Amplitude analysis of the χc1 → ηπ⁺π⁻ decays

Using 448.0 × 10⁶~$ψ(3686)$ events collected with the BESIII detector, an amplitude analysis is performed for ψ(3686)→γχc1, χc1→ηπ⁺π⁻ decays. The most dominant two-body structure observed is a₀(980)±π∓; a₀(980)±→ηπ±. The a₀(980) line shape is modeled using a dispersion relation, and a significant non-zero a₀(980) coupling to the η^π channel is measured. We observe χc1→ a₂(1700)π production for the first time, with a significance larger than 17$σ$. The production of mesons with exotic quantum numbers, JPC=1⁻⁺, is investigated, and upper limits for the branching fractions χc1→ π₁(1400)±π∓, χc1→ π₁(1600)±π∓, and χc1→π₁(2015)±π∓, with subsequent π₁(X)± → ηπ± decay, are determined.