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

Abstract 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 $$ \mathcal{B} $$ (η′ → π + π − e + e − ) = (2.45 ± 0.02(stat.) ± 0.08(syst.)) × 10 −3 and B $$ \mathcal{B} $$ (η′ → π + π − μ + μ − ) = (2.16 ± 0.12(stat.) ± 0.06(syst.)) × 10 −5, and the ratio is B η ′ → π + π − e + e − B η ′ → π + π − μ + μ − = 113.4 ± 0.9 stat . ± 3.7 syst . $$ \frac{\mathcal{B}\left({\eta}^{\prime}\to {\pi}^{+}{\pi}^{-}{e}^{+}{e}^{-}\right)}{\mathcal{B}\left({\eta}^{\prime}\to {\pi}^{+}{\pi}^{-}{\mu}^{+}{\mu}^{-}\right)}=113.4\pm 0.9\left(\textrm{stat}.\right)\pm 3.7\left(\textrm{syst}.\right) $$ . 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 CP η ′ → π + π − e + e − = − 0.21 ± 0.73 stat . ± 0.01 syst . % $$ {\mathcal{A}}_{CP}\left({\eta}^{\prime}\to {\pi}^{+}{\pi}^{-}{e}^{+}{e}^{-}\right)=\left(-0.21\pm 0.73\left(\textrm{stat}.\right)\pm 0.01\left(\textrm{syst}.\right)\right)\% $$ and A CP η ′ → π + π − μ + μ − = 0.62 ± 4.71 stat . ± 0.08 syst . % $$ {\mathcal{A}}_{CP}\left({\eta}^{\prime}\to {\pi}^{+}{\pi}^{-}{\mu}^{+}{\mu}^{-}\right)=\left(0.62\pm 4.71\left(\textrm{stat}.\right)\pm 0.08\left(\textrm{syst}.\right)\right)\% $$ , 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.