Stabilizing Moduli with a Positive Cosmological Constant in Heterotic   M-Theory
  It is shown that strongly coupled heterotic M-theory with anti-five-branes in the S^1/Z_2 bulk space can have meta-stable vacua which break N=1 supersymmetry and have a small, positive cosmological constant. This is demonstrated for the "minimal" heterotic standard model. This vacuum has the exact MSSM matter spectrum in the observable sector, a trivial hidden sector vector bundle and both five-branes and anti-five-branes in the bulk space. The Kahler moduli for which the cosmological constant has phenomenologically acceptable values are shown to also render the observable sector vector bundle slope-stable. A corollary of this result is that strongly coupled M-theory vacua with only five-branes in the S^1/Z_2 interval may have stabilized moduli, but at a supersymmetry preserving minimum with a large, negative cosmological constant. 
Volker Braun, Burt A. Ovrut