Light photinos as dark matter

There are good reasons to consider models of low-energy supersymmetry with very light photinos and gluinos. In most of these models the lightest R-odd, color-singlet state containing a gluino, the R⁰, has a mass in the 1-2 GeV range and the slightly lighter photino γ would survive as the relic R-odd species. For the light photino masses considered here, previous calculations resulted in an unacceptable photino relic abundance. But we point out that processes other than photino self-annihilation determine the relic abundance when the photino and R⁰ are close in mass. Including R⁰ ↔ γ processes, we find that the photino relic abundance is most sensitive to the R⁰-to-γ mass ratio, and within model uncertainties, a critical density in photinos may be obtained for an R⁰-to-γ mass ratio in the range 1.2 to 2.2. We propose photinos in the mass range of 500 MeV to 1.6 GeV as a dark matter candidate, and discuss a strategy to test the hypothesis.