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

Abstract

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 $\mathcal{B}$(η′ → π⁺π⁻e⁺e⁻) = (2.45 ± 0.02(stat.) ± 0.08(syst.)) × 10⁻³ and $\mathcal{B}$(η′ → π⁺π⁻μ⁺μ⁻) = (2.16 ± 0.12(stat.) ± 0.06(syst.)) × 10⁻⁵, and the ratio is $\frac{\mathcal{B}\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}\right)}{\mathcal{B}\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)}=113.4\pm 0.9\left(\text{stat}.\right)\pm 3.7\left(\text{syst}.\right)$. 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/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 ${\mathcal{A}}_{\mathit{CP}}\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}\right)=\left(-0.21\pm 0.73\left(\text{stat}.\right)\pm 0.01\left(\text{syst}.\right)\right)\%$ and ${\mathcal{A}}_{\mathit{CP}}\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)=\left(0.62\pm 4.71\left(\text{stat}.\right)\pm 0.08\left(\text{syst}.\right)\right)\%$, 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/c².