Observation of a strange $J^P=1^{-}$ resonance $K_1^{\ast }(1870)$ in ${\chi }_{c1}\to K^{+}{K}^{-}{\eta }^{\prime }$ decays

Abstract

A partial wave analysis of the process $\psi (2S)\to \gamma {\chi }_{c1}$, ${\chi }_{c1}\to K^{+}{K}^{-}{\eta }^{\prime }$, with the ${\eta }^{\prime }$ meson reconstructed in the ${\pi }^{+}{\pi }^{-}\gamma$ and ${\pi }^{+}{\pi }^{-}\eta$ decay modes, is performed using a sample of $(2712.4\pm 14.3)\times 10^6$ $\psi (2S)$ events collected with the BESIII detector. In the $K{\eta }^{\prime }$ system, a resonant contribution with quantum numbers $J^P=1^{-}$ is observed for the first time and is denoted as the $K_1^{\ast }(1870)$ meson. Its statistical significance is $11.0\sigma$, and its mass and width are measured to be $M=(1869.0\pm 14.6\pm 18.6$) MeV/$c^2$ and $\Gamma =(167.3\pm 34.0\pm 24.2$) MeV, respectively. The branching fraction of the ${\chi }_{c1}\to K^{\mp }{K}_1^{\ast }(1870{)}^{\pm }\to K^{+}{K}^{-}{\eta }^{\prime }$ decay chain is determined to be $(1.48\pm 0.30\pm 0.33)\times 10^{-5}$. The branching fraction of the ${\chi }_{c1}\to K^{+}{K}^{-}{\eta }^{\prime }$ decay is measured to be $(7.90\pm 0.48)\times 10^{-4}$. Several intermediate scalar, vector, and tensor states are required to provide a good description of the data. These results provide new input for understanding strange-meson spectroscopy and investigating possible hybrid-state candidates.

arXiv comment11 pages, 2 figures, 2 tables