Observation of $h_1(1380)$ in the $J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }K\bar{K}\ddot{I}€$ Decay

Abstract

Using $1.31\times 10^9$ $J/\ddot{I}ˆ$ events collected by the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider, we report the first observation of the $h_1(1380)$ in $J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }{h}_1(1380)$ 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 $Γ= (90.3 \pm 9.8 \pm 17.5)\mev$. The product branching fractions, assuming no interference, are determined to be $\mathcal{B}(J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }{h}_1(1380))\times \mathcal{B}(h_1(1380)\to K^{\ast }(892{)}^{+}{K}^{-}+c.c.)=(1.51\pm 0.09\pm 0.21)\times 10^{-4}$ in $\hat{I}{\cdot }^{\prime }{K}^{+}{K}^{-}\ddot{I}{€}^0$ mode and $\mathcal{B}(J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }{h}_1(1380))\times \mathcal{B}(h_1(1380)\to K^{\ast }(892)\bar{K}+c.c.)=(2.16\pm 0.12\pm 0.29)\times 10^{-4}$ in $\hat{I}{\cdot }^{\prime }{K}_S^0{K}^{\pm }\ddot{I}{€}^{\mp }$ mode. The first uncertainties are statistical and the second are systematic. Isospin symmetry violation is observed in the decays $h_1(1380)\to K^{\ast }(892{)}^{+}{K}^{-}+c.c.$ and $h_1(1380)\to K^{\ast }(892{)}^0{\bar{K}}^0+c.c.$. 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 $(35.9\pm 2.6{)}^{\circ }$, consistent with theoretical expectations.