Search for New Light Vector Boson Using J/Ψ at BESIII and Belle II

We investigate various search strategies for light vector boson X in $$ \mathcal{O} $$ (10) MeV mass range using J/Ψ associated channels at BESIII and Belle II: (i) J/Ψ → ηcX with 1010J/Ψs at BESIII, (ii) J/Ψ(ηc + X) + $$ \mathrm{\ell}\overline{\mathrm{\ell}} $$ production at Belle II, and (iii) J/Ψ + X with the displaced vertex in X → e+e− decay are analyzed and the future sensitivities at Belle II with 50 ab−1 luminosity are comprehensively studied. By requiring the displaced vertex to be within the beam pipe, the third method results in nearly background-free analysis, and the vector boson-electron coupling and the vector boson mass can be probed in the unprecedented range, 10−4 ≤ |εe| ≤ 10−3 and 9 MeV ≤ mX ≤ 100MeV with 50 ab−1 at Belle II. This covers the favored signal region of 8Be* anomaly recently reported by Atomki experiment with mX ≃ 17 MeV.