Constraint on unstable photino mass from a superlight gravitino

Abstract

The known lower bound on the unstable photino mass gets invalidated by a gravitino lighter than 10⁻³ eV. Nevertheless, general bounds can be derived by feeding early universe constraints from the primary abundance of He⁴ and the extant lower bound on the gravitino mass into the lifetime for the decay photino → photon + gravitino. The mass-range O(100) eV to O(1) MeV is excluded.