Supersymmetric Intersecting Branes on the Type IIAT6/Bbb Z4Orientifold

We study supersymmetric intersecting D6-branes wrapping 3-cycles in the TypeIIA T^6/Z_4 orientifold background. As a new feature, the 3-cycles in thisorbifold space arise both from the untwisted and the Z_2 twisted sectors. Wepresent an integral basis for the homology lattice, H_3(M,Z), in terms offractional 3-cycles, for which the intersection form involves the Cartan matrixof E8. We show that these fractional D6-branes can be used to constructsupersymmetric brane configurations realizing a three generation Pati-Salammodel. Via brane recombination processes preserving supersymmetry, this GUTmodel can be broken down to a standard-like model.Comment: 48 pages, TeX, harvmac, 8 figures, v4: some signs correcte