Threshold effects in heavy quarkonium spectroscopy

In this contribution, we discuss the possible importance of continuum-coupling (or threshold) effects in heavy quarkonium spectroscopy. Our calculations are carried out in a coupled-channel model, where meson-meson higher Fock (or molecular-type) components are introduced in $Q \bar Q$ bare meson wave functions by means of a pair-creation mechanism. After providing a quick resume of the main characteristics of the coupled-channel model, we briefly discuss its application to the calculation of the masses of heavy quarkonium-like $\chi_{\rm c}(2P)$ and $\chi_{\rm b}(3P)$ states with threshold corrections. We show that the introduction of pair-creation effects in the Quark Model (QM) formalism makes it possible to explain the deviations of $\chi_{\rm c}(2P)$ states' masses from the experimental data, without affecting the good QM description of the properties of $\chi_{\rm b}(3P)$ states.