Why quarkonium hybrid coupling to two $S$-wave heavy-light mesons is not suppressed

We examine the couplings of quarkonium hybrids to heavy-light meson pairs in the Born-Oppenheimer approximation for QCD. The lowest hybrid multiplets consist of bound states of the ${\Pi }_u$ and ${\Sigma }_u^{-}$ potentials. We find that the ${\Sigma }_u^{-}$ potential can couple to pairs of $S$-wave mesons through string breaking, while the ${\Pi }_u$ potential cannot. From this observation, we derive model-independent selection rules that contradict previous expectations that quarkonium hybrids are forbidden to decay into pairs of $S$-wave mesons. These Born-Oppenheimer selection rules are consistent with the partial decay widths of the lowest charmonium hybrid with exotic quantum numbers $J^{PC}=1^{-+}$ recently calculated using lattice QCD.