Narrow Results

The newly observed exotic mesons above threshold were widely discussed in the molecular picture. To understand deeper their structures, we here discuss the spin () of a heavy quark-antiquark pair in an S-wave meson-antimeson system by constructing explicitly the spin wave function. One finds two selection rules for (a) the total angular momentum J is larger than the maximum angular momentum of the light degree of freedom or smaller than the minimum one; (b) J^C = 1⁺, 2^-, 3⁺, ⋯ if the two mesons are different but belong to the same doublet. This feature may be used to constrain possible strong decay channels.