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

With a sample of (10087 ± 44) × 106 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 $B$ (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 $\frac{B\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}\right)}{B\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)}=113.4±0.9\left(\text{stat}.\right)±3.7\left(\text{syst}.\right)$ . 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 ${A}_{\mathrm{CP}}\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}\right)=\left(-0.21±0.73\left(\text{stat}.\right)±0.01\left(\text{syst}.\right)\right)%$ and ${A}_{\mathrm{CP}}\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)=\left(0.62±4.71\left(\text{stat}.\right)±0.08\left(\text{syst}.\right)\right)%$ , 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.