Experimental examination of the Landau-Pomeranchuk-Migdal effect by high-energy electromagnetic cascade showers in lead

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Abstract
To examine the suppression of the bremsstrahlung and pair-creation cross sections predicted by Landau and Pomeranchuk, and calculated quantum mechanically by Migdal, the behavior of cascade showers, especially the width of the transition curve [full width at half maximum (FWHM)] in the 100-TeV region observed in lead emulsion chambers, is compared with results of a Monte Carlo calculation. Nineteen events are examined and it is found that five are unambiguously initiated by hadrons or several γ rays (electrons) of comparable energies. Ten showers among the rest have FWHM larger by two standard deviations than an estimation without Migdal’s correction, if we assume that each of those events is induced by (essentially) a single γ ray or electron. On the other hand, Migdal’s cross sections can give a reasonable account for the observed cascade characteristics even in such cases. Without the Landau-Pomeranchuk-Migdal effect, the only possible way to interpret such characteristics is to assume that the showers we observe are mostly hadron-induced ones. Such a possibility is estimated to be less than 1%. A quantitative experimental test of the effect using accelerator electrons of 600 GeV is considered.