Domain walls and fermion scattering in Grand Unified models

Motivated by Grand Unification, we study the properties of domain walls formed in a model with $SU(5)\times Z_2$ symmetry which is spontaneously broken to $SU(3) \times SU(2) \times U(1)/Z_6$, and subsequently to $SU(3) \times U(1)/Z_3$. Even after the first stage of symmetry breaking, the SU(3) symmetry is broken to $SU(2)\times U(1)/Z_2$ on the domain wall. In a certain range of parameters, flux tubes carrying color- and hyper-charge live on the domain wall and appear as ``boojums'' when viewed from one side of the domain wall. Magnetic monopoles are also formed in the symmetry breaking and those carrying color and hyper-charge can be repelled from the wall due to the Meissner effect, or else their magnetic flux can penetrate the domain wall in quantized units. After the second stage of symmetry breaking, fermions can transmute when they scatter with the domain wall, providing a simpler version of fermion-monopole scattering: for example, neutrinos can scatter into d-quarks, leaving behind electric charge and color which is carried by gauge field excitations living on the domain wall.