Amplitude analysis of the $χ_{c1} \to ηπ^+π^-$ decays

Abstract: Using $448.0 \times 10<sup>6$~$\psi(3686)$ events collected with the BESIII detector, an amplitude analysis is performed for $\psi(3686)\to\gamma\chi_{c1}$, $\chi_{c1}\to\eta\pi<sup>+\pi<sup>-$ decays. The most dominant two-body structure observed is $a_0(980)<sup>{\pm}\pi<sup>{\mp}$; $a_0(980)<sup>{\pm}\to\eta\pi<sup>{\pm}$. The $a_0(980)$ line shape is modeled using a dispersion relation, and a significant non-zero $a_0(980)$ coupling to the $\eta<sup>{\prime}\pi$ channel is measured. We observe $\chi_{c1}\to a_2(1700)\pi$ production for the first time, with a significance larger than 17$\sigma$. The production of mesons with exotic quantum numbers, $J<sup>{PC}=1<sup>{-+}$, is investigated, and upper limits for the branching fractions $\chi_{c1}\to \pi_1(1400)<sup>{\pm}\pi<sup>{\mp}$, $\chi_{c1}\to \pi_1(1600)<sup>{\pm}\pi<sup>{\mp}$, and $\chi_{c1}\to\pi_1(2015)<sup>{\pm}\pi<sup>{\mp}$, with subsequent $\pi_1(X)^{\pm} \to \eta\pi<sup>{\pm}$ decay, are determined.