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 ${\rm D}7_3$-branes are studied. Models with gauge coupling constant unification at an intermediate scale between about $10^{10}$ and $10^{12}$GeV and consistency with the observed value of $\sin^2 \theta_W (M_Z)$ are obtained. Extra vector-like states and extra pairs of Higgs doublets play a crucial role. 
D. Bailin, G. V. Kraniotis, A. Love