Yukawa meson

In 1935, physicist Hideki Yukawa proposed a theory for the strong and weak nuclear forces, predicting a new particle that would act as a carrier for these forces. He called this particle the U-quantum, later named the meson due to its mass being between that of an electron and a proton.1

Yukawa calculated that this particle would have a mass approximately 200 times that of an electron. The discovery of a particle with a similar mass in cosmic rays in 1937 seemed to confirm his theory. However, it was later found that this particle, the muon, was not the carrier of the nuclear forces. The actual carrier, the pion, was discovered in 1947.1

While the pion is now understood not to be the direct carrier of nuclear forces, Yukawa's work laid the foundation for meson theory, which became a significant part of nuclear and high-energy physics.1