Axino mass in supergravity models

We analyze the mass of the axino, the fermionic superpartner of the axion, in general supergravity models incorporating a Peccei-Quinn symmetry and determine the cosmological constraints on this mass. In particular, we derive a simple criterion to identify models with an LSP axino which has a mass of O(m322ƒPQ) = O (keV) and can serve as a candidate for (warm) dark matter. We point out that such models have very special properties and in addition, the small axino mass has to be protected against radiative corrections by demanding small couplings in the Peccei-Quinn sector. Generically, we find an axino mass of order m32. Such masses are constrained by the requirement of an axino decay which occurs before the decoupling of the ordinary LSP. Especially, for a large Peccei-Quinn scale fPQ > 1011 GeV this constraint might be difficult to fulfill.