  A new supersymmetric standard model based on N=1 supergravity is constructed,
aiming at natural explanation for the proton stability without invoking an ad
hoc discrete symmetry through R parity. The proton is protected from decay by
an extra U(1) gauge symmetry. Particle contents are necessarily increased to be
free from anomalies, making it possible to incorporate the superfields for
right-handed neutrinos and an SU(2)-singlet Higgs boson. The vacuum expectation
value of this Higgs boson, which induces spontaneous breakdown of the U(1)
symmetry, yields large Majorana masses for the right-handed neutrinos, leading
to small masses for the ordinary neutrinos. The linear coupling of
SU(2)-doublet Higgs superfields, which is indispensable to the superpotential
of the minimal supersymmetric standard model, is replaced by a trilinear
coupling of the Higgs superfields, so that there is no mass parameter in the
superpotential. The energy dependencies of the model parameters are studied,
showing that gauge symmetry breaking is induced by radiative corrections.
Certain ranges of the parameter values compatible with phenomena at the
electroweak energy scale can be derived from universal values of masses-squared
and trilinear coupling constants for scalar fields at a very high energy scale.
