Light by light scattering at high energy: a tool to reveal new particles

We point out a few remarkable properties of the gamma gamma --> gamma gamma process at high energy, which should allow to search for effects of new particles and interactions. We give illustrations with threshold effects due to pairs of new charged particles (charginos, charged Higgs particles, sfermions), resonance effects due to s-channel production of neutral scalars (standard or supersymmetric neutral Higgs particles or technipions) and unitarity saturating amplitudes due to a strongly interacting sector. The use of polarized photon beams is also briefly discussed. (C) 1999 Elsevier Science B.V. All rights reserved.