Nuclear Interaction ofμ-Mesons Underground

The nuclear interactions of $\mu$-mesons and the genetic relations among their secondary particles are analyzed on the basis of the data obtained with photographic plates exposed underground by George and Evans. The results are used to analyze the experiment performed by Cocconi and Tongiorgi, in which the neutron showers produced in an absorber at different depths of water were studied. The contributions to the neutron showers from various components are compared. It is shown that the $\mu$-mesons directly producing neutron showers in the absorber and the nucleons produced by the $\mu$-mesons in the water above the apparatus and interacting in the absorber give the main contribution to the large neutron showers observed. The multiplicity spectrum of the neutrons is calculated by making use of a simplified model of the nucleon cascade and is shown to depend upon the selection bias of the detecting apparatus. The experimental results are consistently accounted for by assuming for the $\mu$-mesons a nuclear interaction cross section of ∼10 μb per nucleon. The experiments at great depths suggest the predominance of small momentum transfers by nuclear collisions. The large cross section and the small momentum transfer can be described in terms of the virtual photons associated with $\mu$-mesons. These virtual photons produce in nuclear collisions real or virtual $\pi$-mesons which can either escape or be absorbed in the nucleus.