We investigate in detail the role of heavy meson loops in the transition from
JPC=1−− sources to candidates for QCD "exotics", such as Zc(3900),
Zb(10610) and Zb′(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. For instance, we predict
that the signals for Zb and Zb′ should be a lot stronger in
Υ(6S) decays due to this mechanism, if these states are indeed
hadron-hadron resonances. The findings of this work should be valuable for
deepening our understanding of the nature of the mentioned states.Comment: 6 pages, 8 eps figures, version to appear in Phys.Lett.