
5
where ναis the SM neutrino of flavor α=e, µ, τ,σµν =
i
2[γµ, γν], PL= (1 −γ5)/2, Fµν =∂µAν−∂νAµis the
electromagnetic field strength tensor, and Nis a Heavy
Neutral Lepton (HNL).
Note that this Lagrangian is not gauge-invariant and
therefore valid only at energies below the EW scale,
above which we need to consider UV-completions of the
operator in Eq. (12). Here we remain agnostic about
the UV origin of this new interaction and study its phe-
nomenological implications at energies below the EW
scale.
Motivated by the unsolved MiniBooNE [26],
ANITA [27,28], and muon g-2 anomalies [29,30],
the dipole portal provides another way to test the
existence of HNLs and has attracted a lot of attention
recently [15,16,31–48]. Bounds on dαcome from various
laboratory, astrophysical and cosmological observations.
Laboratory constraints come from neutrino oscillation
experiments, dark matter detectors, and the observa-
tion of high-energy neutrinos in neutrino telescope by
studying coherent elastic neutrino-nucleus scattering,
elastic neutrino-electron scattering, deep inelastic inter-
actions, etc. Astrophysical constraints on dαarise from
supernova bursts, Big Bang Nucleosynthesis, or Cosmic
Microwave Background. We refer to Refs. [13,14,46]
for a compilation of various constraints.
A. Phenomenology at DUNE
The DUNE sensitivity to HNLs with a dipole por-
tal interaction has been studied previously in Refs. [15,
16]. The HNL production mechanism studied there was
mainly quasi-elastic (QE) neutrino up-scattering [13],
να+T→N+T, (13)
where T=e, n/p, Ar or atomic nuclei in the crust along
the trajectory of the neutrino beam.
Here, we will include also the deep-inelastic (DIS) con-
tribution, να+p/n →N+X, where Xis an arbitrary
hadronic state, see Fig. 3(a). Furthermore, in addition to
the neutrino up-scattering, we consider the production of
HNLs by decays of short-lived and long-lived mesons [13],
π±,0, K±,0, η, ρ0, see Fig. 3(b), (c) for two example dia-
grams. A detailed discussion of the relevant cross sections
and production rates is given in Appendix B 1. The HNL
production may occur either inside the detector (neutrino
up-scattering) or outside it (both neutrino up-scattering
and meson decays). In the latter case, HNLs need to
reach the detector in order to decay inside it and hence
be detected. As discussed in detail in Appendix B1d, in
most cases the production from meson decays plays only
a sub-leading role and the main production channel is ei-
ther inside or outside up-scattering. The only exception
is HNL production via dτat the ND.
The main HNL decay channels are
N→γ+να, N →l++l−+να,(14)
with l=e, µ. The dominant channel is the mono-photon
channel, with the decay width being
ΓN→ναγ=|dα|2m3
N
4π(15)
for Dirac neutrinos.
Above the di-electron and di-muon mass threshold,
the leptonic channel becomes available. This channel
has been considered in ref. [35] in the context of test-
ing the dipole portal at FASER and in ref. [40] in the
context of the T2K near detector. These processes are
sub-dominant. However, they have the advantage that it
is possible to reconstruct the decay vertex since we have
two charged particles. In the limit mN2ml, the decay
width behaves as
ΓN→ναl+l−≈αEM|dα|2m3
N
12π2log m2
N
m2
l−3(16)
The branching ratios of the leptonic decay modes are
shown in Fig. 4. At large masses mNml, the
suppression of the leptonic decay width with a factor
αEM/3π'10−3compared to the photon channel (15)
gets partially compensated by the logarithm.
The two decay modes lead to different experimental
signatures and imply different requirements for back-
ground rejection. Furthermore, if both decay channels
can be observed, their ratio is a specific prediction of the
model, serving as a smoking-gun signature.
The combination of the different production and decay
processes leads to different signatures of the dipole portal
at DUNE:
1. Monophoton – an event consisting of a single iso-
lated photon appearing inside the detector. This
type of events occur when the HNL is produced
outside of the detector (via e.g. decays of mesons
or by neutrino up-scatterings), then enters the de-
tector, and decays through N→ν+γ.
2. Double-bang – an event inside the detector con-
sisting of two vertices: the one with recoil matter
particles (electrons, nucleons, nuclei) and the one
with a displaced monophoton or a pair of charged
leptons [16,32]. This type of signature appears if
a HNL is produced inside the detector via neutrino
up-scattering, and then travels a distance larger
than the DUNE spatial resolution, which is of order
∆lDUNE '1 cm [17].2
2Whether the recoil particle (and hence the HNL production
point) would be detected depends on the recoil energy of the
target particles. If it is below the DUNE energy detection thresh-
old, it will be not visible. In this case, instead of the double-bang
event, one would see a monophoton from the decaying HNL. In
our current estimates, we assume ideal recoil energy reconstruc-
tion efficiency.