Supersymmetry and SU(2) × U(1) breaking with naturally vanishing vacuum energy

We show how the spontaneous breaking of local N = 1 supersymmetry and of the SU(2) × U(1) gauge symmetry can be simultaneously realized, with naturally vanishing tree-level vacuum energy, in superstring effective supergravities. Both the gravition mass m32 and the electroweak scale mZ are classically undetermined, and slide along moduli directions that include the Higgs flat direction |H10| = |H20|. There are important differences with conventional supergravity goldstino has components along the higgsino direction; SU(2) × U(1) breaking occurs already at the classical level; the scales m32 and mZ, the rauge couplings, the lightest Higgs mass and the cosmological constant are entirely determined by quantum corrections.