SUPERSYMMETRY AND SU(2)XU(1) BREAKING WITH NATURALLY VANISHING VACUUMENERGY

We show how the spontaneous breaking of local N = 1 supersymmetry and of the SU(2) x U(1) gauge symmetry can be simultaneously realized, wit h naturally vanishing tree-level vacuum energy, in superstring effecti ve supergravities. Both the gravitino mass m(3/2) and the electroweak scale mt are classically undetermined, and slide along moduli directio ns that include the Higgs flat direction \H-1(0)\ = \H-2(0)\ There are important differences with conventional supergravity models: the gold stino has components along the higgsino direction; SU(2) x U(1) breaki ng occurs already at the classical level; the scales m(3/2) and M(z), the gauge couplings, the lightest Higgs mass and the cosmological cons tant are entirely determined by quantum corrections.