text: \] All these three decays are made unavoidable in the Green-Schwarz-mechanism-based models and the relative branching ratios are pretty much given. Note that \(W^0\equiv W 3\) is a mixture of \(Z^0\) and \(\gamma\) so all three pairs created from \(Z^0\) and \(\gamma\) would be possible but the Landau-Yang theorem implies that the \(\gamma\gamma\) decay of \(Z'\) is forbidden (the rate is zero) for symmetry reasons.