Spin degeneracy in the heavy meson molecules

In the heavy quark limit, spin degeneracies emerge in the system of heavy hadrons as a consequence of heavy quark symmetry. We study the heavy meson molecules containing hidden heavy quarks in the heavy quark limit and clarify the mass spectrum and spin partners of them. This study also provides the information about the properties of heavy mesons such as the fractions of wavefunctions and the ratios of the productions and decays. The specific exotic hadrons that would be heavy meson molecules are discussed from the point of view of heavy quark spin symmetry.