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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 ͑SM͒ and possible new physics ͑NP͒ 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 1 *V 2 *V 3 *-vertex contribution to the e ϩ e Ϫ →( f f)ϩ( f Ј fЈ) amplitude.

The possible form of new physics ͑NP͒ interactions affecting the ZZZ, ZZ␥, and Z␥␥ vertices is critically examined. Their signatures and the possibilities to study them, through ZZ and Z␥ production, at the CERN e Ϫ e ϩ colliders LEP and LC and at the hadronic colliders, the Fermilab Tevatron and CERN LHC, are investigated. Experimental limits obtained or expected on each coupling are collected. A simple theoretical model based on virtual effects due to some heavy fermions is used for acquiring some guidance on the plausible forms of these NP vertices. In such a case specific relations among the various neutral couplings are predicted, which can be experimentally tested and possibly used to constrain the form of the responsible NP structure.

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 one, we find that only CP conserving neutral gauge couplings are generated, either from the standard quarks and leptons, or from possible New Physics (NP) fermions. Bosonic one-loop diagrams never contribute to these couplings, while the aforementioned fermionic contributions satisfyWe also study examples of two-loop NP effects that could generate non vanishing h γ,Z 4 couplings. We compare quantitative estimates from SM, MSSM and some specific examples of NP contributions, and we discuss their observability at future colliders.

In order to explain possible departures from the standard model ͑SM͒ predictions for bb and cc production at the Z peak, we propose the existence of a ZЈ vector boson with enhanced couplings to quarks. We first show that this proposal is perfectly consistent with the full set of LEP 1 and SLC results. In particular, Z-ZЈ mixing effects naturally explain the fact that ⌫ b and ⌫ c deviate from the SM in opposite directions. We then show that there is a predicted range for enhanced ZЈqq couplings which explains, for a precise and interesting range of ZЈ masses, the excess of dijet events seen at CDF. A ZЈ with such couplings and mass would produce clean observable effects in bb and in total hadronic production at LEP 2.