Disentangling Intertwined Embedded States and Spin Effects in   Light-Front Quantization
  Naive light-front quantization, carried out by a light-front energy integration of covariant amplitudes, is not guaranteed to generate the corresponding Feynman amplitudes. In an explicit example we show that the nonvalence contribution to the minus-component of the EM current of a meson with fermion constituents has a persistent end-point singularity. Only after this term is subtracted, the result is covariant and satisfies current conservation. If the spin-1/2 constituents are replaced by spin zero ones, the singularity does not occur and the result is, without any adjustment, identical to the Feynman amplitude. Numerical estimates of valence and nonvalence contributions are presented for the cases of fermion and boson constituents. 
B.L.G. Bakker and Chueng-Ryong Ji