[en] It is shown that (i) the small but definite discrepancy between the mass-squared Gell-Mann--Okubo mass formula of the 1/2⁺ baryons and experiment, (ii) the trouble of the Σ-Λ degeneracy, and (iii) the deviation of the D/F ratio of the axial-vector semileptonic couplings from the SU(6) value as indicated by the value vertical-bar G/subA//G/subV/ vertical-bar = 0.435 +- 0.035 of the latest high-statistics experiment on the Σ^- → n + e^- + nu-bar decay as compared with the SU(6) value approx. = 0.25, can be explained simultaneously if a ninth I = 0, 1/2⁺ baryon Λ' exists around 1700 MeV with a broad width of about 1 GeV. The hyperon semileptonic decays in the presence of the Λ' are analyzed in detail. The Cabibbo angles are determined to be sintheta/subV/ = 0.227 +- 0.008 and sintheta/subA/ = 0.220 +- 0.020. The theoretical frameworks used are asymptotic SU(3), chiral SU(3) direct-product SU(3) charge algebra, and the simple mechanism of symmetry breaking