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 B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(η′ → π+π−e+e−) = (2.45 ± 0.02(stat.) ± 0.08(syst.)) × 10−3 and B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(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.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\frac{\\mathcal{B}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{e}^{+}{e}^{-}\\right)}{\\mathcal{B}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{\\mu}^{+}{\\mu}^{-}\\right)}=113.4\\pm 0.9\\left(\ extrm{stat}.\\right)\\pm 3.7\\left(\ extrm{syst}.\\right) $$\\end{document}. 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 bη′ = 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 ACPη′→π+π−e+e−=−0.21±0.73stat.±0.01syst.%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\mathcal{A}}_{CP}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{e}^{+}{e}^{-}\\right)=\\left(-0.21\\pm 0.73\\left(\ extrm{stat}.\\right)\\pm 0.01\\left(\ extrm{syst}.\\right)\\right)\\% $$\\end{document} and ACPη′→π+π−μ+μ−=0.62±4.71stat.±0.08syst.%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\mathcal{A}}_{CP}\\left({\\eta}^{\\prime}\ o {\\pi}^{+}{\\pi}^{-}{\\mu}^{+}{\\mu}^{-}\\right)=\\left(0.62\\pm 4.71\\left(\ extrm{stat}.\\right)\\pm 0.08\\left(\ extrm{syst}.\\right)\\right)\\% $$\\end{document}, 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.