The Cosmological Constant in Calabi-Yau 3-fold Compactifications of the   Horava-Witten Theory
  Brane world scenarios offer a way of setting the cosmological constant to zero after supersymmetry breaking provided there is a sufficient number of adjustable integration constants/parameters. In the case of the Horava-Witten theory compactified on a Calabi-Yau threefold, we argue that it is difficult to find enough freedom to get a zero (or small) cosmological constant after supersymmetry breaking. 
S.P. de Alwis and Nikolaos Irges