Formulation of the Quantum Theory of Electromagnetic In- teraction" Phys.Rev. 80 (1950) 440-457 DOI: 10.1103/PhysRev.80.440 [49] P. G. 0. Freund and Y. Nambu, "Scalar field coupled to the trace of the energy-momentum tensor" Phys. Rev. 174 (1968) 1741-1743. Also see Appendix II in R. H. Kraichnan, "Specia