In 1937 Neddermeyer and Anderson discovered the "mesotron" in cosmic rays by using a cloud chamber with a magnetic field and a 1cm platinum plate inside, and measuring the curvature of the tracks on both sides of the plate.
They measured dE/dx vs E, assuming E=p.
They observed two types of particles:"shower" and "penetrating" particles.
The "shower" particles were electrons and positrons. The "penetrating" particles were heavier than electron, but lighter than protons (since protons would ionize more, due to smaller β).
Later in the same year, Street and Stevenson extracted the mass of the new particle, by a simultaneous measurement of momentum and ionization.
When stopped in matter, positive and negative particles should behave differently.
- positive: should decay
- negative: should be captured into atomic-like orbits, but with very small radii (overlapping the nucleus).
This makes a difference for pions and muons: pions would interact with the nucleus and would be absorbed before decay.
Conversi, Pancini and Piccioni used different materials. They found that positive particles always decayed when stopped in matter, while negative particles where absorbed by the nuclei in iron but decayed in carbon.
In 1947 Perkins observed in photographic emulsion an event consistent with the Yukawa picture: a slow negative particle stopped in the emulsion, was absorbed by the nucleus, and the nucleus was blasted apart and three fragments were observed in the emulsion.
The incoming track was identified by the fact that grain density and scattering increased as the track reached the vertex (dE/dx increases with traversed path).
Its ionization was too big and the scattering too small to be an electron, and the scattering too big to be a proton. So it was intermediate in mass.
The mass was extracted from the range energy-curve, calculated for several intermediate mass hypotheses by scaling the curve for protons.
Lattes, Occhialini and Powell found the connection between the Conversi, Pancini and Piccioni result and the Perkins result, by observing the decay pion->muon.
The observed decay product appeared to have fixed range in the emulsion.
This indicates that it was always produced with the same energy and the decay
was into two particles, one of which is invisible (Pauli's neutrino). The pion
(seen by Perkins) decayed into muon and neutrino.
Explaination of the discrepancy iron/carbon in Conversi, Pancini and Piccioni result:
Also negative muons (not only pions) can interact with the nucleus (proton is transformed into neutron and a neutrino is emitted) like electrons but the probability of absorption depends on the Z of the nucleus. For high-Z nuclei the muon orbit overlaps the nucleus and the muon is absorbed.