Production and constraints for a massive dark photon at electron-positron colliders

Dark sector may couple to the Standard Model via one or more mediator particles. We discuss two types of mediators: the dark photon $A^{\mathrm{\prime }}$ and the dark scalar mediator $\varphi$. The total cross-sections and various differential distributions of the processes $e^{+} e^{-} \rightarrow q \bar{q} A^{\prime}$and$e^{+} e^{-} \rightarrow q \bar{q} \phi$($q=u,~d,~c,~s$and$b$quarks)arediscussed\mathrm{.}Wefocusonthestudyoftheinvisible$A^{\prime}$duetothecleanerbackgroundatfuture$e^{+} e^{-}$ colliders. It is found that the kinematic distributions of the two-jet system could be used to identify (or exclude) the dark photon and the dark scalar mediator, as well as to distinguish between them. We further study the possibility of a search for dark photons at a future CEPC experiment with $\sqrt{s}=$ 91.2 GeV and 240 GeV. With CEPC running at $\sqrt{s}=$ 91.2 GeV, it would be possible to perform a decisive measurement of the dark photon (20 GeV $<m_{A^{\mathrm{\prime }}}<$ 60 GeV) in less than one operating year. The lower limits of the integrated luminosity for the significance $S\mathrm{/}\sqrt{B}=$ 2$\sigma$, 3$\sigma$ and 5$\sigma$ are presented.