The imaginary part of the fourth-order Bhabha (e⁺-e^-) scattering matrix element interferes with the (real) second-order matrix element, to produce a sixth-order dependence of the cross section on the spin of one of the particles (after summing over the spins of the other three particles). The process of extracting the imaginary part of the fourth-order matrix element is presented in some detail in one of the graphs (vacuum polarization).
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Frønsdal et al. (1961) studied this question.
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