isospinviolating

Isospinviolating refers to processes or effects that do not conserve isospin, the approximate SU(2) symmetry that treats up and down quarks as alike under strong interactions. In the real world, this symmetry is broken by two main sources: the difference between the up and down quark masses and electromagnetic interactions, which couple differently to these quarks. As a result, strong interaction amplitudes are modified and certain transitions that would be forbidden under exact isospin symmetry can occur, albeit with small probabilities.

Two broad manifestations are commonly discussed. First, mass splittings within isospin multiplets, such as the neutron–proton mass difference, arise from both quark mass differences and electromagnetic effects. Second, isospinviolating decays and mixings occur, for example eta → 3 pions (η → π+ π− π0), which would be forbidden by isospin conservation in the limit of equal up and down quark masses, but proceed due to the u–d mass difference. Electromagnetic effects also produce phenomena like rho–omega mixing, where the vector mesons with different isospin content mix through photon interactions.

The study of isospinviolating processes provides insight into the light quark mass ratio m_u/m_d and the role of electromagnetism in hadronic physics. They are analyzed with theoretical tools such as chiral perturbation theory and lattice QCD that include QED, and are constrained by precision measurements of hadron masses and decay rates. While typically small compared with isospin-conserving strong processes, isospinviolating effects are important for a complete understanding of strong and electromagnetic interactions.