Is the Standard Model the maximum Gauge Symmetry?

We explicitly demonstrate that though CP2 does not admit spinors, CP2 ⊗ S 2 ∼= SU(3) ⊗ SU(2) for the compactified six-dimensional internal space of the RNS model does, because it is a Ricci-flat Calabi-Yau manifold corresponding to three generations of fermions and seems to be the only choice for nontrivial fermion generations. The Chan-Paton factors for the SU(3) symmetry are fully Neumann and correspond to the dynamics of the system, while those of U(2) ∼= SU(2)⊗U(1) of the Standard Model can only be introduced through d-branes. The full spectrum, though not supersymmetric in the true sense, has equal numbers of bosonic and fermionic degrees of freedom. Hence, the zero-point energy cancels, and there is no Casimir force between parallel d-branes.