Chiral Symmetry Breaking and Pion Decay in a Magnetic Field

The pattern of chiral symmetry breaking is exploited to compute vector and axial-vector pion matrix elements in a uniform magnetic field. Our results are model independent, and thereby constitute low-energy theorems that must be obeyed by QCD in external magnetic fields. Chiral perturbation theory and lattice QCD results are compared, for which there is some tension. As an application, the matrix elements are utilized to compute pion decay rates in a magnetic field.