Chiral Fermions from Compactification of O(32) and E(8) ⊗ E(8) String Theories

Abstract

Compactification to four dimensions of a low-energy approximation to ten-dimensional O(32) string theory leads only to grand unified theories with trivial replication of families. The E(8) \ensuremath{\bigotimes} E(8) string is more promising and leads in four dimensions to SU(8) with six families. If compactification breaks the symmetry to lower-rank groups, e.g., SU(7) or E(6), there may be any number of families. In the SU(8) and SU(7) models there is nontrivial family symmetry.