An Accurate Measurement of the Landau-Pomeranchuk-Migdal Effect

In the 1950s, Landau, Pomeranchuk, and Migdal (LPM) predicted that the cross section for bremsstrahlung from highly relativistic particles in dense media would be suppressed due to interference between amplitudes of nearby interactions. We have measured the production rates of 5 to 500 MeV photons from 8 and 25 GeV electrons traversing thin gold and carbon targets. We observe, to within 5% accuracy, suppression at the level predicted by Migdal. For extremely thin targets, we observe ``edge effects'' where the LPM suppression is reduced near the edges of the targets. We also point out the imprecise foundation of the LPM theory.