Light-meson spectroscopy

The authors survey the current status of light-meson spectroscopy, beginning with a general introduction to meson spectroscopy and its importance in understanding the physical states of quantum chromodynamics. Phenomenological models of hadron spectroscopy are described with particular emphasis on the constituent-quark model and the qualitative features it predicts for the meson spectrum. The authors next discuss expectations for hadrons lying outside the quark model, such as hadron states with excited gluonic degrees of freedom. These states include so-called hybrids and glueballs, as well as multiquark states. The established meson states are compared to the quark-model predictions and most meson states are found to be well described by the quark model. However, a number of states in the light-quark sector do not fit in well, suggesting the existence of hadronic states with additional degrees of freedom. The review ends with a brief description of future directions in meson spectroscopy.