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 $200\lesssim m_H \lesssim 500$ 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 H0, provided it is sufficiently heavy. The effect is somewhat reduced, though, by the smallness of the H0 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 the SUSY case, 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.

The Standard Model contributions to the processes γγ→γγ, γZ and ZZ at sufficiently high energies, are found to be helicity conserving and almost purely imaginary. This is due to the W-loop contribution, which is much bigger than the fermionic ones at such energies. Thus the structure of these amplitudes acquires an impressive simplicity at high energies. Nothing like this appears in other process like e.g. the production of a pair of neutral Higgs bosons.

We compute the MSSM one-loop contributions to the asymptotic energy behaviour of fermion-antifermion pair production at future lepton-antilepton colliders. Besides the conventional logarithms of Renormalization Group origin, extra SUSY linear logarithmic terms appear of "Sudakov-type". In the TeV range their overall effect on a variety of observables can be quite relevant and drastically different from that obtained in the SM case.

We establish the general off-shell structure of the three neutral gauge boson self-couplings V*1V*2V*3, with applications to the Z*Z*Z*, Z*Z*γ*,γ*γ*Z* cases. New coupling forms appear which do not exist when two gauge bosons are on shell. We give the contribution arising from a fermionic triangle loop. It covers both the standard model (SM) and possible new physics (NP) contributions such as those arising in the minimal supersymmetric SM. For what concerns NP contributions with a high scale, we discuss the validity of an effective Lagrangian involving a limited set of parameters. Finally we write the general expression of the V*1V*2V*3-vertex contribution to the e+e--->(ff¯)+(f'f¯') amplitude.

Including the radiative perturbative corrections and the short distance tachyonic gluon mass effects which mimic the ones of UV renormalons, we re-estimate the decay amplitudes, masses and widths of light hybrid mesons from QCD spectral sum rules. We show that the effects are tiny and confirm the previous lowest order results. We discuss the phenomenological impacts of our results for the vector hybrids.

We examine the standard and the new physics (NP) contributions to the ZZZ, ZZγ, and Zγγ neutral gauge couplings. At the one-loop level, if we assume that there is no CP violation contained in NP beyond the standard model (SM) one, we find that only CP conserving neutral gauge couplings are generated, either from the standard quarks and leptons, or from possible new physics fermions. Bosonic one-loop diagrams never contribute to these couplings, while the aforementioned fermionic contributions satisfy hZ3~=-fγ5, hZ4=hγ4=0. We also study examples of two-loop NP effects that could generate nonvanishing hγ,Z4 couplings. We compare quantitative estimates from SM, minimal supersymmetric standard model (MSSM), and some specific examples of NP contributions, and we discuss their observability at future colliders.