Horizontal air showers induced by muons with energy above 1013 eV

Abstract

We have been observing horizontal air showers to study the high-energy interactions of muons. Nine horizontal air showers of size greater than 104 and zenith angle above 70° were observed. Two of these showers showed evidence that they are not pure electromagnetic showers but nucleonic showers. One shower contained a nuclear-active particle which produced a nuclear interaction in the cloud chamber, and the other shower contained a muon which penetrated about 103 g cm−2 of iron absorber. The size distribution of the horizontal showers was compared with the calculated one assuming that the nuclear interaction of high-energy muons was negligible. The discrepancy between the experimental distribution and the calculated one is reduced if the nuclear interaction of high-energy muons is taken into account, which supports our view that the horizontal air showers (at least some fraction of them) are produced by nuclear interactions of high-energy muons.