Killing vectors in asymptotically flat space--times: II. Asymptotically   translational Killing vectors and the rigid positive energy theorem in higher   dimensions
  We show that the borderline cases in the proof of the positive energy theorem for initial data sets, on spin manifolds, in dimensions $n\ge 3$, are only possible for initial data arising from embeddings in Minkowski space-time. 
Piotr T. Chrusciel, Daniel Maerten