Systematic study of the singularity mechanism in heavy quarkonium decays

Understand

We investigate in detail the role of heavy meson loops in the transition from $J^{PC}=1^{--}$ sources to candidates for QCD "exotics", such as $Z_c(3900)$, $Z_b(10610)$ and $Z_b'(10650)$.

• We demonstrate that, if a vector state strongly couples to a heavy meson pair in an $S$--wave and this system decays to another heavy meson pair (e.g.
• via pion emission), again in an $S$-wave, the pertinent diagrams get enhanced significantly, if the intermediate states are (near) on--shell and have small relative momenta.
• In a limited kinematic range this mechanism generates "singularity regions" that lead to the creation of a large number of low energy heavy meson pairs, providing an ideal environment for the formation of hadron-hadron bound states or resonances.