Lattice QCD evaluation of baryon magnetic-moment sum rules

Magnetic-moment combinations and sum rules are evaluated using recent results for the magnetic moments of octet baryons determined in a numerical simulation of quenched QCD. The model-independent results of the lattice calculations remove some of the confusion and contradiction surrounding past magnetic-moment sum-rule analyses. The lattice results reveal the underlying quark dynamics investigated by magnetic-moment sum rules and indicate the origin of magnetic-moment quenching for the nonstrange quarks in $\Sigma$. In contrast with previous sum-rule analyses, the lattice results indicate the magnetic moments of nonstrange quarks in $\Xi$ are more likely enhanced than quenched relative to that in the nucleon. In most cases, the spin-dependent dynamics and center-of-mass effects giving rise to baryon dependence of the quark moments are seen to be sufficient to violate the sum rules in agreement with experimental measurements. In turn, the sum rules are used to further examine the results of the lattice simulation. The Sachs sum rule suggests that quark loop contributions, not included in present lattice calculations, may play a key role in removing the discrepancies between lattice and experimental ratios of magnetic moments. This is supported by other sum rules sensitive to quark loop contributions. A measure of the isospin symmetry breaking in the effective quark moments due to quark loop contributions is in agreement with model expectations.