where F
0
µν
, H
h
, and N are respectively hidden sector field strengths, Higgses, and fermions.
Typically the impact of each of these portals is separately treated, as each one leads to
distinct search strategies.
In this paper we study the impact of the photon portal for light DM, in which the
SM photon kinetically mixes with a U(1) dark photon [1]. The implications of γ − γ
0
kinetic mixing for DM has been widely studied [2–8]. At the phenomenological level, the
photon portal gives rise to two main classes of probes: (1) direct detection signals from
DM-proton or DM-electron scattering, and (2) the production of DM at accelerators and
colliders. Given the strong direct detection constraints, we will focus on the sub-GeV
regime for DM. Notice that the strength of the direct detection constraints for > GeV DM
masses is partly thanks to the coherent enhancement of the DM scattering on the nucleus.
In light of the recent discovery of coherent neutrino-nucleus scattering [9] by the
COHERENT collaboration, we ask what the COHERENT data brings to bear on pho-
ton portal models of light DM. The possibility of producing and detecting light DM at
coherent neutrino-nucleus experiments was studied in [10]. We additionally study the abil-
ity of reactor neutrino experiments like TEXONO to constrain light DM from their electron
recoil events. The mass reach of TEXONO extends to ∼ MeV masses, while COHERENT’s
stopped pion source can access DM masses out to ∼ 65 MeV.
The remainder of this paper is organized as follows. In section 2 we introduce the
model of study with a kinetically mixed dark photon interacting with pairs of DM par-
ticles. In section 3 we examine the sensitivity at TEXONO to dark photons produced
via, γe
−
→ V
0
e
−
, with V
0
decaying to DM which then produces electronic recoil events.
In section 4 we look at the sensitivity at COHERENT to producing dark photons from
neutral pion decay. At COHERENT the rate is dominated by the coherently enhanced
nuclear recoil events. In section 5 we show the derived COHERENT constraints on light
DM in the context of the existing constraints on light DM finding that COHERENT ex-
cludes previously allowed thermal relic parameter space for . 30 MeV masses. Finally in
section 6 we conclude and comment on the potential for future limits on the model.
2 Light DM with dark photon portal
We assume that the hidden sector U(1) gauge group spontaneously breaks to give the dark
photon V
0
µ
a mass. Then the relevant terms of the Lagrangian for DM interacting with a
kinetically mixed photon are
L ⊃ L
X
−
1
4
F
0
µν
F
0µν
+
1
2
m
2
V
0
V
0µ
V
0
µ
− F
µν
F
0µν
(2.1)
where the DM portion of the Lagrangian is
L = i
¯
X
/
DX − m
X
¯
XX (2.2)
with D
µ
≡ ∂
µ
− ig
X
V
0
µ
being the covariant derivative and g
X
the gauge coupling. Strictly
speaking we suppose that the dark photon kinetically mixes with the SM hyper-charge
field strength, which then induces mixing with both the Z boson and the SM photon after
– 2 –