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A
bstract
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
−
→ γ
A
′
→ γ
μ
+
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 2
σ
exclusion limits on the mixing parameter are
ϵ
≲ 10
−3
-10
−2
. The CEPC with
s
=
240
GeV and FCC-ee with
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
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
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
−
→ γ
A
′
with dark photon decay into several other channels at future
e
+
e
−
colliders.