chargesymmetry

Charge symmetry is a concept in quantum chromodynamics and nuclear physics referring to the invariance of the strong interaction under interchange of protons and neutrons (equivalently, up and down quarks). It is a particular aspect of isospin symmetry: a 180-degree rotation in isospin space that would make protons and neutrons interchangeable if the electromagnetic interaction and the small difference between up and down quark masses were neglected. In practice, charge symmetry is an approximate symmetry of the strong force.

Origin and scope: The strong interaction is nearly insensitive to electric charge, which leads to near equality between neutron and proton up to small corrections. However, charge symmetry is broken by two main sources: the mass difference between up and down quarks (m_u ≠ m_d) and electromagnetic effects, which couple differently to protons and neutrons. This charge-symmetry breaking (CSB) can be studied by comparing proton-proton, neutron-neutron, and neutron-proton interactions and by examining small differences in nuclear binding energies and reaction observables.

Implications and evidence: Under perfect charge symmetry, pp, nn, and np interactions would be identical after removing Coulomb effects. In reality, CSB is observed at the percent level in certain observables, such as differences in scattering lengths and in the binding-energy differences of mirror nuclei (the Nolen-Schiffer phenomenon). Experiments probing CSB include measurements of asymmetries in pion production and analyzing powers in nucleon scattering. Studying charge symmetry helps isolate contributions from strong, electromagnetic, and quark-mass effects in hadronic systems.