Finite quark mass effects in the improved ladder Bethe-Salpeter amplitudes

We study the finite quark mass effects of the low-energy QCD using the impr oved ladder Schwinger-Dyson and Bethe-Salpeter equations which are derived in the manner consistent with the vector and axial-vector Ward-Takahashi id entities. The nonperturbative mass-independent renormalization allows us to calculate the quark condensate for a nonzero quark mass. We explicitly sho w that the partial conservation of axial-vector current relation holds. The key ingredients are the Cornwall-Jackiw-Tomboulis effective action, the ge neralized Noether current, and the introduction of the regularization funct ion to the Lagrangian. Reasonable values of the pion mass, the pion decay c onstant, and the quark condensate are obtained with a rather large Lambda(Q CD). The pion mass square and the pion decay constant are almost proportion al to the current quark mass up to the strange quark mass region. It sugges ts that the chiral perturbation is applicable up to the strange quark mass region. We study the validity of the approximation often used in solving th e Bethe-Salpeter equations too. [S0556-2813(99)03703-6].