Measurement of ηc(1S) , ηc(2S) , and nonresonant η′π+π− production via two-photon collisions

We report the measurement of $\gamma \gamma \to \eta_{c}(1S), \eta_{c}(2S) \to \eta' \pi^+ \pi^-$ with $\eta'$ decays to $\gamma \rho$ and $\eta \pi^+ pi^-$ using 941 fb$^{-1}$ of data collected with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The $\eta_{c}(1S)$ mass and width are measured to be $M = [2984.6\pm0.7 (stat.)\pm2.2 (syst.)] MeV/c^{2}$ and $\Gamma = [30.8^{+2.3}_{-2.2}~(stat.) \pm 2.5~(syst.)]$ MeV, respectively. First observation of $\eta_{c}(2S) \to \eta' pi^+ pi^-$ with a significance of 5.5$\sigma$ including systematic error is obtained, and the $\eta_{c}(2S)$ mass is measured to be $M = [3635.1\pm3.7~(stat.)\pm2.9~(syst.)] MeV/c^{2}$. The products of the two-photon decay width and branching fraction (B) of decays to $\eta' pi^+ pi^-$ are determined to be $\Gamma_{\gamma \gamma}B = [65.4\pm2.6~(stat.)\pm6.9~(syst.)]$ eV for $\eta_{c}(1S)$ and $[5.6^{+1.2}_{-1.1}~(stat.)\pm1.1~(syst.)]$ eV for $\eta_{c}(2S)$. A new decay mode for the $\eta_{c}(1S) \to \eta' f_{0}(2080)$ with $f_{0}(2080) \to \pi^+ pi^-$ is observed with a statistical significance of 20$\sigma$. The $f_{0}(2080)$ mass and width are determined to be $M = [2083^{+63}_{-66}~(stat.)\pm 32~(syst.)] MeV/c^{2}$ and $\Gamma = [178^{+60}_{-178}~(stat.) \pm 55~(syst.)]$ MeV. The cross sections for $\gamma \gamma \to \eta' pi^+ pi^-$ and $\eta' f_{2}(1270)$ are measured for the first time.