The heavy neutral Higgs signature in the gamma gamma to Z Z process

If the Standard Model (SM) Higgs particle is sufficiently heavy, then its contribution to gamma gamma to Z Z should be largely imaginary, interfering with the also predominantly imaginary SM "background" generated by the W-loop. For standard model Higgs masses in the region 200 \lsim m_H \lsim 500 GeV, this interference is found to be constructive and increasing the Higgs signal. In the minimal SUSY case an interference effect should also appear for the contribution of the heavier CP-even neutral Higgs boson H^0, provided it is sufficiently heavy. The effect is somewhat reduced though, by the smallness of the H^0 width and the gamma gamma and ZZ branching ratios. The interference is again found to be constructive for part of the parameter space corresponding to sfermion masses at the TeV scale and maximal stop mixing. For both the SM and SUSY cases, regions of the parameter space exist though, where the interference may be destructive. It is therefore essential to take these effects into account when searching for possible scalar Higgs-like candidates. To this aim, we present the complete analytic expressions for both resonance and background amplitudes.