Observation of an Isoscalar Resonance with Exotic ${J}^{PC}={1}^{-+}$ Quantum Numbers in $J/\psi \to \gamma \eta {\eta }^{\prime }$

Using a sample of $\left(10.09±0.04\right)×{10}^{9}\text{}\text{}J/\psi $ events collected with the BESIII detector operating at the BEPCII storage ring, a partial wave analysis of the decay $J/\psi \to \gamma \eta {\eta }^{\prime }$ is performed. The first observation of an isoscalar state with exotic quantum numbers ${J}^{PC}={1}^{-+}$, denoted as ${\eta }_{1}\left(1855\right)$, is reported in the process $J/\psi \to \gamma {\eta }_{1}\left(1855\right)$ with ${\eta }_{1}\left(1855\right)\to \eta {\eta }^{\prime }$. Its mass and width are measured to be $\left(1855±{9}_{-1}^{+6}\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $\left(188±{18}_{-8}^{+3}\right)\text{}\text{}\mathrm{MeV}$, respectively, where the first uncertainties are statistical and the second are systematic, and its statistical significance is estimated to be larger than $19\sigma $.