Study of nonlinear QED effects in interactions of terawatt laser with high energy electron beam

Nonlinear Compton scattering and electron-positron pair production have been observed in collisions of low-emittance 46.6 GeV electron beam with terawatt laser pulses from an Nd:glass laser at the experiment E144 at the Final Focus Test Beam at SLAG. In nonlinear Compton scattering up to four laser photons interact with an electron. The positrons are interpreted as arising from a two step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the first laboratory evidence for inelastic light-by-light scattering involving only real photons. Results are in agreement with theoretical calculations of strong-field QFD. This work was supported in part by DOF grant DF-FG05-91FR40627.