Supersymmetry and SU(2)xU(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 gravitino mass m 3 / 2 and the electroweak scale m Z are classically undetermined, and slide along moduli directions that include the Higgs flat direction | H 01 | = | H 02 | . There are important differences with conventional supergravity models: the goldstino has components along the higgsino direction; SU (2) × U (1) breaking occurs already at the classical level; the scales m 3 / 2 and m Z , the gauge couplings, the lightest Higgs mass and the cosmological constant are entirely determined by quantum corrections.