Lorentz contraction of an electric field around relativistic electron beams

Spatio-temporal terahertz electric field profiles around 35 MeV electron bunches propagating in the air are measured by the echelon-based single-shot electro-optic measurement with a sub-picosecond temporal resolution. We demonstrates that the Lorentz contraction of the electric field in the direction perpendicular to the pass of the electron beams is directly observed.