| Summary: | To this date, experimental searches for exotics (fractionally charged states) have yielded negative results, thus imposing strong limitations on their existence at low energies. In this work we argue that a possible scenario that could explain the experimental data is that such states are simply not present at the low energy limit. This thesis presents the phenomenology of the first string model which does not have any exotic states at the massless level, and where only the top quark Yukawa coupling exists at the tri-linear level superpotential. More specifically we present its spectrum, cubic superpotential and a viable semi-realistic phenomenological scenario which is supported by a specific set of F - and D-fl.at solutions. The discovery of this model is the result of the statististical exploration of a class of heterotic Pati-Salam vacua, out of which we managed to extract three generation exophobic models with the required representations needed to induce spontaneous breaking to the Standard Model. We also exhibit the derivation of the analytic formulae that permitted the exact identification of several properties of a string vacuum, and thus allowed its distinction among supersymmetric vacua that share the same gauge group.
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