The heavy quarkonium spectrum above the open flavor threshold

The plethora of charmonium (and some bottomonium) resonances discovered during the last decade, the so called XYZ states [1], challenge our current understanding of QCD. An important ingredient to describe those states is the interaction of heavy quarkonia with heavy-light meson pairs. We propose to include a realistic estimate of this interaction in the calculation of the heavy quarkonium spectrum. The estimate should be first obtained by fitting lattice QCD data on the string breaking phenomena of refs. [2,3], and extrapolating them to realistic values of the light quark masses [4]. The outcome should be then incorporated in a coupled-channel Schrödinger equation and the spectrum calculated numerically.