Study of η′ → π⁺π⁻l⁺l⁻ decays at BESIII

With a sample of (10087 ± 44) × 10⁶J/ψ events accumulated with the BESIII detector, we analyze the decays η′ → π⁺π⁻l⁺l⁻(l = e, μ) via the process J/ψ → γη′. The branching fractions are measured to be B(η′ → π⁺π⁻e⁺e⁻) = (2.45 ± 0.02(stat.) ± 0.08(syst.)) × 10⁻³ and B(η′ → π⁺π⁻μ⁺μ⁻) = (2.16 ± 0.12(stat.) ± 0.06(syst.)) × 10⁻⁵, and the ratio is Bη^′→π⁺π⁻e⁺e⁻Bη^′→π⁺π⁻μ⁺μ⁻=113.4±0.9stat.±3.7syst. In addition, by combining the η′ → π⁺π⁻e⁺e⁻ and η′ → π⁺π⁻μ⁺μ⁻ decays, the slope parameter of the electromagnetic transition form factor is measured to be b_η′ = 1.30 ± 0.19 (GeV/c²)⁻², which is consistent with previous measurements from BESIII and theoretical predictions from the VMD model. The asymmetry in the angle between the π⁺π⁻ and l⁺l⁻ decay planes, which has the potential to reveal the CP-violation originating from an unconventional electric dipole transition, is also investigated. The asymmetry parameters are determined to be A_CPη^′→π⁺π⁻e⁺e⁻=−0.21±0.73stat.±0.01syst.% and A_CPη^′→π⁺π⁻μ⁺μ⁻=0.62±4.71stat.±0.08syst.%, implying that no evidence of CP-violation is observed at the present statistics. Finally, an axion-like particle is searched for via the decay η′ → π⁺π⁻a, a → e⁺e⁻, and upper limits of the branching fractions are presented for the mass assumptions of the axion-like particle in the range of 0 − 500 MeV/c².