Heterotic Vacua from general (non-) Abelian Bundles
  We report on the construction of four-dimensional string vacua by considering general abelian and non-abelian bundles on an internal Calabi-Yau for both heterotic theories. The structure of the resulting gauge sector is extremely rich and gives rise to many new model building possibilities. We analyse the chiral spectrum including the contribution from heterotic five-branes and provide the general consistency conditions. The one-loop corrected supersymmetry condition on the bundles is found to be that of pi-stability. As an application we present a supersymmetric Standard-Model like example for the SO(32) string with U(n) bundles on an elliptically fibered Calabi-Yau. 
Timo Weigand