Supersymmetric standard models on D-branes

Type IIB superstring models with the standard model gauge group on D3-branes and with massless matter associated with open strings joining D3-branes to D3-branes or D3-branes to D73-branes are studied. Models with gauge coupling constant unification at an intermediate scale between about 1010 and 1012 GeV and consistency with the observed value of sin2θW(MZ) are obtained. Extra vector-like states and extra pairs of Higgs doublets play a crucial role.