Yukawa hypothesis

The Yukawa hypothesis was a hypothesis proposed by Japanese physicist Hideki Yukawa that postulated the existence of a force carrier of the strong interaction, analogous to what the photon is for the electromagnetic interaction.

For example, the elastic scattering of a proton and a neutron, $n + p \rightarrow n + p$, would be mediated by this particle.

The yet-to-be-discovered particle was called a pion (symbol: $\pi$), thought to be a meson with a mass of approximately 140 MeV. Specifically, he postulated three types: $\pi^{+}$, $\pi^{-}$ and $\pi^{0}$, with different electric charge to account for different interactions. The existence of the pion would be later confirmed experimentally, winning Yukawa Nobel Prize for physics.

What he didn't expect was that the force carrier of strong force would actually be something else. It would later be discovered that the actual gauge boson of the strong force is a different particle known as the gluon.