  We demonstrate the existence of heterotic-string vacua that are free of
massless exotic fields. The need to break the non-Abelian GUT symmetries in k=1
heterotic-string models by Wilson lines, while preserving the GUT embedding of
the weak-hypercharge and the GUT prediction sin^2\theta_w(M(GUT))=3/8,
necessarily implies that the models contain states with fractional electric
charge. Such states are severely restricted by observations, and must be
confined or sufficiently massive and diluted. We construct the first
quasi-realistic heterotic-string models in which the exotic states do not
appear in the massless spectrum, and only exist, as they must, in the massive
spectrum. The SO(10) GUT symmetry is broken to the Pati-Salam subgroup. Our PS
heterotic-string models contain adequate Higgs representations to break the GUT
and electroweak symmetry, as well as colour Higgs triplets that can be used for
the missing partner mechanism. By statistically sampling the space of
Pati-Salam vacua we demonstrate the abundance of quasi--realistic three
generation models that are completely free of massless exotics, rendering it
plausible that obtaining realistic Yukawa couplings may be possible in this
space of models.

High Energy Physics - Theory;High Energy Physics - Phenomenology