In 1970, Kobayashi and Maskawa concluded that an effective six-quark vertex with a determinantal form is necessary in the chiral effective models to account for the large mass of X meson, which is now called the η ′. The determinantal interaction has an SU(3)L ⊗SU(3)R symmetry but not U(3)L ⊗ U(3)R, and, hence accounts for the explicit breaking of U(1)A symmetry in quantum chromodynamics (QCD); the vertex was later derived by ’t Hooft as an instanton-induced quark interaction. The vertex, which may be called the Kobayashi-Maskawa-’t Hooft (KMT) term, is widely used in quantitative analyses of hadron physics and QCD phase transitions at finite temperature and density. An account is made for the KMT term with recent extensive applications. Described are also personal experiences with Professor Maskawa and Professor Kobayashi, including an encounter with Professor Maskawa when the author first presented his work on the KMT term. The pseudoscalar meson η ′ (958) was called X in the past for some time. It was difficult to understand its large mass within the chiral SU(3)L ⊗ SU(3)R with an explicit symmetry breaking term −ǫ0S0 − ǫ8S8, which may have been identifie
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