Abstract
We perform a completeO(α4) calculation of the coupling of theZ boson to three photons via both fermion andW boson loops and keeping the full dependence on the quark andW masses. To evaluate theW boson contribution to the fourth rank polarization tensor we use the unitary gauge. We find that the contributions from fermion and boson loops are remarkably similar. Expressions for the helicity amplitudes are presented. The results are applied to the decayZ→γγγ where we find a partial width of about 1.35 eV form top>91 GeV and sin2 θ W , of which theW boson loops account for approximately 0.3 eV mainly through their interference with the fermion loops.
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