Leading-order actions of Goldstino fields

This paper starts with a self-contained discussion of the so-called Akulov-Volkov action S-AV, which is traditionally taken to be the leading-order action of the Goldstino field. Explicit expressions for S-AV and its chiral version S-AV(ch) are presented. We then turn to the issue on how these actions are related to the leading-order action S-NL proposed in the newly proposed constrained superfield formalism. We show that S-NL may yield S-AV/S-AV(ch) or a totally different action S-KS, depending on how the auxiliary field in the former is integrated out. However, S-KS and S-AV/S-AV(ch) always yield the same S-matrix elements, as one would have expected from general considerations in quantum field theory.