Almost the supersymmetric Standard Model from intersecting D6-branes on the Z_6' orientifold

Intersecting stacks of supersymmetric fractional branes on the Z_6' orientifold may be used to construct the supersymmetric Standard Model. If a,b are the stacks that generate the SU(3)_{colour} and SU(2)_L gauge particles, then, in order to obtain {\em just} the chiral spectrum of the (supersymmetric) Standard Model (with non-zero Yukawa couplings to the Higgs mutiplets), it is necessary that the number of intersections a \cap b of the stacks a and b, and the number of intersections a \cap b' of a with the orientifold image b' of b satisfy (a \cap b,a \cap b')=(2,1) or (1,2). It is also necessary that there is no matter in symmetric representations of the gauge group, and not too much matter in antisymmetric representations, on either stack. Fractional branes having all of these properties may be constructed on the Z_6' orientifold. We construct a (four-stack) model with two further stacks, each with just a single brane, which has precisely the matter spectrum of the supersymmetric Standard Model, including a single pair of Higgs doublets. However, the gauge group is SU(3)_{\rm colour} x SU(2)_L x U(1)_Y x U(1)_H. Only the Higgs doublets are charged with respect to U(1)_H.