By adding electromagnetic effects to quantum chromodynamics calculations, theorists have achieved a leap in accuracy.
The mass difference between the neutron and proton—about 0.14%—is known experimentally with an impressive precision of 4 parts in 10 million. But calculating that difference from scratch via quantum chromodynamics (QCD), the theory of the strong force, is another matter altogether.
The simplest description of neutrons and protons posits them as bound states of three “valence” quarks (up, up, down for protons and up, down, down for neutrons). Analogous to the way photons mediate the electromagnetic force between charged electrons, gluons mediate the strong force between quarks, which carry color charge. (See the article by Frank Wilczek, Physics Today, August 2000, page 22.) But unlike neutral photons, gluons also carry color charge and therefore interact with each other. One consequence is that perturbation theory, so successful for quantum electrodynamics (QED), fails spectacularly for QCD at the GeV energy characteristic of neutrons and protons.
To read more, click here.