Effects of meson loops on a spectrum of quark states

In recent years, many so called ''XYZ'' states have been discovered, which were not anticipated from conventional quark models. In their simplest form, quark models describe the confining interaction between effective constituent quarks which leads to the formation of hadrons as bound states. However it has been argued for a long time that when building quark models with coupled channels, unitarization cannot be neglected. In addition to introducing an imaginary part to the masses and resulting in considerable mass shifts, it has been found to have many interesting and unexpected consequences. To come to an understanding of the heavy quarkonium spectrum and the nature of the ''XYZ'' mesons, it is essential to systematically investigate how the inclusion of meson loops effects a quark spectrum. As a step in this direction, we study simple unitarized models. We demonstrate that in general quark states do not tend to get very broad if their coupling to the continuum increases, but instead they decouple from the latter in the large coupling limit. However a few of them behave very differently and demonstrate a kind of collectivity. While the actual calculations are done within particular, very simplified models, we argue that the findings might well be general.