  A search for radiative decays of the $P$-wave spin singlet charmonium
resonance $h_c$ is performed based on $4.48 \times 10^{8}$ $\psi'$ events
collected with the BESIII detector operating at the BEPCII storage ring. Events
of the reaction channels $h_{c} \rightarrow \gamma \eta'$ and $\gamma \eta$ are
observed with a statistical significance of $8.4 \sigma$ and $4.0 \sigma$,
respectively, for the first time. The branching fractions of $h_{c} \rightarrow
\gamma \eta'$ and $h_{c} \rightarrow \gamma \eta$ are measured to be
$\mathcal{B}(h_{c} \rightarrow \gamma \eta')=(1.52 \pm 0.27 \pm
0.29)\times10^{-3}$ and $\mathcal{B}(h_{c} \rightarrow \gamma \eta)=(4.7 \pm
1.5 \pm 1.4)\times10^{-4}$, respectively, where the first errors are
statistical and the second are systematic uncertainties.

High Energy Physics - Experiment