Symmetries $\mathrm{SU}{\mathrm{}\left(3\right)\mathrm{}}_c\times \mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_L\times \mathrm{U}\mathrm{}\left(1\right)\mathrm{}\times \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$ arising from a specific class of models involving the spontaneous breakdown of SO(10) are considered. In these, two neutral vector bosons $Z_1$ and $Z_2$ arise. The $Z_1$ is expected to be very similar to the $Z_0$ of the standard $\mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_L\times \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$ model. The most restrictive constraints, arising from the Novosibirsk experiment on parity violation in atomic bismuth, imply $0.98\le \frac{M\left(Z_1\right)}{M\left(Z_0\right)}\le 1$. The $Z_2$ can be as light as $\mathrm{}(2.5-3)M\left(Z_0\right)$. Its effects in neutral-current interactions, couplings, and production mechanisms are examined.

• Received 28 December 1981

DOI:https://doi.org/10.1103/PhysRevD.25.3036