Exotic and excited-state radiative transitions in charmonium from lattice QCD

We compute, for the first time using lattice QCD methods, radiative transition rates involving excited charmonium states, states of high spin and exotics. Utilizing a large basis of interpolating fields we are able to project out various excited state contributions to three-point correlators computed on quenched anisotropic lattices. In the first lattice QCD calculation of the exotic $$1^{-+}$$ $$\eta_{c1}$$ radiative decay, we find a large partial width $$\Gamma(\eta_{c1} \to J/\psi \gamma) \sim 100 \,\mathrm{keV}$$. We find clear signals for electric dipole and magnetic quadrupole transition form factors in $$\chi_{c2} \to J/\psi \gamma$$, calculated for the first time in this framework, and study transitions involving excited $$\psi$$ and $$\chi_{c1,2}$$ states. We calculate hindered magnetic dipole transition widths without the sensitivity to assumptions made in model studies and find statistically significant signals, including a non-exotic vector hybrid candidate $Y_{\mathrm{hyb?}} \to \et