Charmonium Production in Photon-Photon Collisions
We have searched for the two-photon production of the ${\ensuremath{\eta}}_{c}$, ${\ensuremath{\chi}}_{0}$, and ${\ensuremath{\chi}}_{2}$ charmonium states at the SLAC ${e}^{+}{e}^{\ensuremath{-}}$ collider PEP in the channels $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{K}^{\ifmmode\pm\else\textpm\fi{}}{K}_{S}^{0}{\ensuremath{\pi}}^{\ensuremath{\mp}}$, $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{K}^{+}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, and $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{K}^{+}{K}^{\ensuremath{-}}{K}^{+}{K}^{\ensuremath{-}}$. We identify four ${\ensuremath{\eta}}_{c}$ candidates in the ${K}^{+}{K}^{\ensuremath{-}}{K}^{+}{K}^{\ensuremath{-}}$ channel on a negligible background; the one $\ensuremath{\varphi}\ensuremath{\varphi}$ event among them implies a 95%-confidence-level (C.L.) lower limit for ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}({\ensuremath{\eta}}_{c})$ of 1.7 keV. In the other channels we find no evidence for any of the three states. We establish 95%-C.L. upper limits ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}({\ensuremath{\eta}}_{c})<15.5$ keV, ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}({\ensuremath{\chi}}_{0})<17.0$ keV, and ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}({\ensuremath{\chi}}_{2})<4.2$ keV. From all channels combined, we obtain the value ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}({\ensuremath{\eta}}_{c})={6.4}_{\ensuremath{-}3.4}^{+5.0}$ keV.