Phenomenology of fermionic strings

We investigate the low-energy phenomenology of string models constructed in the fermionic formulation. Models are constructed in which the gauge group SU(3) × SU(2) × U(1) ×... is obtained directly from free world-sheet fermions. We also construct models in which SU(3) × SU(2) × U(1) ×... is obtained by symmetry breaking along completely flat directions in the effective potential; this corresponds to world-sheet interactions. In this category we construct models in which SU(5) is broken to SU(3) × SU(2) × U(1) by a Higgs boson in the adjoint representation and in which SO(4) × SO(6) is broken by the spinor representation. We analyze the effects of the spin structure, massless moduli, and Fayet-Iliopoulos $D$ terms on low-energy phenomenology. In particular we consider the renormalization-group flow of the couplings, proton decay, mass matrices, Higgs-boson mass, and charge quantization. We briefly compare these results with the flipped SU(5) approach and other constructions such as orbifolds and Calabi-Yau manifolds.