Speaker
Description
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
$\mathcal{B}\left[\chi_{c1}\rightarrow\pi_{1}(1600)^{\pm}\pi^{\mp} \right] \times \mathcal{B}\left[\pi_{1}(1600)^{\pm}\rightarrow\pi^{\pm}\eta^{\prime}\right] = \left( 4.30 \pm 0.14 ({\rm stat})^{+1.04}_{-1.03}({\rm syst})~ \right) \times 10^{-4}$.
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