New Scenario for String Unification

We present a new scenario for gauge coupling unification in flipped SU(5) string models, which identifies the $M_{32}$ scale of SU(3) and SU(2) unification with the empirical $M_{\mathrm{LEP}}\sim {10}^{15-16}\mathrm{GeV}$ scale, and the $M_{51}$ scale of SU(5) and U(1) unification with the theoretical $M_{\mathrm{string}}\sim 5\times {10}^{17}\mathrm{GeV}$ string unification scale. The vacuum shift necessary for the cancellation of the anomalous ${\mathrm{U}}_A\left(1\right)$ and an SU(4) hidden sector with fractionally charged particles play a crucial role in the dynamical determination of all intermediate mass scales in this scenario.