Charge symmetry and the nucleon-nucleon interaction

Shortly after the discovery of the neutron by Chadwick in 1932 the great similarity between the neutron and proton compelled Heisenberg to introduce the concept of isospin invariance. According to this, the neutron and the proton are considered as two degenerate states of the nucleon. It is one of the first symmetries postulated in elementary particle physics. The study of symmetries is a powerful tool for bringing order into the complex phenomena of subatomic physics. The extent to which they are broken brings new insights into fundamental processes. Isospin invariance implies the full equivalence of neutrons and protons. A weaker symmetry is charge symmetry, which is a mirror symmetry where invariance is expected under the change of all neutrons into protons and all protons into neutrons. This article reports a recent high-precision measurement at TRIUMF of the extent of charge symmetry violation in the simplest nuclear system: the two-nucleon system, specifically the neutron-proton system. The mirror of charge symmetry interchanges the neutron and proton, but if one of the particles is labelled by being in a state of definite (ordinary) spin, the mirror inversion leads to a comparison of two different experimental situations, namely, scattering of spin-polarized neutrons from unpolarized protons compared to scattering of unpolarized neutrons from polarized protons. Charge symmetry is preserved by the strong interaction but broken weakly by the electromagnetic interaction. This experiment yields a value for the extent of this breaking that is consistent with theoretical calculations based on meson exchange. (6 refs., 5 figs., tab.)