Proton Stability and Superstring Z'
  Recently it was argued that proton lifetime limits impose that the scale of quantum gravity must be above 10^{16} GeV. By studying the proton stability in the context of realistic heterotic-string models, I propose that proton longevity necessitates the existence of an additional U(1)_{Z'} symmetry, which is of non-GUT origin and remains unbroken down to intermediate, or low, energies. It is shown that the realistic string models frequently give rise to U(1) symmetries, which suppress the proton decay mediating operators, with, or without, the possibility of R-parity violation. By studying the F- and D-flat directions, I examine whether the required symmetries remain unbroken in the supersymmetric vacuum and show that in some examples they can, whereas in others they cannot. The proton decay rate is proportional to the U(1)_{Z'} symmetry breaking scale. Imposing the proton lifetime limits I estimate the U(1)_{Z'} breaking scale and show that if substantial R-parity violation is present the associated Z' is within reach of forthcoming collider experiments 
Alon E. Faraggi