Threshold Structures in Hadron Spectrum and Hadronic Molecules

Numerous candidates for exotic hadrons have been detected experimentally in the past two decades, predominantly near the threshold of a pair of hadrons. This study aims to investigate the overall behavior of near-threshold line shapes in invariant mass distributions. It is noteworthy that the threshold cusp might manifest as a peak only in channels with attractive interaction. The assertion is made that there should be near-threshold structures for any heavy-quark and heavy-antiquark hadron pairs exhibiting attractive interaction at the threshold, as observed in the invariant mass distribution of heavy quarkonium and light hadrons that couple to the open-flavor hadron pair. Furthermore, we have conducted an analysis of potential hadronic molecules comprising pairs of heavy hadrons, utilizing the Bethe-Salpeter equation with constant interactions derived from the one-boson-exchange model. Observed candidates for these hadronic molecules are in good agreement with our predicted spectrum.