Low-Scale SU(4)_W Unification

We embed the minimal left-right model SU(2)_L*SU(2)_R* U(1)_{B-L} into an SU(4)_W gauge group, and break the unified group via five-dimensional S^1/(Z_2*Z_2) orbifolding. Leptons are fitted into SU(4)_W multiplets and located on a symmetry preserving O brane, while quarks are placed onto an O' brane where the symmetry is broken. This approach predicts \sin^2\theta_W=0.25 for the weak mixing angle at tree level and leads to a rather low weakly (strongly) coupled unification scale of order 3*10^2 TeV (several TeV) with supersymmetry, or as low as several TeV in the non-supersymmetric case. Another symmetry breaking chain with the low-energy gauge group SU(2)_L*U(1)_{3R}*U(1)_{B-L} can also give rise to a weak mixing angle \sin^2\theta_W=0.25 at tree level after gauge symmetry breaking by orbifolding. Such theories with low-scale unification have interesting phenomenological consequences.