Sparticle spectrum and dark matter in type I string theory with an intermediate scale

The supersymmetric particle spectrum is calculated in type I string theories formulated as orientifold compactifications of type IIB string theory. A string scale at an intermediate value of $10^{11}-10^{12}$ GeV is assumed and extra vector-like matter states are introduced to allow unification of gauge coupling constants to occur at this scale. The qualitative features of the spectrum are compared with Calabi-Yau compactification of the weakly coupled heterotic string and with the eleven dimensional supergravity limit of $M$-theory. Some striking differences are observed. Assuming that the lightest neutralino provides the dark matter in the universe, further constraints on the sparticle spectrum are obtained. Direct detection rates for dark matter are estimated.