We examine the possibilities for two extra Z bosons in ${\mathrm{E}}_6$ electroweak models, assuming electroweak symmetry breaking occurs via the 27 representation of ${\mathrm{E}}_6$, as required by superstring theories. We compare scenarios where ${\mathrm{E}}_6$ is broken to a rank-6 or rank-5 group, and discuss means by which the different models may be distinguished.

• Received 9 October 1986

DOI:https://doi.org/10.1103/PhysRevD.35.1005