Precision Tests in e+e- and gamma-gamma processes and Virtual New Physics Effects

Contribution to LP99, International Symposium on Lepton-Photon Interactions, Stanford, Aug.1999. Summary of works done in collaboration with M. Beccaria, P. Ciafaloni, D. Comelli, G.J. Gounaris, J. Layssac, P. Porfyriadis, S. Spagnolo, C. Verzegnassi. We report on several works dealing with precison tests of the Standard Model (SM) and beyond, in the processes $e^+e^-\to f\bar f$ (f= lepton or quarks) and $\gamma\gamma \to \gamma\gamma, \gamma Z$. We first point out a set of remarkable properties of the SM amplitudes and observables at high energies, at tree level and at 1-loop, like "Sudakov" behaviour or the dominance of purely imaginary non-flip amplitudes due to the WWWW box. We then consider various types of virtual NP contributions due to supersymmetry, anomalous gauge boson couplings, technicolour and extended gauge structures. Effects at LEP2 and LC (in particular with a laser backscattering $\gamma\gamma$ mode) are discussed. We point out specific features like clear threshold enhancements.