Leading-order actions of Goldstino fields

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