Amplitude analysis of the χc1→ηπ+π− decays

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Abstract

Using $448.0\ifmmode\times\else\texttimes\fi{}1{0}^{6}$ $\ensuremath{\psi}(3686)$ events collected with the BESIII detector, an amplitude analysis is performed for $\ensuremath{\psi}(3686)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{c1}$, ${\ensuremath{\chi}}_{c1}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ decays. The most dominant two-body structure observed is ${a}_{0}(980{)}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$; ${a}_{0}(980{)}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$. The ${a}_{0}(980)$ line shape is modeled using a dispersion relation, and a significant nonzero ${a}_{0}(980)$ coupling to the ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\pi}$ channel is measured. We observe ${\ensuremath{\chi}}_{c1}\ensuremath{\rightarrow}{a}_{2}(1700)\ensuremath{\pi}$ production for the first time, with a significance larger than $17\ensuremath{\sigma}$. The production of mesons with exotic quantum numbers, ${J}^{PC}={1}^{\ensuremath{-}+}$, is investigated, and upper limits for the branching fractions ${\ensuremath{\chi}}_{c1}\ensuremath{\rightarrow}{\ensuremath{\pi}}_{1}(1400{)}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$, ${\ensuremath{\chi}}_{c1}\ensuremath{\rightarrow}{\ensuremath{\pi}}_{1}(1600{)}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$, and ${\ensuremath{\chi}}_{c1}\ensuremath{\rightarrow}{\ensuremath{\pi}}_{1}(2015{)}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$, with subsequent ${\ensuremath{\pi}}_{1}(X{)}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ decay, are determined.

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