In the standard model the proton is protected from decay naturally by gauge
symmetries, whereas in the ordinary minimal supersymmetric standard model an ad
hoc discrete symmetry is imposed for the proton stability. We present a new
supersymmetric model in which the proton decay is forbidden by an extra U(1)
gauge symmetry. Particle contents are necessarily increased to be free from
anomalies, incorporating right-handed neutrinos. Both Dirac and Majorana masses
are generated for neutrinos, yielding non-vanishing but small masses. The
superpotential consists only of trilinear couplings and the mass parameter
μ of the minimal model is induced by spontaneous breaking of the U(1)
symmetry.Comment: 10 pages, Revte