Isobaric analog state

Nuclear quantum states in different nuclear isobars belonging to the same isospin multiplet, with equal total isospin and approximately the same spatial and spin structure. An isospin raising or lowering operator connects the states in the limit of exact isospin symmetry. Nuclear Coulomb energy and other isospin-breaking effects shift their energies and modify this ideal relationship.

Compare an unknown allowed branch of parent $P$ with a reference allowed decay of mean lifetime ${\tau }_R$ and unit branching fraction. Its branching fraction is the parent mean lifetime ${\tau }_P$ times the inferred partial rate. The phase-space ratio $f_X/f_R$ and the total vector-plus-axial strengths $S_X/S_R$ enter separately. Approximating both strengths by their Fermi transition contributions and both phase-space integrals by $Q^5$ gives a useful scale, but does not establish a unique branch fraction if the axial strengths are unknown.