Spinless Mesons in the Chiral-Symmetry Scheme

The multiplet of the pseudoscalar mesons $K$, $\pi$, $\eta$, and ${\eta }^{\prime }$ with their scalar partners is analyzed as a reducible representation of chiral $S{U}_2\otimes S{U}_2$. We introduce a spontaneous breaking mechanism of this symmetry, leaving isospin and hypercharge as residual invariance. We consider also in this scheme a Lagrangian model, which exhibits various remarkable group-theoretical properties, and leads to numerical values, mass, and mixing-angle relations, in very good agreement with experimental data.