Aspects of string phenomenology at the self dual radius

This dissertation is primarily focused on the discussion of heterotic string phenomenology derived from the free fermionic approach. Two models addressing different phenomenological issues, namely optical unification as a solution to the factor of twenty difference between GUT and string scale unification, and observable/hidden sector mirror models that contain an unavoidable gauge (and matter) mirror symmetry breaking for which the broken matter provides a potential dark matter candidate. Lastly, a geometrical interpretation for simultaneous D- and F-flat directions is presented.; Chapter two presents the first model constructed from free fermionic strings in which requirements for optical unification [1] may be satisfied. Free fermionic string models generically produce intermediated scale particles, for which specific sets can act as a diverging lens causing the string scale unification point to appear to unify at the GUT scale unification point.; In chapter three, a model is discussed in which mirror symmetric boundary conditions imposed on world-sheet fermions necessarily require asymmetric GSO projections destroying mirror symmetry and enhancing hidden sector gauge groups. Specifically, an expected non-chiral Pati-Salam mirror universe model is transformed into a chiral model with enhanced hidden sector gauge symmetry and reduced observable sector gauge symmetry: [SU(4) C ⊗ SU(2)L ⊗ SU(2)R]O ⊗ [SU(4)C ⊗ SU(2) L ⊗ SU(2)R] H, is necessarily transformed into a chiral [SU(4) C ⊗ SU(2)L) O ⊗ [SO(10) ⊗ SU(2) R]H.; Finally, a discussion is presented in chapter four in which it is hoped that a geometrical interpretation can be given when both D- and F-flat constraints are simultaneously satisfied that will provide new insight into the classification of these flat directions enabling a clearer recognition for when these flat directions exist. This approach is specifically applied to the groups SU(2) and SO(2n), relevant to the MSSM and flipped SU(5) models.