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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 stronglycouples to a heavy meson pair in an $S$--wave and this system decays to anotherheavy meson pair (e.g. via pion emission), again in an $S$-wave, the pertinentdiagrams get enhanced significantly, if the intermediate states are (near)on--shell and have small relative momenta. In a limited kinematic range thismechanism generates "singularity regions" that lead to the creation of a largenumber of low energy heavy meson pairs, providing an ideal environment for theformation of hadron-hadron bound states or resonances. For instance, we predictthat the signals for $Z_b$ and $Z_b'$ should be a lot stronger in$\Upsilon(6S)$ decays due to this mechanism, if these states are indeedhadron-hadron resonances. The findings of this work should be valuable fordeepening our understanding of the nature of the mentioned states.

Systematic study of the singularity mechanism in heavy quarkonium decays