Study of Ua(1) Breaking in Hadron Physics

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Chiral effective theories of QOD are applied to the hadron spectra and properties of hadrons at finite temperature and baryon density.We constructed an extended NJL model including a U(1)A breaking six-quark interaction vertex and found that a strong axial U(1) breaking is preferred phenomenological both in the meson and baryon sectors We also applied the improved ladder approximation in the Bethe-Salpeter equation for QCD and analyzed the pseudoscalar meson spectrum. The renormalization procedure is carried out for finite quark mass and an effective axialvector current is introduced so that the axial Ward-Takahashi identity is satisfied. Numerical calculation shows that the masses and decay constants of mesons are reproduced fairly well and the Gell-Mann-Oaks-Renner relation is satisfied for finite quark mass up to a hundred MeV.W also applied the QCD sum rule to the positive and negative parity baryons and showed explicitly that their mass difference comes from the chiral symmetry breaking. Two distinct assignments of chiral symmetry is shown to exhibit different behaviors of hadrons.