Unquenching and unitarising mesons in quark models and on the lattice

TL;DR

This paper reviews how unitarised quark models and recent lattice calculations show that mesons below decay thresholds are influenced by virtual meson loops, challenging the traditional static quark-antiquark picture.

Contribution

It compares old and new quark models with recent lattice results, highlighting the importance of meson-loop effects in meson spectroscopy.

Findings

Unitarised models predict significant meson-loop contributions.

Lattice calculations confirm the importance of meson loops for certain narrow states.

Traditional static models may be insufficient for accurate meson descriptions.

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.