Amplitude analysis of the ${\ensuremath{\chi}}_{c1}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ decays

Amplitude analysis of the $χ_{c 1} \to η π^{+} π^{-}$ decays

M. Ablikim et al. (BESIII Collaboration)

Phys. Rev. D 95, 032002 – Published 13 February 2017

Abstract

Using $448.0 \times 1 0^{6}$ $ψ (3686)$ events collected with the BESIII detector, an amplitude analysis is performed for $ψ (3686) \to γ χ_{c 1}$ , $χ_{c 1} \to η π^{+} π^{-}$ decays. The most dominant two-body structure observed is $a_{0} (980)^{\pm} π^{\mp}$ ; $a_{0} (980)^{\pm} \to η π^{\pm}$ . The $a_{0} (980)$ line shape is modeled using a dispersion relation, and a significant nonzero $a_{0} (980)$ coupling to the $η^{'} π$ channel is measured. We observe $χ_{c 1} \to a_{2} (1700) π$ production for the first time, with a significance larger than $17 σ$ . The production of mesons with exotic quantum numbers, $J^{P C} = 1^{- +}$ , is investigated, and upper limits for the branching fractions $χ_{c 1} \to π_{1} (1400)^{\pm} π^{\mp}$ , $χ_{c 1} \to π_{1} (1600)^{\pm} π^{\mp}$ , and $χ_{c 1} \to π_{1} (2015)^{\pm} π^{\mp}$ , with subsequent $π_{1} (X)^{\pm} \to η π^{\pm}$ decay, are determined.