LIGHT PHOTINOS AS DARK MATTER

There are good reasons to consider models of low-energy supersymmetry with very light photinos and gluinos. In a wide class of 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, $\pho$, 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 $R^0$ are close in mass. Including $\r0\longleftrightarrow\pho$ processes, we find that the photino relic abundance is most sensitive to the $\r0$-to-$\pho$ mass ratio, and within model uncertainties, a critical density in photinos may be obtained for an $\r0$-to-$\pho$ 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.