Observation of the Exotic State $\pi_{1}(1600)$ in $\psi(2S)\rightarrow\gamma\chi_{c1},\chi_{c1}\rightarrow\pi^{+}\pi^{-}\eta'$

Abstract

A partial wave analysis of the process $\psi(2S)\rightarrow\gamma\chi_{c1}, \chi_{c1}\rightarrow\pi^+\pi^-\eta^{\prime}$ is performed using $(2712.4\pm14.3)\times10^{6}$ $\psi(2S)$ events collected with the BESIII detector. An isovector state with exotic quantum numbers $J^{PC}=1^{-+}$, denoted as $\pi_{1}(1600)$, is observed for the first time in the charmonium decay of $\chi_{c1}\rightarrow\pi_{1}^{\pm}(1600)\pi^{\mp}$, $\pi_{1}^{\pm}(1600)\rightarrow\pi^{\pm}\eta^{\prime}$ with a statistical significance over $21\sigma$. Its mass and width are determined to be $1828 \pm 8 ({\rm stat})^{+11}_{-33}({\rm syst})~\mathrm{MeV}/c^2$ and $638 \pm 26 ({\rm stat})^{+35}_{-86}({\rm syst})~\mathrm{MeV}$, respectively, using a relativistic Breit-Wigner function with a mass-dependent width. The corresponding product of branching fractions is determined to be $\mathcal{B}\left[\chi_{c1}\rightarrow\pi_{1}(1600)^{\pm}\pi^{\mp} \right] \times \mathcal{B}\left[\pi_{1}(1600)^{\pm}\rightarrow\pi^{\pm}\eta^{\prime}\right] = \left( 4.30 \pm 0.14 ({\rm stat})^{+1.04}_{-1.03}({\rm syst})~ \right) \times 10^{-4}$.