Using $1.31\times {10}^{9}J/\psi$ events collected by the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider, we report the first observation of the $h_1(1380)$ in $J/\psi \to {\eta }^{\prime }{h}_1(1380)$ 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 $M=(1423.2\pm 2.1\pm 7.3)\mathrm{MeV}/c^2$ and $\mathrm{\Gamma }=(90.3\pm 9.8\pm 17.5)\mathrm{MeV}$. The product branching fractions, assuming no interference, are determined to be $\mathcal{B}(J/\psi \to {\eta }^{\prime }{h}_1(1380))\times \mathcal{B}(h_1(1380)\to K^{*}(892{)}^{+}{K}^{-}+\mathrm{c}.\mathrm{c}.)=(1.51\pm 0.09\pm 0.21)\times {10}^{-4}$ in ${\eta }^{\prime }{K}^{+}{K}^{-}{\pi }^0$ mode and $\mathcal{B}(J/\psi \to {\eta }^{\prime }{h}_1(1380))\times \mathcal{B}(h_1(1380)\to K^{*}(892)\overline{K}+\mathrm{c}.\mathrm{c}.)=(2.16\pm 0.12\pm 0.29)\times {10}^{-4}$ in ${\eta }^{\prime }{K}_S^0{K}^{\pm }{\pi }^{\mp }$ mode. The first uncertainties are statistical and the second are systematic. Isospin symmetry violation is observed in the decays $h_1(1380)\to K^{*}(892{)}^{+}{K}^{-}+\mathrm{c}.\mathrm{c}.$ and $h_1(1380)\to K^{*}(892{)}^0{\overline{K}}^0+\mathrm{c}.\mathrm{c}.$. 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 $(35.9\pm 2.6)°$, consistent with theoretical expectations.

• Received 17 April 2018

DOI:https://doi.org/10.1103/PhysRevD.98.072005

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Published by the American Physical Society