Radiative transitions in charmonium from lattice QCD

Radiative transitions between charmonium states offer an insight into the internal structure of heavy-quark bound states within QCD. We compute, for the first time within lattice QCD, the transition form factors of various multipolarities between the lightest few charmonium states. In addition, we compute the experimentally unobservable, but physically interesting vector form factors of the eta(c), J/psi, and chi(c0). To this end we apply an ambitious combination of lattice techniques, computing three-point functions with heavy domain-wall fermions on an anisotropic lattice within the quenched approximation. With an anisotropy xi=3 at a(s)similar to 0.1 fm we find a reasonable gross spectrum and a hyperfine splitting similar to 90 MeV, which compares favorably with other improved actions. In general, after extrapolation of lattice data at nonzero Q(2) to the photopoint, our results agree within errors with all well-measured experimental values. Furthermore, results are compared with the expectations of simple quark models where we find that many features are in agreement; beyond this we propose the possibility of constraining such models using our extracted values of physically unobservable quantities such as the J/psi quadrupole moment. We conclude that our methods are successful and propose to apply them to the problem of radiative transitions involving hybrid mesons, with the eventual goal of predicting hybrid meson photoproduction rates at the GlueX experiment.