Unquenching and unitarising mesons in quark models and on the lattice

Abstract

Mesons with masses below their lowest OZI-allowed strong-decay thresholds have very small widths. Thus, it is usually believed that they can be safely treated as pure quark-antiquark bound states in spectroscopy models. However, unitarised and coupled-channel models from decades ago already indicated that this may not be the case, owing to significant virtual meson-loop contributions. Recent unquenched lattice calculations that include two-meson interpolators besides the usual $q\bar{q}$ ones confirm the latter conclusion, in particular for the enigmatic narrow $D_{s0}^\star(2317)$, $D_{s1}(2460)$, and $X(3872)$ states. Here, we briefly review some predictions of some old and new quark models that go beyond the static description of mesons, also in comparison with up-to-date lattice results.