact tetraquarks, and QCD sum rules. The instructor discusses the theoretical framework, Green functions, and the existence of narrow exotic states. Emphasis is placed on the behavior of T-phile diagrams, the large-Nc properties of meson-meson interactions, and the constraints on tetraquark masses and couplings. The lecture concludes by examining the implications of T-phile Green functions on tetraquark properties and the survival of molecular states at large Nc.