Observation of ${\eta }_c^{\prime }$ Production in $\gamma \gamma$ Fusion at CLEO

We report on the observation of the ${\eta }_c^{\prime }(2{}^{1}S_0)$, the radial excitation of the ${\eta }_c(1{}^{1}S_0)$ ground state of charmonium, in the two-photon fusion reaction $\gamma \gamma \to {\eta }_c^{\prime }\to K_S^0{K}^{\pm }{\pi }^{\mp }$ in $13.6{\mathrm{fb}}^{-1}$ of CLEO II/II.V data and $13.1{\mathrm{fb}}^{-1}$ of CLEO III data. We obtain $M({\eta }_c^{\prime })=3642.9\pm 3.1(\mathrm{stat})\pm 1.5(\mathrm{syst})\mathrm{MeV}$ and $M({\eta }_c)=2981.8\pm 1.3(\mathrm{stat})\pm 1.5(\mathrm{syst})\mathrm{MeV}$. The corresponding values of hyperfine splittings between ${}^{1}S_0$ and ${}^{3}S_1$ states are $\Delta M_{\mathrm{hf}}(1S)=115.1\pm 2.0\mathrm{MeV}$ and $\Delta M_{\mathrm{hf}}(2S)=43.1\pm 3.4\mathrm{MeV}$. Assuming that the ${\eta }_c$ and ${\eta }_c^{\prime }$ have equal branching fractions to $K_{S}K\pi$, we obtain ${\Gamma }_{\gamma \gamma }({\eta }_c^{\prime })=1.3\pm 0.6\mathrm{keV}$.

Figure 1

Invariant mass distributions for $K_S^0{K}^{\pm }{\pi }^{\mp }$ events from (top) the CLEO II data and (bottom) the CLEO III data. The curves in the figures are results of fits described in the text.