Nonperturbative calculation of symmetry breaking in quantum field theory
  A new version of the delta expansion is presented, which, unlike the conventional delta expansion, can be used to do nonperturbative calculations in a self-interacting scalar quantum field theory having broken symmetry. We calculate the expectation value of the scalar field to first order in delta, where delta is a measure of the degree of nonlinearity in the interaction term. 
Carl M. Bender and Kimball A. Milton