Pion mass and decay constant

Independent of assumptions about the form of the quark-antiquark scattering kernel we derive the explicit relation between the pion Bethe-Salpeter amplitude, Γπ, and the quark propagator in the chiral limit; Γπ necessarily involves a non-negligible γ5γ·P term (P is the pion four-momentum). We also obtain exact expressions for the pion decay constant, fπ, and mass, both of which depend on Γπ; and demonstrate the equivalence between fπ and the pion Bethe-Salpeter normalisation constant in the chiral limit. We stress the importance of preserving the axial-vector Ward-Takahashi identity in any study of the pion itself, and in any study whose goal is a unified understanding of the properties of the pion and other hadronic bound states.