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 epsilon. A non-zero also induces a mixing between A' and Z if dark photon mass m(A') is not zero. This mixing can be large when m(A') is close to m(z) even if the parameter epsilon is small. Many efforts have been made to constrain the parameter for a low dark photon mass m(A') compared with the Z boson mass m(z). We study the search for dark photon in e(+)e(-) -> gamma A' -> gamma mu(+)mu(-) for a dark photon mass m(A'), as large as kinematically allowed at future e(+)e(-) colliders. For large m(A'), care should be taken to properly treat possible large mixing between A' and Z. We obtain sensitivities to the parameter for a wide range of dark photon mass at planed e(+)e(-) colliders, such as Circular Electron Positron Collider (CEPC), Intemational Linear Collider (ILC) and Future Circular Collider (FCC-ee). For the dark photon mass 20 GeV less than or similar to m(A') less than or similar to 330 GeV, the 2 sigma exclusion limits on the mixing parameter are epsilon less than or similar to 10(-3)-10(-2). The CEPC with root s = 240 GeV and FCC-cc with root s = 160 GeV are more sensitive than the constraint from current LHCb measurement once the dark photon mass m(A') greater than or similar to 50 GeV. For m(A') greater than or similar to 220 GeV, the sensitivity at the FCC-cc with root s = 350 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 root s = 240 GeV and 5 ab(-1) can be even better than that at 13 TeV LHC with 3 ab(-1) for m(A') greater than or similar to 180 GeV. We also comment on sensitivities of e(+)e(-) -> gamma A' with dark photon decay into several other channels at future e(+)e(-) colliders.