Measurements of the Mass, Total Width, and Two-Photon Partial Width of the ${\eta }_c$ Meson

Using $13.4{\mathrm{fb}}^{-1\mathrm{}}$ of data collected with the CLEO detector at the Cornell Electron Storage Ring, we have observed 300 events for the two-photon production of ground-state pseudoscalar charmonium in the decay ${\eta }_c\to K_S^0{K}^{\mp }{\pi }^{\pm }$. We have measured the ${\eta }_c$ mass to be $[2980.4\pm 2.3(\mathrm{stat})\pm 0.6(\mathrm{syst}\left)\right]\mathrm{MeV}$ and its full width as $[27.0\pm 5.8(\mathrm{stat})\pm 1.4(\mathrm{syst}\left)\right]\mathrm{MeV}$. We have determined the two-photon partial width of the ${\eta }_c$ meson to be $[7.6\pm 0.8(\mathrm{stat})\pm 0.4(\mathrm{syst})\pm 2.3(\mathrm{br}\left)\right]\mathrm{keV}$, with the last uncertainty associated with the decay branching fraction.