Using a sample of $1.31\times10^{ 9} ~J/\psi$ events collected with the BESIII detector, we perform a study of $J/\psi\to\gamma K\bar{ K}\eta'$. The $X(2370)$ is observed in the $K\bar{ K}\eta'$ invariant-mass distribution with a statistical significance of 8.3$\sigma$. Its resonance parameters are measured to be $M = 2341.6 \pm 6.5\text{ (stat.)} \pm 5.7\text{ (syst.)}$~MeV/$c^{ 2}$ and $\Gamma = 117 \pm 10 \text{ (stat.)} \pm 8 \text{ (syst.)}$~MeV. The product branching fractions for $J/\psi\to \gamma X(2370), X(2370) \to K^{ +} K^{ -}\eta'$ and $J/\psi\to \gamma X(2370), X(2370) \to K_{ S}^{ 0} K_{ S}^{ 0}\eta'$ are determined to be $(1.79 \pm 0.23 \text{ (stat.)} \pm 0.65 \text{ (syst.)}) \times 10^{ -5}$ and $(1.18 \pm 0.32\ text{ (stat.)} \pm 0.39 \text{ (syst.)}) \times 10^{ -5}$, respectively. No evident signal for the $X(2120)$ is observed in the $K\bar{ K}\eta'$ invariant-mass distribution. The upper limits for the product branching fractions of $\mathcal{ B}(J/\psi \to \gamma X(2120)\to\gamma K^{ +} K^{ -} \eta')$ and $\mathcal{ B}(J/\psi\to\gamma X(2120)\to\gamma K_{ S}^{ 0} K_{ S}^{ 0} \eta')$ are determined to be $1.49\times10^{ -5}$ and $6.38\times10^{ -6}$ at the 90% confidence level, respectively.
Categories: hep-ex