Evidence for the η c ( 2 S ) → π + π − η decay

The decay ηc⁡(2⁢S) → π+π-η is searched for through the radiative transition ψ(3686) → γηc⁡(2⁢S) using 448 million ψ(3686) events accumulated at the BESIII detector. The first evidence of ηc⁡(2⁢S) → π+π-η is found with a statistical significance of 3.5⁢σ. The product of the branching fractions of ψ⁡(3686) → γηc⁡(2⁢S) and ηc⁡(2⁢S) → π+π-η is measured to be Br⁡(ψ⁡(3686) → γηc⁡(2⁢S)) × Br⁡(ηc⁡(2⁢S) → π+π-η) = (2.97 ± 0.81 ± 0.26) × 10-6, where the first uncertainty is statistical and the second one is systematic. The branching fraction of the decay ηc⁡(2⁢S) → π+π-η is determined to be Br⁡(ηc⁡(2⁢S) → π+π-η) = (42.4 ± 11.6 ± 3.8 ± 30.3) × 10-4, where the third uncertainty is transferred from the uncertainty of the branching fraction of ψ⁡(3686)→ γηc⁡(2⁢S).