Measurement of the η transition form factor through η' → π⁺π^-η decay

2025/06/10 by BESIII Collaboration, Ablikim, M., Achasov, M. N. +678
#FOS: Physical sciences #High Energy Physics - Experiment (hep-ex)

paper · doi:10.48550/arxiv.2506.08576

Abstract

Based on a sample of (1.0087±0.0044)×10¹⁰ J/ψ events collected at BESIII, the transition form factor of the η meson is extracted by analyzing J/ψ→γη',~η'→π⁺π^-η,~η→γl⁺l^- (l=e, μ) events. The measured slope of the transition form factor is Λ^-2=1.645±0.093_{\rm stat.}± 0.024_{\rm sys.} (GeV/c²)^-2 for the di-electron channel and Λ^-2=1.645±0.343_{\rm stat.}±0.017_{\rm sys.} (GeV/c²)^-2 for the di-muon channel. The branching fractions for η→γe⁺e^- and η→γμ⁺μ^- are measured to be B(η→γe⁺e^-)=(6.79±0.04_{\rm stat.}±0.36_{\rm sys.})× 10^-3 and B(η→γμ⁺μ^-)=(2.97±0.11_{\rm stat.}±0.07_{\rm sys.})× 10^-4. By combining with the results based on the J/ψ→γη,~η→γe⁺e^- events from the previous BESIII measurement, we determine Λ^-2=1.707±0.076_{\rm stat.}±0.029_{\rm sys.} (GeV/c²)^-2 and B(η→γe⁺e^-)=(6.93±0.28_{\rm tot.})× 10^-3. In addition, we search for the dark photon (A') using the combined events. No significant signal is observed, and the upper limits on B(η→γA',~A'→ e⁺e^-) are set at 90% confidence level for different A' mass hypotheses.

Citations

• Anomalous decays of pseudoscalar mesons

Related

• Measurement of the Electromagnetic Transition Form-factors in the decays η'→π⁺π^-l⁺l^-
• Study of η→π⁺π^-l⁺l^-
• Improved measurement of the decays η' → π⁺π^-π⁺⁽⁰⁾π^-(0) and search for the rare decay η' → 4π⁰
• Measurement of the branching fraction of η→ μ⁺ μ^- and search for η→ e⁺ e^-
• Observation of the decay J/ψ→ e⁺ e^- η(1405) with η(1405) → π⁰ f₀(980)