Observation of the Dalitz decayη′→γe+e−

Abstract

We report the first observation of the Dalitz decay η 0 → γe þ e − , based on a data sample of 1.31 billion J=ψ events collected with the BESIII detector. The η 0 mesons are produced via the J=ψ → γη 0 decay process. The ratio Γðη 0 → γe þ e − Þ=Γðη 0 → γγÞ is measured to be ð2.13 AE 0.09ðstatÞ AE 0.07ðsysÞÞ × 10 −2 . This corresponds to a branching fraction Bðη 0 → γe þ e − Þ ¼ ð4.69 AE 0.20ðstatÞ AE 0.23ðsysÞÞ × 10 −4 . The transition form factor is extracted and different expressions are compared to the measured dependence on the e þ e − invariant mass. The results are consistent with the prediction of the vector meson dominance model.

Search for the decay η′→γγη

“…The most stringent upper limit, reported by GAMS-4π setup, on the branching fraction of this decay is 8 × 10 −4 at the 90% confidence level (CL) [10]. The BESIII experiment using J=ψ radiative decays has observed a series of η 0 new decay modes [11][12][13][14][15][16][17], and in this paper we present a search for η 0 → γγη in the J=ψ radiative decay.…”

η′ and η mesons with connection to anomalous glue

“…is often quoted for the decays. Values extracted for the η from timelike decays are b η = 1.60 ± 0.17 ± 0.08 GeV −2 and Λ = 0.79 ± 0.04 ± 0.02 GeV from BES-III (Ablikim et al, 2015b), with Λ close to the ω and ρ masses which appear with vector meson dominance of the virtual photon. In the space-like region the CELLO Collaboration found b η = 1.60 ± 0.16 GeV −2 (Behrend et al, 1991).…”

The hadronic light-by-light contribution to the muon’s anomalous magnetic moment

“…(58,59) for the η, and η states [80,81], the LMD + V parameterization of Eqs. (60,61) for the π 0 [80,81], the Padé estimate [88], as well as the dispersive [97,98] and lattice [115,116] estimates for the π 0 pole. Lastly, we also show the interpolating parameterization of Eq.…”