The Family Group in Grand Unified Theories

Abstract

We review the known ways of incorporating and breaking symmetries in a renormalizable way. We summarize the various grand unified theories based on $SU_5, SO_{10}$, and $E_6$ as family enlargement groups. An $SU_5$ model with an $SU_2$ gauged family group is presented as an illustration. In it, the e-family (i.e., e,u and d) is classically massless and acquires calculable mass corrections. The family group is broken by the same agent that does the superstrong breaking. Finally, we sketch a way of unifying the family group with $SU_5$ into $SU_8$.