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

Abstract：

Dark sector may couple to the Standard Model via one or more mediator particles. We discuss two types of mediators: the dark photon $ A^{\prime} $ and the dark scalar mediator $ \phi $ . 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 bquarks) are discussed. We focus on the study of the invisible $ A^{\prime} $ due to the cleaner background at future $ 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^{\prime}} < $ 60 GeV) in less than one operating year. The lower limits of the integrated luminosity for the significance $ S/\sqrt{B} = $ 2 $ \sigma $ , 3 $ \sigma $ and 5 $ \sigma $ are presented.