Covariant Solutions of the Bethe-Salpeter Equation
  There is a need for covariant solutions of bound state equations in order to construct realistic QCD based models of mesons and baryons. Furthermore, we ideally need to know the structure of these bound states in all kinematical regimes, which makes a direct solution in Minkowski space (without any 3-dimensional reductions) desirable. The Bethe-Salpeter equation (BSE) for bound states in scalar theories is reformulated and solved for arbitrary scattering kernels in terms of a generalized spectral representation directly in Minkowski space (K. Kusaka et al., PRD 51, 7026 '95). This differs from the conventional Euclidean approach, where the BSE can only be solved in ladder approximation after a Wick rotation. 
A.G. Williams, K. Kusaka and K.M. Simpson