Coulomb distortion measurements by comparing electron and positron quasielastic scattering off 12 C and 208 Pb

Positron and electron quasielastic cross sections on ${}^{12}\mathrm{C}$ and ${}^{208}\mathrm{Pb}$ were measured at the Saclay linear accelerator (ALS) to test Coulomb corrections to the plane-wave Born approximation. The positron and electron response functions at the same effective kinematics were found to be equal within a 3% experimental uncertainty, which allows us to discriminate between different theoretical models of Coulomb distortions. Our result strongly supports the effective momentum approximation, even for nuclei as heavy as ${}^{208}\mathrm{Pb}$.