  Mesonic resonances are generally observed in data as narrow, moderately
broad, or wide peaks in scattering or production processes. In the eyes of
nearly all experimentalists, any suchlike bump is a true resonance as soon as
its statistical significance exceeds certain minimal values. However, this
simple point of view ignores possible effects from competing hadronic channels
and the opening of the corresponding thresholds. On the other hand, most
theoretical hadron-model builders consider mesons merely bound states of a
quark and an antiquark, or of more exotic combinations sometimes involving
valence gluons as well. Also the latter description is much too naive, since
considerable mass shifts or even the dynamical generation of extra states due
to unquenching are equally ignored.
  In the present paper, a largely empirical yet very successful approach to
meson spectroscopy is revisited, in which all the above phenomena can be
accounted for non-perturbatively, with concrete examples of some enigmatic
mesonic states described in detail. First, the X(4260) charmonium enhancement
is argued to be a non-resonant structure resulting from depletion effects due
to competing channels and resonances. Then, the X(3872) charmonium-like meson
is described as a unitarised $J^{PC}=1^{++}$ $c\bar{c}$ state. Also, the
unusual pattern of masses and widths of the open-charm axial-vector mesons
$D_1(2420)$, $D_1(2430)$, $D_{s1}(2536)$, and $D_{s1}(2460)$ is shown to follow
from highly non-perturbative coupled-channel and mixing effects. Finally, first
indications of a very light scalar boson are presented, on the basis of
published BABAR data.

High Energy Physics - Phenomenology;High Energy Physics - Experiment