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

Abstract

Abstract 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 m A ′ is not zero. This mixing can be large when m A ′ is close to m Z even if the parameter ϵ is small. Many efforts have been made to constrain the parameter ϵ 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 − → γA ′ → γμ + μ − 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 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 ≲ m A ′ ≲ 330 GeV, the 2σ exclusion limits on the mixing parameter are ϵ ≲ 10−3-10−2. The CEPC with $$ \sqrt{s}=240 $$ s = 240 GeV and FCC-ee with $$ \sqrt{s}=160 $$ s = 160 GeV are more sensitive than the constraint from current LHCb measurement once the dark photon mass m A ′ ≳ 50 GeV. For m A ′ ≳ 220 GeV, the sensitivity at the FCC-ee with $$ \sqrt{s}=350 $$ 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 $$ \sqrt{s}=240 $$ s = 240 GeV and 5ab−1 can be even better than that at 13TeV LHC with 3ab−1 for m A ′ ≳ 180 GeV. We also comment on sensitivities of e + e − → γA ′ with dark photon decay into several other channels at future e + e − colliders.