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 $\mathrm{D}{7}_3$-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.