# Evidence for $e^{+}e^{-} \to \gamma\eta_c(1S)$ at center-of-mass energies between 4.01 and 4.60 GeV

## TL;DR

This paper reports the first evidence of the process e+e- to gamma eta_c(1S) at energies between 4.01 and 4.60 GeV, with a peak cross section of about 2.11 pb assuming the Y(4260) resonance, using BESIII data.

## Contribution

First experimental evidence for e+e- to gamma eta_c(1S) in the specified energy range, with cross section measurement under Y(4260) assumption.

## Key findings

- Marginal signal significance at individual energies (≤3.0σ).
- Combined analysis yields a 4.2σ significance for the process.
- Measured peak cross section of approximately 2.11 pb.

## Abstract

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$\sigma$.