Search for a hypothetical gauge boson and dark photons in charmonium transitions

We report a direct search for a new gauge boson, X, with a mass of 17 MeV=c2, which could explain the anomalous excess of eþe− pairs observed in the 8Be nuclear transitions. The search is conducted in the charmonium decays χcJ → XJ=ψðJ ¼ 0; 1; 2Þ via the radiative transition ψð3686Þ → γχcJ using ð2712.4 ± 14.3Þ × 106 ψð3686Þ events collected with the BESIII detector at the BEPCII collider. No significant signal is observed, and the updated upper limit on the coupling strength of charm quark and the new gauge boson, ϵc, is set to be jϵcj < 1.2 × 10−2 at 90% confidence level. We also report new constraints on the mixing strength ϵ between the Standard Model photon and dark photon γ0 in the mass range from 5 to 300 MeV=c2. The upper limits at 90% confidence level vary within ð2.5–17.5Þ × 10−3 depending on the γ0 mass.