Observation of Nonlinear Effects in Compton Scattering

Nonlinear Compton scattering has been observed in the collision of a low-emittance 46.6-GeV electron beam with terawatt pulses from a Nd:glass laser at 1054 and 527 nm wavelengths in an experiment at the Final Focus Test Beam at SLAC. Peak laser intensities of 10¹⁸ W/cm² have been achieved, corresponding to a value of 0.6 for the parameter η = eE_rms/mω₀c. Results are presented for multiphoton Compton scattering in which up to four laser photons interact with an electron, in agreement with theoretical calculations.