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.