Scattering of free electrons by free electrons

The scattering of single free electrons by free electrons has been measured directly. The methods of measurement included changing the incident relative velocities of the electrons by changing either the angle between the beams or the beam energies and then identifying uniquely the outgoing electron pairs with energy and coincidence time detection. The measured energy and angular differential scattering cross sections over the relative interaction velocity range from 0.3×10⁷ to 3.0×10⁷ m/s for forward and backward scattering angles which were calibrated absolutely to within ±{}2% and with a statistical uncertainty within ±{}2% at an angle of 30^∘{} between the beams and an observational angle of 60^∘{}. Subsequent relative data showed a dependence on (relative velocity)^-4 and cos²(beam intersection angle) scattering parameters as expected for an exact quantum mechanical description for pure Coulomb field M{}ller scattering. While the precision and reproducibility of all data are at the percent level, four of the 48 data points vary significantly from the expected quantum values and are not included in our overall experimental uncertainty.