Phenomenological constraints on grand unification in superstring-inspired models.

Abstract

From the viewpoint of unification we investigate various superstring-inspired ${\mathrm{E}}_{6}$ models which feature no intermediate symmetry-breaking scale, and which contain only one extra U(1) low-energy gauge group. Inputting the current experimentally allowed ranges for the coupling constants at the W scale, we show that unification is achieved most readily when the so-called ``survivors'' (particles which do not belong to an ordinary ${\mathrm{E}}_{6}$ generation) are SU(2) doublets, and is difficult to achieve without them.