| Popis: |
We present the construction of heterotic string models built using the free fermionic formulation, and focus on how additional U(1)s may arise. We motivate an anomaly free combination, U(1)_\zeta, as a proton lifetime preserving symmetry external to a left--right symmetric gauge group. This same combination is found to nullify lepton number in U(1)_{B-L} to form a leptophobic combination, also in left--right symmetric models, which we compare to other leptophobic U(1) combinations constructed in the context of different gauge groups. We then accommodate U(1)_\zeta as a proton lifeguard symmetry in an effective field theory. We present a comparative study of how gauge coupling unification constraints may be satisfied when SO(10)xU(1)_\zeta\not\subset E_6 and when the U(1)_\zeta charges do have an E_6 embedding. We show that without such an embedding, current values of \sin^2\theta_W(M_Z) and \alpha_3(M_Z) rule these models out. We go on to discuss how viable string models with this property included may be constructed. |
