Quark-antiquark bound states

This brief review discusses various aspects of the description of hadrons as bound states of quarks at the simpler example of mesons, which are regarded as composite of a (constituent) quark and antiquark. It is shown how a recently derived relativistic virial theorem may be used in order to cast some light on the interplay between the (semi-)relativistic and (maybe only effective) nonrelativistic treatment of bound states. Furthermore, a new relativistic approach to fermion-antifermion bound states is briefly sketched and applied in order to predict analytically a few selected general features of the meson spectrum. 27 refs.; 2 figs.; 1 tab