Heavy exotic hadrons as hadronic molecules

Recent observations of peak-like structures near two-hadron thresholds imply that their structures are dominated by molecules of two hadrons. To have such states realized, suitable conditions are needed for the interaction between the constituent hadrons. Because many of them are open charm hadrons, quark dynamics associated with the presence of heavy quarks and chiral dynamics associated with light quarks are expected to work coherently. To implement these features, we have constructed a coupled channel model for the hidden-charm pentaquarks without and with strangeness whose quark contents are ccbar-qqq or ccbar-sqq (q = u, d quarks); Pc and Pcs [1, 2]. The model includes all possible open charm hadron channels which also couples to five-quark compact states. The model incorporates naturally one-pion exchange force at long distances and virtual excitations of the compact five quark states in two-hadrron scattering channels, both of which provides attractive interaction that is responsible for the generation of the molecular states near two-hadron thresholds. By tuning only one parameter in the model, it is possible to explain the existing data of Pc and Pcs including many of their masses and decay widths, and to further predict other molecular states. We emphasize that the long range dynamics associated with chiral symmetry and short range one of quark dynamics due to the internal structure of hadrons are important. The present analysis was done for hadrons containing charm quarks, but some relevance to those containing strange quarks will be also discussed.