A SUPERSTRING MODEL WITH R SYMMETRIES AND LOW B−L BREAKING

The phenomenology of a three-generation superstring model arising from the compactification of the ten-dimensional heterotic superstring theory on the Tian-Yau manifold with a discrete Z2×Z3 symmetry is discussed. The Z2 is used to define a matter parity and the Z3 generates an R symmetry. Presence of the discrete R symmetry enforces the existence of exactly flat directions in the superpotential that motivate us to break the SU(3) 3 to SU(3) × SU(2) L× SU(2) R× U(1) B−L at a scale M X closely associated with the compactification scale MC~1018 GeV , since the R symmetry and B−L breaking need to arise from unknown string physics and for the successful suppression of proton decay rate due to certain dimension-five operators. When the R symmetry is broken at a scale MR~1014 GeV , the matter parity is broken spontaneously as well, guiding us to the scale of the breaking to the standard model, MB−L~103 GeV , making the additional Z boson accessible at accelerator energies. When the spectrum of particles is derived from the mass matrices, the model emerges as the first model with known particles having nonstandard Z2 matter-parity assignments. The transformation properties of the standard model fermions under the discrete symmetries of the theory give rise to an extremely interesting pattern for their mass generation. The model has acceptable properties vis à vis proton decay lifetime considerations and renormalization group considerations. This is a superstring model where the lightest supersymmetric particle, favored to be the photino, decays with a cosmologically interesting lifetime.