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 O(10) MeV mass range using J/Ψ associated channels at BESIII and Belle II: (i) J/Ψ → η_cX with 10¹⁰J/Ψs at BESIII, (ii) J/Ψ(η_c + X) + ℓ ℓ ¯ 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 ≤ |ε_e| ≤ 10⁻³ and 9 MeV ≤ m_X ≤ 100MeV with 50 ab⁻¹ at Belle II. This covers the favored signal region of ⁸Be^* anomaly recently reported by Atomki experiment with m_X ≃ 17 MeV.