Observation of a relativistic analog of the Brewster effect in electron-photon scattering.

The scattering of light by relativistic charged particles is a fundamental process in electrodynamics, underpinning phenomena ranging from cosmic ray interactions to laboratory X-ray generation. While the spectral and temporal properties of this process are well understood, the polarization-dependent response as a function of incidence angle has remained largely unexplored. Here we report the experimental observation of a relativistic analog to the Brewster effect in the interaction between a high-brightness relativistic electron beam and an optical laser pulse at shallow incidence. For incident radiation polarized in the plane of scattering, Lorentz-transformations predict a distinct interaction angle at which photon emission is suppressed in the forward direction, in close analogy with Brewster reflection at dielectric interfaces. These results connect a central concept of classical optics to the relativistic electron-photon scattering and highlight polarization as a controllable degree of freedom in light-matter interactions involving high-energy particle beams.