Almost the supersymmetric Standard Model from the Z{sub 6}{sup '} orientifold

Intersecting stacks of supersymmetric fractional branes on the Z{sub 6}{sup '} orientifold may be used to construct the supersymmetric Standard Model. We construct a four-stack model, which has the matter spectrum of the supersymmetric Standard Model, including a single pair of Higgs doublets, plus three neutrino-singlet states. However, the gauge group is SU(3){sub colour}xSU(2){sub L}xU(1){sub Y}xU(1), and there is no overall cancellation of Ramond-Ramond tadpoles.