Observation of $e^{+}{e}^{-}\to \gamma {\chi }_{c1}$ and search for $e^{+}{e}^{-}\to \gamma {\chi }_{c0},\gamma {\chi }_{c2},$ and $\gamma {\eta }_c$ at $\sqrt{s}$ near 10.6 GeV at Belle

Abstract

Using data samples of 89.5~fb${}^{-1}$, 711.0~fb${}^{-1}$, and 121.4~fb${}^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider at center-of-mass energies 10.52 GeV, 10.58 GeV, and 10.867 GeV, respectively, we study the exclusive reactions $e^{+}{e}^{-}\to \gamma {\chi }_{cJ}$ ($J=0,1,2$) and $e^{+}{e}^{-}\to \gamma {\eta }_c$. A significant $\gamma {\chi }_{c1}$ signal is observed for the first time at $\sqrt{s}=10.58$ GeV with a significance of $5.1\sigma$ including systematic uncertainties. No significant excesses for $\gamma {\chi }_{c0}$, $\gamma {\chi }_{c2}$, and $\gamma {\eta }_c$ final states are found, and we set 90\% credibility level upper limits on the Born cross sections (${\sigma }_{\mathrm{B}}$) at 10.52 GeV, 10.58 GeV, and 10.867~GeV. Together with cross sections measured by BESIII at lower center-of-mass energies, the energy dependency of ${\sigma }_{\mathrm{B}}(e^{+}{e}^{-}\to \gamma {\chi }_{c1})$ is obtained.

Comments: 8 pages, 4 figures