Physics Letters B 802 (2020) 135182
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Viable secret neutrino interactions with ultralight dark matter
James M. Cline
McGill University, Department of Physics, 3600 University St., Montréal, QC H3A2T8, Canada
a r t i c l e i n f o a b s t r a c t
Article history:
Received
9 August 2019
Received
in revised form 24 October 2019
Accepted
18 December 2019
Available
online 9 January 2020
Editor:
B. Grinstein
Several anomalies in neutrino oscillation experiments point to the existence of a ∼ 1eV sterile neutrino
ν
s
mixing with ν
e
at the level of U
e4
∼
=
0.1, but such a neutrino is strongly disfavored by constraints
on additional light degrees of freedom (δN
eff
) and total neutrino mass (
ν
m
ν
) from cosmology. “Secret
neutrino interactions” that have been invoked to suppress the cosmological production of ν
s
typically
falter, but recently it was pointed out that ν
s
could get a large mass in the early universe by coupling
to ultralight dark matter φ, which can robustly suppress its production. The model has essentially
two free parameters: m
φ
, and m
s,0
, the mass of the sterile neutrino at early times, enhanced by its
coupling to φ. I determine the parameter regions allowed by limits on δN
eff
and
ν
m
ν
from the
cosmic microwave background and big bang nucleosynthesis, using a simplified yet accurate treatment of
neutrino oscillations in the early universe. This mechanism could have an important impact on laboratory
experiments that suggest oscillations with sterile neutrinos.
© 2020 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Short baseline (SBL) neutrino oscillation experiments at nu-
clear
reactors suggest at 3σ an eV-scale sterile neutrino ν
s
that
mixes with ν
e
[1–5]. A persistent deficit of low-energy solar ν
e
flux in gallium experiments lends support to this interpretation.
The NEOS [6] and DANSS [7] experiments that also search for ν
e
-ν
s
oscillations observe features that could be consistent with the SBL
anomalies, though are not yet conclusive. Recent fits to the data fa-
vor
a mass m
4
=1.1eV and mixing matrix element U
e4
=0.11 [8].
Moreover there are hints from other experiments, LSND [9] and
MiniBooNE [10], of ν
μ
→ν
e
oscillations via a sterile neutrino with
similar mass and mixing parameters. The sterile neutrino intepre-
tation
of ν
μ
→ ν
e
is clouded by constraints on ν
μ
-ν
s
oscillations
from MINOS [11] and IceCube [12,13]. In this work I focus on the
simpler ν
e
-ν
s
scenario that could explain the SBL deficits. The KA-
TRIN
experiment will provide an independent probe in the near
future [14].
A
generic challenge to the existence of sterile neutrinos in the
indicated mass and mixing range are their oscillations in the early
universe that would fully equilibrate the sterile species [15–17].
This is strongly excluded by big bang nucleosynthesis (BBN) and
cosmic microwave background (CMB) constraints on additional ef-
fective
neutrino species, δN
eff
, as well as the sum of neutrino
E-mail address: jcline@physics.mcgill.ca.
masses
m
ν
. Some means of suppressing oscillations in the early
universe while allowing them at the present time is needed.
The
use of sterile neutrino interactions to inhibit oscillations
has a long history [18–20]. With respect to the current anomalies,
refs. [21,22]suggested that self-interactions of the sterile neutrino
could impede the oscillations and thereby satisfy the cosmologi-
cal
constraints. This mechanism is referred to as “secret neutrino
interactions,” despite the efforts of PRL to censor the name. Subse-
quent
investigation showed that although the self-interactions in
this context could prevent ν
4
production until freezeout of the
active neutrinos, in accordance with bounds on N
eff
, at lower tem-
peratures
their self-scattering combines with oscillations to con-
vert
active neutrinos to ν
4
and violate the CMB bound on
m
ν
.
[23–27]. (An exception is found for self-interactions mediated by a
light gauge boson of mass 10 MeV [28].)
It
was recently pointed out that an effective realization of secret
interactions is to couple ν
s
to ultralight bosonic dark matter φ [29].
In that case the scalar behaves as a coherent condensate, that has
not yet started oscillating at early times. It can easily give a large
mass to ν
s
during this epoch, inhibiting the oscillations. Once the
Hubble rate drops below m
φ
, the field oscillates and redshifts with
scale factor as a
−3/2
as the universe expands. Its contribution to
m
s
quickly disappears, leaving only the bare Lagrangian mass of
∼ 1eV. The “secret interaction” moniker is especially appropriate
in this case, since the required coupling of ν
s
to φ was shown to
be exceedingly weak, λ ∼ 10
−23
. Similar interactions of light dark
https://doi.org/10.1016/j.physletb.2019.135182
0370-2693/
© 2020 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.