Domain walls in SU(5) - art. no. 123506

We consider the grand unified SU(5) model with a small or vanishing cubic t erm in the adjoint scalar field in the potential. This gives the model an a pproximate or exact Z(2) symmetry whose breaking leads to domain walls. The simplest domain wall has the structure of a kink across which the Higgs fi eld changes sign (Phi --> -Phi) and inside which the full SU(5) is restored . The kink is shown to be perturbatively unstable for all parameters. We th en construct a domain wall solution that is lighter than the kink and show it to be perturbatively stable for a range of parameters. The symmetry in t he core of this domain wall is smaller than that outside. The interactions of the domain wall with magnetic monopoles are discussed and it is shown th at magnetic monopoles with certain internal space orientations relative to the wall pass through the domain wall. Magnetic monopoles in other relative internal space orientations are likely to be swept away on collision with the domain walls, suggesting a scenario where the domain walls might act li ke optical polarization filters, allowing certain monopole "polarizations" to pass through but not others. As SU(5) domain walls will also be formed a t small values of the cubic coupling, this leads to a very complicated pict ure of the evolution of defects after the grand unified phase transition.