Searching for the light leptophilic gauge boson $Z_x$ via four-lepton final states at the CEPC

Abstract

We investigate the possibility of detecting the leptophilic gauge boson $Z_x$ predicted by the $U(1)_{L_e-L_\mu}$ model via the processes $e^+e^-\rightarrow\ell^+\ell^-Z_x(Z_x\rightarrow \ell^+\ell^-)$ and $e^+e^-\rightarrow \ell^+\ell^-Z_x(Z_x\rightarrow \nu_\ell\bar{\nu_\ell})$ at the Circular Electron Positron Collider (CEPC) with the center of mass energy $\sqrt s=240$ GeV and the luminosity $\mathcal{L}=5.6$ $\mathrm{ab^{-1}}$. We give the expected sensitivities of the CEPC to the parameter space at $1\sigma$, $2\sigma$, $3\sigma$ and $5\sigma$ levels.