Search for new light vector boson using $J/\Psi$ at BESIII and Belle II

Abstract

We investigate various search strategies for light vector boson $X$ in $\mathcal{O}(10)\mathrm{MeV}$ 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 at BESIII, (ii) $J/\Psi ({\eta }_c+X)+\ell \bar{\ell }$ production at Belle~II, and (iii) $J/\Psi +X$ with the displaced vertex in $X\to e^{+}{e}^{-}$ decay are analyzed and the future sensitivities at Belle II with 50 ${\mathrm{ab}}^{-1}$ 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, $10^{-4}\le |{\epsilon }_e|\le 10^{-3}$ and $9\mathrm{MeV}\le m_X\le 100\mathrm{MeV}$ with 50 ${\mathrm{ab}}^{-1}$ at Belle II. This covers the favored signal region of ${}^8{\mathrm{Be}}^{\ast }$ anomaly recently reported by Atomki experiment with $m_X\simeq 17\mathrm{MeV}$.

16 pages, 8 figures, and 6 tables

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