Observation of $h_1(1380)$ in the $J\mathrm{/}\psi \to {\eta }^{\mathrm{\prime }}K\bar{K}\pi$ Decay

Using $1.31\times {10}^9$ $J\mathrm{/}\psi$ events collected by the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider, we report the first observation of the $h_1(1380)$ in $J\mathrm{/}\psi \to {\eta }^{\mathrm{\prime }}{h}_1(1380)$ with a significance of more than ten standard deviations. The mass and width of the possible axial-vector strangeonium candidate $h_1(1380)$ are measured to be $M = (1423.2 \pm 2.1 \pm 7.3)\mevcc$and$\Gamma = (90.3 \pm 9.8 \pm 17.5)\mev$. The product branching fractions, assuming no interference, are determined to be $$\mathcal{B}$(J/\psi \to \eta^{\prime}h_1(1380)) \times $\mathcal{B}$(h_1(1380) \to K^{}(892)^{+} K^{-} +c.c.) = (1.51 \pm 0.09 \pm 0.21) \times 10^{-4}$in$\eta^{\prime}K^+K^-\pi^0$modeand$$\mathcal{B}$(J/\psi \to \eta^{\prime}h_1(1380)) \times $\mathcal{B}$(h_1(1380) \to K^{}(892)\bar K +c.c.) = (2.16 \pm 0.12 \pm 0.29) \times 10^{-4}$in$\eta^{\prime}K_S^0K^{\pm}\pi^{\mp}$ mode. The first uncertainties are statistical and the second are systematic. Isospin symmetry violation is observed in the decays $h_1(1380)\to K^{\ast }(892{)}^{+}{K}^{-}+c\mathrm{.}c\mathrm{.}$ and $h_1(1380)\to K^{\ast }(892{)}^0{\bar{K}}^0+c\mathrm{.}c\mathrm{.}$. Based on the measured $h_1(1380)$ mass, the mixing angle between the states $h_1(1170)$ and $h_1(1380)$ is determined to be $(35.9\pm 2.6{)}^{\circ }$, consistent with theoretical expectations.