A Study of High Field Quantum Electrodynamics in the Collision of High Energy Electrons with a Terawatt Laser

An experiment is described which studied quantum electrodynamic interactions under conditions of extremely high elds, along with a review of the relevant theory. The high elds were created by an intense, tightly-focused pulse of laser light at green or infrared wavelengths, into which was sent an ultra-relativistic electron beam of 46.6-GeV energy. The relevant theory is that of an electron in an electromagnetic wave so intense that the electron's mass is e ectively shifted by the transverse momentum imparted to it by the wave, and the electron encounters eld strengths comparable to the Schwinger critical eld strength of 511 kV per Compton wavelength. An electron in the intense wave may radiate a photon and balance 4-momentum by absorbing multiple photons from the laser, which can lead to real photons with energies above the kinematic limit for conventional Compton scattering. All particles have signi cant probability of scattering multiple times while in the focus of the laser, including the photons radiated by the electrons, which may convert into electron-positron pairs, again with absorption of multiple photons from the laser. This experiment was able to measure the rates and spectra of positrons, electrons, and photons emerging from the interaction region. Results from both experiment and theoretical simulations are presented and compared. The results from the electron and positron measurements are compatible with the accepted theory, within experimental uncertainties due mainly to the laser intensity measurement. The photon spectrum shows the correct shape, but the ratio of rates in the linear and two-absorbedphoton portions of the spectrum does not vary as expected with the laser intensity, suggesting a disagreement with the accepted theory, with a signi cance of roughly two standard deviations. A follow-up experiment would be in order.