Observation of electromagnetic interactions of high energy muons deep underground

The first experimental measurement of the spectrum of cascade showers produced by cosmic ray muon interactions deep underground in the transferred energy range 0.4–200 GeV has been performed by means of the NUSEX calorimeter at a depth of about 5000 hg/${\mathrm{cm}}^2$, the average muon energy being close to 360 GeV. The spectrum of energy transfers is in good agreement with calculations based on the commonly used theoretical formulas for muon interaction cross sections and on a conservative picture of the formation of muon energy spectrum at great depths. The present data do not confirm the deficit of the cross section at low energy transfers for the 1 TeV muon energy recently reported by the CCFR Collaboration. A possible explanation of the difference between the observed and the expected stochastic energy loss distributions presented by the CCFR group is discussed.