Goldstino and sgoldstino in microscopic models and the constrained superfields formalism

We examine the exact relation between the superconformal symmetry breaking chiral superfield (X) and the goldstino superfield in microscopic models of an arbitrary Kahler potential (K) and in the presence of matter fields. We investigate the decoupling of the massive sgoldstino and scalar matter fields and the offshell/onshell-SUSY expressions of their superfields in terms of the fermions composites. For general K of two superfields, we study the properties of the superfield X after integrating out these scalar fields, to show that in the infrared it satisfies (offshell) the condition $X^3=0$ and $X^2\not=0$. We then compare our results to those of the well-known method of constrained superfields discussed in the literature, based on the conjecture $X^2=0$. Our results can be used in applications, to couple offshell the (s)goldstino fields to realistic models such as the MSSM.