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

A bstract With a sample of (10087 ± 44) × 10 6 J/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 B (η ′ → π + π − e + e −) = (2. 45 ± 0. 02 (stat. ) ± 0. 08 (syst. ) ) × 10 − 3 and B B (η ′ → π + π − μ + μ −) = (2. 16 ± 0. 12 (stat. ) ± 0. 06 (syst. ) ) × 10 − 5, and the ratio is B ({^ ^+^-e^+e^-) }B ({^ ^+^-^+^-) }=113. 4 0. 9 (stat. ) 3. 7 (syst. ) B η ′ → π + π − e + e − B η ′ → π + π − μ + μ − = 113. 4 ± 0. 9 stat. ± 3. 7 syst. . 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 /c 2) − 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 A₂₏ (^ ^+^-e^+e^-) = (-0. 21 0. 73 (stat. ) 0. 01 (syst. ) ) \% A CP η ′ → π + π − e + e − = − 0. 21 ± 0. 73 stat. ± 0. 01 syst. % and A₂₏ (^ ^+^-^+^-) = (0. 62 4. 71 (stat. ) 0. 08 (syst. ) ) \% A CP η ′ → π + π − μ + μ − = 0. 62 ± 4. 71 stat. ± 0. 08 syst. %, 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 /c 2.