Hidden Sectors from Multiple Line Bundles for the B-L$B-L$ MSSM

We give a formalism for constructing hidden sector bundles as extensions of sums of line bundles in heterotic M-theory. Although this construction is generic, we present it within the context of the specific Schoen threefold that leads to the physically realistic B-L$B-L$ MSSM model. We discuss the embedding of the line bundles, the existence of the extension bundle, and a number of necessary conditions for the resulting bundle to be slope-stable and thus N=1$N=1$ supersymmetric. An explicit example is presented, where two line bundles are embedded into the SU(3)$SU(3)$ factor of the E6xSU(3)$E_{6} \times SU(3)$ maximal subgroup of the hidden sector E-8 gauge group, and then enhanced to a non-Abelian SU(3)$SU(3)$ bundle by extension. For this example, there are in fact six inequivalent extension branches, significantly generalizing that space of solutions compared with hidden sectors constructed from a single line bundle.