Neutrons and protons form the stable baryons – these are the most direct manifestations of the multiple facets of the Standard Model, with important strong, electromagnetic and weak properties. Understanding baryons then goes a long way towards understanding how the Standard Model leads to the properties of our world.
The treatment of baryons does not work as well in chiral perturbation theory as does that of mesons. I find this difference interesting and it has taught me more about effective field theory. One aspect is that chiral SU(3) has a terrible time giving an expansion in the kaon mass, when treated with dimensional regularization, while we know that SU(3) is very lightly broken in practice. It turns out that this is because the chiral loops include at much shorter distances that those where the chiral degrees of freedom are valid, leading to large spurious contributions. Barry and I proposed the idea of
“long distance regularization” to include only those parts of loop integrals where the chiral couplings are valid. This has to lead to a modified form of power counting, but certainly leads to much better phenomenology.
Due to interest in potential violations of the equivalence principle (a connection that I am still working on) I got interested in the physics of nuclear binding, in particular the question of how it depends on quark masses. Originally, I expected that I would need to do a somewhat “artistic” treatment of this difficult subject. However, due to the advances of others using effective field theory methods, it turned out to be possible to do a pretty reasonable scientific treatment. Along the way, I learned a lot about the nature of the main component of the nuclear force, so-called sigma exchange. I then used this in exploring the chiral limit of QCD and the case of heavier masses, where nuclear binding disappears remarkably quickly.
These explorations were quite enjoyable. Because the methods used are more familiar in particle physics than in nuclear physics, they are somewhat outside of the mainstream of nuclear physics. I hope to return to this topic to extend the treatment into one that can be more readily applied by nuclear theorists.