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 $\mathcal{O}(10)~{\rm MeV}$ mass range using $J/\Psi$ associated channels at BESIII and Belle II: (i) $J/\Psi \to \eta_c X$ with $10^{10} J/\Psi$s BESIII, (ii) (\eta_c +X) +\ell \bar{\ell}$ production Belle~II, (iii) +X$ the displaced vertex $X\to e^+e^-$ decay are analyzed future sensitivities II 50 ${\rm ab}^{-1}$ luminosity comprehensively studied. By requiring to be within beam pipe, third method results nearly background-free analysis, boson-electron coupling can probed unprecedented range, $10^{-4}\leq |\varepsilon_e| \leq 10^{-3}$ $9~{\rm MeV}\leq m_X\leq 100 {\rm II. This covers favored signal region of $^8{\rm Be}^*$ anomaly recently reported by Atomki experiment $m_X \simeq 17~{\rm MeV}$.