An Experimental Review of Charmonium
  This review briefly outlines recent experimental results from the charmonium system. These include new measurements of the M1 radiative transition rates $B(\psi(1S,2S)\to\gamma\eta_c(1S))$; new insights into the mass of the $\eta_c(1S)$; the observation of $J/\psi(1S)$ decays to three photons; new measurements of the two-photon widths of the $\chi_{cJ}(1P)$; the observation of radiative decays of the $\chi_{cj}(1P)$ to light quarks; a precise measurement of the $h_c(1P)$ mass; and an update on properties of the $\eta_c(2S)$. Each result adds piecemeal to our understanding of the strong force. 
R. E. Mitchell