Search for a heavy dark photon at future e+e− colliders

A coupling of a dark photon A′ from a U(1)A′ with the standard model (SM) particles can be generated through kinetic mixing represented by a parameter ϵ. A non-zero ϵ also induces a mixing between A′ and Z if dark photon mass mA′ is not zero. This mixing can be large when mA′ is close to mZ even if the parameter ϵ is small. Many efforts have been made to constrain the parameter ϵ for a low dark photon mass mA′ compared with the Z boson mass mZ . We study the search for dark photon in e+e− → γA′ → γμ+μ− for a dark photon mass mA′ as large as kinematically allowed at future e+e− colliders. For large mA′, care should be taken to properly treat possible large mixing between A′ and Z. We obtain sensitivities to the parameter ϵ for a wide range of dark photon mass at planed e+e− colliders, such as Circular Electron Positron Collider (CEPC), International Linear Collider (ILC) and Future Circular Collider (FCC-ee). For the dark photon mass 20 GeV ≲ mA′ ≲ 330 GeV, the 2σ exclusion limits on the mixing parameter are ϵ ≲ 10−3-10−2. The CEPC with s=240\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt{s}=240 $$\end{document} GeV and FCC-ee with s=160\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt{s}=160 $$\end{document} GeV are more sensitive than the constraint from current LHCb measurement once the dark photon mass mA′ ≳ 50 GeV. For mA′ ≳ 220 GeV, the sensitivity at the FCC-ee with s=350\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt{s}=350 $$\end{document} GeV and 1.5 ab−1 is better than that at the 13 TeV LHC with 300 fb−1, while the sensitivity at the CEPC with s=240\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt{s}=240 $$\end{document} GeV and 5ab−1 can be even better than that at 13TeV LHC with 3ab−1 for mA′ ≳ 180 GeV. We also comment on sensitivities of e+e− → γA′ with dark photon decay into several other channels at future e+e− colliders.