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

TL;DR

This paper explores innovative search strategies for a light vector boson in the 10-100 MeV mass range using $J/$ channels at BESIII and Belle II, aiming to probe new parameter space and address recent anomalies.

Contribution

It introduces new experimental methods, including displaced vertex analysis, to significantly improve sensitivity to light vector bosons at Belle II.

Findings

Nearly background-free analysis for displaced vertices within the beam pipe.

Probing vector boson-electron coupling in the range $10^{-4}$ to $10^{-3}$.

Coverage of the $^8$Be* anomaly favored region around 17 MeV.

Abstract

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 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 ${\rm ab}^{-1}$ 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, $10^{-4}\leq |\varepsilon_e| \leq 10^{-3}$ and $9~{\rm MeV}\leq m_X\leq 100 {\rm MeV}$ with 50 ${\rm ab}^{-1}$ at Belle II. This covers the favored signal region of $^8{\rm Be}^*$ anomaly recently reported by Atomki experiment with $m_X \simeq 17~{\rm MeV}$.