The gravitational content of lorentzian complex structures
  The definition of a positive energy is investigated in a renormalizable 4-dimensional generally covariant model, which depends on the lorentzian complex structure and not the metric of spacetime. The gravitational content of the lorentzian complex structures is revealed by identifying the spacetime with special 4-dimensional surfaces of the G{2,2} Grassmannian manifold. The lorentzian complex structure is found to be a codimension-4 CR structure and its classification is studied using the Chern-Moser and Cartan methods. The spacetime metric is found to be a Fefferman-like metric of this codimension-4 CR structure. The open CR manifolds "hanging" from the points of the U(2) characteristic boundary of the SU(2,2) classical domain belong into representations of the Poincar\'e group and are related to the particle spectrum of the model. 
C. N. Ragiadakos
hep-th gr-qc