Measurement of the Electromagnetic Transition Form-factors in the decays \(\eta'\rightarrow\pi^+\pi^-l^+l^-\)

• Published in: arXiv.org
• Main Authors: Ablikim, M, Bertani, M, Bettoni, D, Chang, J F, Chen, C, Chen, S J, Chen, S L, Chen, Z Y, Dai, H L, Deng, C Q, Denig, A, Ding, B, Ding, Y, Dong, L Y, Du, S X, Fang, J, Feng, J H, Gao, H, Gao, Y N, Garbolino, S, Gilman, A, Gradl, W, Gu, Y T, Guskov, A, He, K K, Heng, Y K, Huang, Y P, Hüsken, N, Jeong, J H, Q Ji, Jiang, T J, Kang, X S, Khachatryan, V, Khoukaz, A, Kühn, W, Lenz, T, C Li, Li, J S, Li, Ke, Li, L K, Li, S X, Liang, C, Liang, H, Limphirat, A, Liu, C X, Liu, H, Lu, H J, Lu, Y P, Lu, Z H, Miao, H, Niu, Q L, Pan, X, Pathak, A, Patteri, P, Plura, S, Qian, W B, Qiao, C F, Ruan, S N, Salone, N, Shen, H F, Shi, B A, Shi, J L, Shi, Q Q, Song, W M, Sun, L, Sun, Y J, Sun, Z Q, Teng, J X, Uman, I, Wang, Bo, Wang, J P, Wang, Y F, Wang, Yaqian, Wang, Yi, Wang, Z L, C Wu, Wu, X H, Y Wu, Wu, Y J, Xie, Z P, Xu, Q J, W Xu, Yang, Tao, Yang, Z W, T Yu, Yuan, C Z, Zeng, Y, Zhang, B X, Zhang, J, Zhang, J Q, Zhang, P, Zhang, Q Y, Zhang, Z H, Zhao, J Y, Zhao, Lei, Zhao, Y X, Zhemchugov, A, Zhou, H, Zhou, L P, J Zu
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 03.02.2024
• Subjects: Asymmetry; Electric dipoles; Form factors; Fractions; Hypothetical particles; Parameters; Particle decay
• ISSN: 2331-8422

With a sample of \((10087\pm44)\times10^{6}\) \(J/\psi\) events accumulated with the BESIII detector, we analyze the decays \(\eta'\rightarrow\pi^+\pi^-l^+l^-(l=e,\) \(\mu)\) via the process \(J/\psi\rightarrow\gamma\eta'\). The branching fractions are measured to be \(\mathcal{B}(\eta'\rightarrow\pi^+\pi^-e^+e^-)=(2.45\pm0.02(\rm{stat.})\pm0.08(\rm{syst.})) \times10^{-3}\) and \(\mathcal{B}(\eta'\rightarrow\pi^+\pi^-\mu^+\mu^-)=(2.16\pm0.12(\rm{stat.})\pm0.06(\rm{syst.}))\times10^{-5}\), and the ratio is \(\frac{\mathcal{B}(\eta'\rightarrow\pi^{+}\pi^{-}e^{+}e^{-})}{\mathcal{B}(\eta'\rightarrow\pi^{+}\pi^{-}\mu^{+}\mu^{-})} = 113.4\pm0.9(\rm{stat.})\pm3.7(\rm{syst.})\). In addition, by combining the \(\eta'\rightarrow\pi^+\pi^-e^+e^-\) and \(\eta'\rightarrow\pi^+\pi^-\mu^+\mu^-\) decays, the slope parameter of the electromagnetic transition form factor is measured to be \(b_{\eta'}=1.30\pm0.19\ (\mathrm{GeV}/c^{2})^{-2}\), which is consistent with previous measurements from BESIII and theoretical predictions from the VMD model. The asymmetry in the angle between the \(\pi^+\pi^-\) 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 \(\mathcal{A}_{CP}(\eta'\rightarrow\pi^+\pi^-e^+e^-)=(-0.21\pm0.73(\rm{stat.})\pm0.01(\rm{syst.}))\%\) and \(\mathcal{A}_{CP}(\eta'\rightarrow\pi^+\pi^-\mu^+\mu^-)=(0.62\pm4.71(\rm{stat.})\pm0.08(\rm{syst.}))\%\), implying that no evidence of \(CP\)-violation is observed at the present statistics. Finally, an axion-like particle is searched for via the decay \(\eta'\rightarrow\pi^+\pi^-a, a\rightarrow e^+e^-\), and upper limits of the branching fractions are presented for the mass assumptions of the axion-like particle in the range of \(0-500\ \mathrm{MeV}/c^{2}\).