Covariant Classification of q q bar-Meson Systems and Existence of New Scalar and Axial-Vector Mesons

The level classification of quark-antiquark systems is generally made by the LS-coupling scheme, resorting to the non-relativistic quark model. However, it has been well known that the pi-meson with the (L,S)=(0,0) shows also the properties as a Nambu-Goldstone boson in the case of spontaneous breaking of a relativistic symmetry, the chiral symmetry. In this talk I present a covariant classification scheme for describing both the non-relativistic and the relativistic q q bar-mesons and point out possibility for existence of the new scalar meson nonet (to be assigned to the sigma nonet) and also of the new axial-vector meson nonet, ``chiralons,'' which should be discriminated from the conventional ^3P_0 and ^3P_1 states, respectively.