Observation of X(2370) and search for X(2120) in $$J/\psi \rightarrow \gamma K{\bar{K}} \eta '$$: BESIII Collaboration

Using a sample of $$1.31\times 10^{9} ~J/\psi $$ events collected with the BESIII detector, we perform a study of $$J/\psi \rightarrow \gamma K{\bar{K}}\eta '$$. 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 $$M=2341.6\pm 6.5 \, \text {(stat.)} \pm 5.7 \, \text {(syst.)}~ \hbox {MeV}/c^{2}$$ and $$\Gamma = 117\pm 10 \, \text {(stat.)}\pm 8 \, \text {(syst.)}~\hbox {MeV}$$. The product branching fractions for $$J/\psi \rightarrow \gamma X(2370),X(2370)\rightarrow K^{+}K^{-}\eta '$$ and $$J/\psi \rightarrow \gamma X(2370),X(2370)\rightarrow K_{S}^{0}K_{S}^{0}\eta '$$ 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 X(2120) is observed in the $$K{\bar{K}}\eta '$$ invariant-mass distribution. The upper limits for the product branching fractions of $${\mathcal {B}}(J/\psi \rightarrow \gamma X(2120)\rightarrow \gamma K^{+} K^{-} \eta ')$$ and $${\mathcal {B}}(J/\psi \rightarrow \gamma X(2120)\rightarrow \gamma K_{S}^{0} K_{S}^{0} \eta ')$$ are determined to be $$1.49\times 10^{-5}$$ and $$6.38\times 10^{-6}$$ at the 90% confidence level, respectively.