Explaining the Many Threshold Structures in the Heavy-Quark Hadron Spectrum

Tremendous progress has been made experimentally in the hadron spectrum containing heavy quarks last two decades. It is surprising that many resonant structures are around thresholds of a pair hadrons. There should be threshold cusp at any $S$-wave threshold. By constructing nonrelativistic effective field theory with open channels, we discuss generalities behavior, and offer an explanation abundance near-threshold peaks quarkonium regime. We show can up as peak only for channels attractive interaction, width inversely proportional to reduced mass relevant argue there heavy-quark heavy-antiquark hadrons, which have interaction threshold, invariant distribution light hadrons couple open-flavor pair. The structure becomes more pronounced if pole. Predictions possible pairs also given ground state Precisely measuring will play important role revealing heavy-hadron interactions, thus understanding puzzling hidden-charm hidden-bottom structures.