We present first evidence for the process $e^+e^-\to \gamma\eta_c(1S)$ at six center-of-mass energies between 4.01 and 4.60~GeV using data collected by the BESIII experiment operating at BEPCII. These data sets correspond to a total integrated luminosity of 4.6~fb$^{ -1}$. We measure the Born cross section at each energy using a combination of twelve $\eta_c(1S)$ decay channels. Because the significance of the signal is marginal at each energy ($\le3.0\sigma$), we also combine all six energies under various assumptions for the energy-dependence of the cross section. If the process is assumed to proceed via the $Y(4260)$, we measure a peak Born cross section $\sigma_{ \mathrm{ peak}}(e^+e^-\to\gamma\eta_c(1S)) = 2.11 \pm 0.49 (\mathrm{ stat.}) \pm 0.36 (\mathrm{ syst.})$~pb with a statistical significance of 4.2$\sigma$.
Categories: hep-ex