How did cosmic-ray research help discover the positron and the muon?

Between the 1930s and 1950s, cosmic rays served as the primary source of high-energy particles for scientists, as they were the only available means to study subatomic interactions at such scales. The discovery of the positron and the muon was instrumental in establishing the field of particle physics.1

The Positron

In 1932, physicist Carl David Anderson utilized a cloud chamber—an invention by C.T.R. Wilson that tracks ionizing radiation—to study cosmic-ray composition. By analyzing cloud-chamber photographs, Anderson identified particle tracks with an orientation indicating a positive charge but a mass too small to be a proton. He concluded these were positrons, the first discovered antiparticle, confirming theoretical predictions made by P.A.M. Dirac.234

The Muon

In 1936, Anderson and Seth Neddermeyer discovered the muon while investigating cosmic-ray "showers". Initially, researchers believed this particle was the one predicted by Yukawa Hideki to mediate the strong force within the atomic nucleus. Subsequent research revealed that the muon is actually a lepton that does not participate in strong interactions, distinguishing it from the pion discovered later.52