The box diagram in Yukawa theory
  We present a light-front calculation of the box diagram in Yukawa theory. The covariant box diagram is finite for the case of spin-1/2 constituents exchanging spin-0 particles. In light-front dynamics, however, individual time-ordered diagrams are divergent. We analyze the corresponding light-front singularities and show the equivalence between the light-front and covariant results by taming the singularities. 
Bernard L. G. Bakker and Jorn K. Boomsma (Department of Physics and   Astrophysics, Vrije Universiteit) and Chueng-Ryong Ji ...