Mesons

np30/Meson1.htm, S.A.Moszkowski 2-18-99

IV. MESONS

Our ideas about the origin of nuclear forces have undergone an interesting evolution.
During the 1920's, the interactions in atomic systems became understood on the basis of the newly developed quantum mechanics. Even before then, it was known that in the hydrogen atom, we have a simple Coulomb interaction, except for small corrections.
In complex atoms and molecules, we have to take into account the symmetry properties of the wavefunction. This leads to exchange forces, and a microscopic understanding of the chemical bond. Such bonds lead to saturation, only a certain maximum number of particles can interact with each other, so that the binding energy is proportional to the number of particles.
When Heisenberg started to study the mechanism of the nuclear force, he used the chemical bond as an analogy. Even after it was determined that the neutron is an elementary particle, rather an a proton-electron system) , he thought of the neutron-proton interaction as due to exchange of an electron. This electron would, of course, have to have Bose statistics. Heisenberg's ideas helped to pave the way for Yukawa's proposal of meson exchange. With the development of Quantum electrodynamics, it had been established that the Coulomb interaction in atoms is due to the exchange of massless photons. Yukawa proposed that nuclear forces are due to the exchange of massive photons. He postulated that, since the nuclear forces were already known to have a range r_0 of about 1 fm (10^-15m), the mass of the meson would have to be of order hbar/(r_0*c), which is about 100 MeV. Indeed, soon afterward Yukawa's proposal, a new particle of the right mass was discovered in Cosmic rays. This is the particle which became known as the muon. However, further studies showed that this particle interacted far too weakly with nuclei to be considered as a possible carrier of the nuclear force. From 1936 on this was a major paradox, which was resolved only in 1948 with the discovery of the pion, another meson of about the right mass but with strong interactions with nucleons.
So the pion could be considered as a carrier of the nuclear force. This was the end of the story at the time, but there have been major new developments since then.
See hyperlink to modern times: Quark Chemistry