Shedding light on charmonium

We investigate E1 radiative transitions within charmonium in a relativistic approach based on light-front QCD. In quantum field theory, two sets of processes are pure E1: ${\chi }_{c0}\to J\mathrm{/}\psi \gamma$ ($\psi \to {\chi }_{c0}\gamma$) and $h_c\to {\eta }_c\gamma$ (${\eta }_c^{\mathrm{\prime }}\to h_c\gamma$), both involving the $P$-wave charmonia. We compute the E1 radiative decay widths as well as the corresponding transition form factors of various processes including those involving $2P$ states. These observables provide an access to the microscopic structures of the $P$-wave charmonium. We show that our parameter-free predictions are in excellent agreement with the experimental measurements as well as lattice simulations whenever available.