AN ACCURATE MEASUREMENT OF THE LANDAU-POMERANCHUK-MIGDAL EFFECT

In the 1950s, Landau, Pomeranchuk, acid Migdal (LPM) predicted that th e cross section for bremsstrahlung from highly relativistic particles in dense media would be suppressed due to interference between amplitu des of nearby interactions. We have measured the production rates of 5 to 500 MeV photons from 8 and 25 GeV electrons traversing thin gold a nd carbon targets. We observe, to within 5% accuracy, suppression at t he 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 th eory.