Solving the gravitino problem by the axino

Abstract

In a large class of supersymmetric (SUSY) axion models the mass of the axino ã (a fermionic superpartner of the axion) is predicted as m a ̃ ≲ O(1) keV. Thus, the axino is the lightest SUSY particle (LSP). We pointed out that such a light axino provides a natural solution to the gravitino problem, if the gravitino is the next LSP. We derive a constraint on the reheating temperature T R of inflation, T R ≲10 15 GeV for the gravitino mass m 3/2≃100 GeV, which is much weaker than that obtained in the minimal SUSY standard model.