Observation of $h_1(1380)$ in the $J/ψ\to η^{\prime} K\bar K π$ Decay

Abstract: Using $1.31 \times 10<sup>9$ $J/\psi$ events collected by the BESIII detector at the BEPCII $e<sup>+e<sup>-$ collider, we report the first observation of the $h_1(1380)$ in $J/\psi \to \eta^{\prime} h_1(1380)$ 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 $$\mathcal{B}(J/\psi \to \eta^{{\prime}h_1(1380))} \times \mathcal{B}(h_1(1380) \to K^{{*}(892)^{+}} K^{-} +c.c.) = (1.51 \pm 0.09 \pm 0.21) \times 10<sup>{-4}$$ in $\eta<sup>{\prime}K<sup>+K<sup>-\pi<sup>0$ mode and $$\mathcal{B}(J/\psi \to \eta^{{\prime}h_1(1380))} \times \mathcal{B}(h_1(1380) \to K^{*}(892)\bar K +c.c.) = (2.16 \pm 0.12 \pm 0.29) \times 10<sup>{-4}$$ in $\eta<sup>{\prime}K_S<sup>0K<sup>{\pm}\pi<sup>{\mp}$ mode. The first uncertainties are statistical and the second are systematic. Isospin symmetry violation is observed in the decays $$h_1(1380) \to K^{{*}(892)^{+}} K^{-} +c.c.$$ and $$h_1(1380) \to K^{{*}(892)^{0}\bar} K^{0} +c.c.$$. 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 $(35.9\pm2.6)<sup>{\circ}$, consistent with theoretical expectations.