Meson spectroscopy: too much excitement and too few excitations

TL;DR

This paper reviews meson spectroscopy, critiques the traditional potential models, and advocates for unquenched approaches that better explain observed mesonic resonances and discrepancies in experimental data.

Contribution

It introduces an unquenched framework for meson spectroscopy, addressing limitations of the funnel potential and explaining puzzling mesonic states more effectively.

Findings

Traditional models show significant discrepancies with experimental spectra.

Unquenched approaches successfully describe certain mesonic resonances.

Neglecting coupled-channel effects leads to incomplete understanding of meson states.

Abstract

We briefly review the general status of meson spectroscopy, especially in light of the often made claim that there are too many observed resonances to be accounted for as $q\bar{q}$ states. Also, the adequacy of the usual Coulomb-plus-linear, alias "funnel", confining potential for reproducing the experimental spectra of light, heavy-light, and heavy mesons is critically analysed. Thus, many serious discrepancies are observed and discussed. As possible causes, we suggest the neglect of unitarisation and other coupled-channel effects, as well as the deficiency of the funnel potential itself. In order to illustrate our alternative, "unquenched" approach, we present some recent examples of successfully described puzzling mesonic enhancements and resonances, such as the charmonium states X(4260) and X(3872), as well as the axial-vector charmed mesons $D_1(2420)$, $D_1(2430)$, $D_{s1}(2536)$, and $D_{s1}(2460)$.