Studies of nonlinear QED in collisions of 46.6 GeV electrons with intense laser pulses

We report on measurements of quantum electrodynamic processes in an intense electromagnetic wave, where nonlinear effects (both multiphoton and vacuum polarization) are prominent. Nonlinear Compton scattering and electron-positron pair production have been observed in collisions of 46.6 GeV and 49.1 GeV electrons of the Final Focus Test Beam at SLAC with terawatt pulses of 1053 nm and 527 nm wavelengths from a Nd:glass laser. Peak laser intensities of ~0.5×10¹⁸ W/cm² have been achieved, corresponding to a value of ~0.4 for the parameter η=eE_rms/mω₀c and to a value of ~0.25 for the parameter Υ_e=E^*_rms/E_crit=eE^*_rmsħ/m²c³, where E^*_rms is the rms electric field strength of the laser in the electron rest frame. We present data on the scattered electron spectra arising from nonlinear Compton scattering with up to four photons absorbed from the field. A convolved spectrum of the forward high energy photons is also given. The observed positron production rate depends on the fifth power of the laser intensity, as expected for a process where five photons are absorbed from the field. The positrons are interpreted as arising from the collision of a high-energy Compton scattered photon with the laser beam. The results are found to be in agreement with theoretical predictions.