# Measurement of ${\eta }_{c}\left(1S\right)$, ${\eta }_{c}\left(2S\right)$, and nonresonant ${\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ production via two-photon collisions

09 October 2018

Abstract: We report the measurement of $\gamma \gamma \to {\eta }_{c}\left(1S\right),{\eta }_{c}\left(2S\right)\to {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ with ${\eta }^{\prime }$ decays to $\gamma \rho$ and $\eta {\pi }^{+}{\pi }^{-}$ using $941\text{}\text{}{\mathrm{fb}}^{-1}$ of data collected with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{-}$ collider. The ${\eta }_{c}\left(1S\right)$ mass and width are measured to be $M=\left[2984.6±0.7\text{}\left(\mathrm{stat}\right)±2.2\text{}\left(\mathrm{syst}\right)±0.3\text{}\left(\mathrm{model}\right)\right]\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $\Gamma =\left[30.{8}_{-2.2}^{+2.3}\text{}\left(\mathrm{stat}\right)±2.5\text{}\left(\mathrm{syst}\right)±1.4\text{}\left(\mathrm{model}\right)\right]\text{}\text{}\mathrm{MeV}$, respectively. First observation of ${\eta }_{c}\left(2S\right)\to {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ with a significance of $5.5\sigma$ including systematic error is obtained, and the ${\eta }_{c}\left(2S\right)$ mass is measured to be $M=\left[3635.1±3.7\text{}\left(\mathrm{stat}\right)±2.9\text{}\left(\mathrm{syst}\right)±0.4\text{}\left(\mathrm{model}\right)\right]\text{}\text{}\mathrm{MeV}/{c}^{2}$. The products of the two-photon decay width and branching fraction ($B$) of decays to ${\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ are determined to be ${\Gamma }_{\gamma \gamma }$${\Gamma }_{\gamma \gamma }B=\left[65.4±2.6\text{}\left(\mathrm{stat}\right)±7.8\text{}\text{}\left(\mathrm{syst}\right)\right]\text{}\text{}\mathrm{eV}$ for ${\eta }_{c}\left(1S\right)$ and $\left[{5.6}_{-1.1}^{+1.2}\text{}\left(\mathrm{stat}\right)±1.1\text{}\left(\mathrm{syst}\right)\right]\text{}\text{}\mathrm{eV}$ for ${\eta }_{c}\left(2S\right)$. The cross sections for $\gamma \gamma \to {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ and ${\eta }^{\prime }{f}_{2}\left(1270\right)$ are measured for the first time.

Published in: Physical Review D 98 (2018)