The $\gamma \gamma \to Z Z$ process and the search for virtual SUSY effects at a $\gamma \gamma$ Collider

Abstract.

We study the helicity amplitudes of the process $\gamma \gamma \to Z Z$ in the Standard Model at high energy. These amplitudes receive contributions from the W and charged quark and lepton loops, analogous to those encountered in the $\gamma \gamma \to \gamma\gamma, \gamma Z$ cases studied before. But $\gamma \gamma \to ZZ$ also receives contributions from the Higgs s-channel poles involving the effective Higgs-$\gamma \gamma$ vertex. At energies $\gsim 300 GeV$, the amplitudes in all three processes are mainly helicity-conserving and almost purely imaginary; which renders them a very useful tool in searching for New Physics. As an example, a SUSY case is studied, and the signatures due to the virtual effects induced by a chargino-, charged slepton- or a lightest stop-loop in $\gamma \gamma \to ZZ$, are explored. These signatures, combined with the analogous ones in $\gamma \gamma \to \gamma \gamma$ and $\gamma \gamma \to \gamma Z$, should help identifying the nature of possible New Physics particles.