We consider popular extensions of the standard model which include new heavy $Z^0$ gauge bosons, and examine their effect on parity violation in elastic electron-proton scattering at intermediate energies. These extensions all involve an effective low-energy symmetry group SU(3${)}_{\mathrm{C}}$⊗SU(2${)}_{\mathrm{L}}$ ⊗U(1${)}_{\mathrm{Y}}$⊗(U)$1_{\mathrm{Y}\mathcal{’}}$. In several special cases motivated by grand unified theories based on SO(10) and ${\mathrm{E}}_6$, we find that a parity measurement with an accuracy of a few percent would raise the existing mass bounds on a heavy $Z^0$, and would reduce the range of allowed mixing angles by as much as a factor of 5. In some cases, the lower bound on a heavy $Z^0$ mass would be raised by more than 100 GeV over current limits.

• Received 29 August 1988

DOI:https://doi.org/10.1103/PhysRevD.39.163