Z-BOSON DECAY INTO PHOTONS

We perform a complete O(alpha4) calculation of the coupling of the Z b oson to three photons via both fermion and W boson loops and keeping t he full dependence on the quark and W masses. To evaluate the W boson contribution to the fourth rank polarization tensor we use the unitary gauge. We find that the contributions from fermion and boson loops ar e remarkably similar. Expressions for the helicity amplitudes are pres ented. The results are applied to the decay Z-->gammagammagamma where we find a partial width of about 1.35 eV for m(top) > 91 GeV and sin2t heta(W) = 0.23, of which the W boson loops account for approximately 0 .3 eV mainly through their interference with the fermion loops.