Abstract
Using a data sample of $(1310.6\pm7.0)\times10^{6}$ $J/\psi$ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J/\psi \to \eta' e^+e^-$ with two dominant $\eta'$ decay modes, $\eta' \to \gamma \pi^+ \pi^-$ and $\eta' \to \pi^+\pi^-\eta$. The branching fraction is determined to be $\mathcal{B}(J/\psi \to \eta' e^+e^-) = (6.59\pm0.07\pm0.17) \times 10^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon ($\gamma '$) is performed via $J/\psi \to\eta' \gamma ', \gamma' \to e^{+}e^{-}$. Excluding the $\omega$ and $\phi$ mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/$c^{2}$. We set upper limits at the 90\% confidence level on $\mathcal{B}(J/\psi \to \eta' \gamma ')\times\mathcal{B}(\gamma ' \to e^+e^-)$, $\mathcal{B}(J/\psi \to\eta' \gamma'$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.
Highlights
The electromagnetic (EM) Dalitz decay of a vector meson (V) to a pseudoscalar meson (P) and a pair of leptons (l 1⁄4 e, μ), V → Plþl−, provides important information on the interaction at the V-P transition vertex [1], where the lepton pair in the final state originates from a virtual photon
Using a data sample of ð1310.6 Æ 7.0Þ × 106 J=ψ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay J=ψ → η0eþe− with two dominant η0 decay modes, η0 → γπþπ− and η0 → πþπ−η
We report on the updated branching fraction (BF) measurement of J=ψ → η0eþe− and a search for a dark photon through J=ψ → η0γ0, γ0 → eþe−, with 405 pb−1 eþe− collision data containing ð1310.6 Æ 7.0Þ × 106 J=ψ events [27] collected by BESIII
Summary
The electromagnetic (EM) Dalitz decay of a vector meson (V) to a pseudoscalar meson (P) and a pair of leptons (l 1⁄4 e, μ), V → Plþl−, provides important information on the interaction at the V-P transition vertex [1], where the lepton pair in the final state originates from a virtual photon. The simple realizations of these models usually consist of an extra U(1) gauge group, with a corresponding massive vector boson force carrier, called a dark photon (γ0), which is neutral under the SM gauge symmetries, but couples to the SM photon via kinetic mixing [21] and decays into SM particles. Such models provide a natural scenario for dark matter interactions. Together with the study of J=ψ → ηeþe− [28] with the same data set, it is the first time that the dark photon is searched for through the charmonium decays
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