Observation of the Dalitz Decay $\eta' \to \gamma e^+e^-$

We report the first observation of the Dalitz decay η′→γe+e-, based on a data sample of 1.31 billion J/ψ events collected with the BESIII detector. The η′ mesons are produced via the J/ψ→γη′ decay process. The ratio Γ(η′→γe+e-)/Γ(η′→γγ) is measured to be (2.13±0.09(stat)±0.07(sys))×10-2. This corresponds to a branching fraction B(η′→γe+e-)=(4.69±0.20(stat)±0.23(sys))×10-4. The transition form factor is extracted and different expressions are compared to the measured dependence on the e+e- invariant mass. The results are consistent with the prediction of the vector meson dominance model.

Note:

11 pages,7 figures

13.40.Gp
13.20.-v
13.40.Hq
14.40.Be
experimental results
form factor: transition
model: meson dominance
eta(958): decay
branching ratio
vector meson

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