Pseudoscalar-meson mixing problem, spectral-function sum rules, and chiral symmetry

The pseudoscalar-meson spectrum is first discussed in connection with SU(4) extended spectral-function sum rules. The spectrum can be fitted into the sum rule to within a well-known ambiguity in assigning the η^' particle. In order to remove this ambiguity, the η^' decay width as well as its mass are next investigated in the effective-chiral-Lagrangian approach. Experimental data seem to favor the assignment of η^' = X(958).