What does true meson spectroscopy encompass?
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Based on collaborative work of Eef van Beveren and George Rupp


Contrary to widespread belief, there are not too many observed
mesonic resonances to be accounted for by normal quark-antiquark
states, which is the usually invoked argument to justify the
introduction of (crypto)exotic configurations. The reasons why
newly detected mesonic enhancements often seem to be incompatible
with $q\bar{q}$ states can be manyfold:

  (i) the underlying confining potential may be different from
      what is generally taken for granted;
 (ii) mass shifts due to unitarisation ( or "unquenching") are
      mostly neglected;
(iii) unitarisation sometimes even yields extra, dynamically
      generated resonances, which nevertheless have a $q\bar{q}$
      source;
 (iv) the opening of strong decay thresholds generally distorts
      the line shapes of nearby resonances, or can even by
      themselves give rise to enhancements that look like
      resonances;
  (v) large inelasticity effects between observed OZI-forbidden
      decays and non-observed OZI-allowed ones can lead to
      signal depletion at true resonances and/or thresholds,
      resulting in non-resonant apparent enhancements in between.

In this talk, an assessment will be made of the status of meson
spectroscopy in the light of the above mechanisms, with several
concrete examples of light and heavy-light mesons as well as
charmonium and bottomonium states. Some candidates for exotic
mesons will be critically discussed. Finally, suggestions will
be made how the COMPASS collaboration may contribute to settle
some of the outstanding issues in the light-quark sector.