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)xU(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_1^0| = |H_2^0|. There are important differences with conventional supergravity models: the goldstino has components along the higgsino direction; SU(2) x 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.