Nuclear force

In physics, the nuclear force is the force holding assemblies of protons and neutrons together.

History
In 1935 Yukawa invented the meson theory for explaining the forces holding an assemblage of neutrons and protons together. The mesons were postulated to exist in three forms: &pi;+, &pi;0, and &pi;–, all of spin zero, today called pions. The behavior of nuclear forces was explained as an exchange of pions. An example is shown in the figure, where the neutron (lower right) emits a pion of negative electric charge to become a positive proton (upper right), while the proton (lower left) absorbs the pion to become a neutron (upper left).

Incidental experimental observation of the muon or &mu;-meson in 1936 appeared to provide the &pi;– meson needed by the theory, but soon it was found that the muon did not interact with the nucleus and had spin 1/2 (it is a lepton). Later, in 1947, the actual &pi;-meson or pion was discovered, and the pion did interact with the nucleus. Today all three pions have been observed, &pi;+,&pi;0, and &pi;–, the last being the neutral pion found in 1950.

Modern formulation
Today a more refined theory of nuclear interactions is based upon the Standard Model. In that model, pions are considered to be quark-antiquark pairs. The &pi;– is a d-quark combined with a ū-antiquark, and a &pi;+ is the other way around, making each the antiparticle of the other. The &pi;0 is a u-quark combined with a ū-antiquark, and therefore its own antiparticle. Because the pions are colorless (the anti-quark in a pion always has the anti-color to the quark, regardless of its flavor), their exchange between nucleons is not a simple color exchange, but a residual color force sometimes referred to as a color van der Waals force, an analogy to the weak electromagnetic attraction between electric charge-neutral atomic complexes. In any event, nuclear forces are not considered fundamental today, but are a consequence of the underlying strong forces between quarks, also called chromodynamic forces or color forces. On that basis, nuclear forces are fundamentally based upon color, and only approximated by the Yukawa theory.