The tau forward-backward Asymmetry within Grand Unified Theories

We will study some possible non universal corrections that could affect single measurements of the Weinberg angle sin^2(theta) (leaving the full average essentially unchanged). We will concentrate on the class of the models that introduces a new light gauge boson and an extended Higgs sector, but we restrict the Higgs charges choice in order to automatically achieve zero mixing at the tree level between the Z_0 and the Z'. This will more naturally keep small the universal effects with a relatively light Z', but it will imply only vector-like coupling between this boson and the standard fermions. A SO(10) gauge boson which distinguishes between families can have such properties and could explain a deviation in the tau forward-backward asymmetry without affecting the tau polarization measurements at LEP.