In 1935 Yukawa proposed in his paper that there exist a meson field, that is, a field that is different from the electromagnetic field, whose quanta, the mesons have a finite mass, which is coupled to protons and neutrons in a manner analogous to the coupling of photons to charged particles. The exchange of these quanta will then give rise to an interaction between nucleons. In this way, interactions as different as electromagnetism and the nuclear forces would at least share a common universal mechanism.
The mesons predicted by Yukawa were in the end found in 1947. The long range part of the nucleon-nucleon force is due to pi-mesons, whose mass was found to be 140 MeV. They were found to be pseudoscalar, and like photons, they can be emitted in collisions or transitions.
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