
Physics Letters B 783 (2018) 76–81
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Physics Letters B
www.elsevier.com/locate/physletb
Signatures of self-interacting dark matter in the matter power
spectrum and the CMB
Ran Huo
a,∗
, Manoj Kaplinghat
b
, Zhen Pan
c
, Hai-Bo Yu
a
a
Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA
b
Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
c
Department of Physics, University of California, Davis, CA 95616, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
22 April 2018
Accepted
5 June 2018
Available
online 19 June 2018
Editor:
H. Peiris
We consider a self-interacting dark matter model in which the massive dark photon mediating the
self-interaction decays to light dark fermions to avoid over-closing the universe. We find that if the
model is constrained to explain the dark matter halos inferred for spiral galaxies and galaxy clusters
simultaneously, there is a strong indication that dark matter is produced asymmetrically in the early
universe. It also implies the presence of dark radiation, late kinetic decoupling for dark matter, and a
suppressed linear power spectrum due to dark acoustic damping. The Lyman-α forest power spectrum
measurements put a strong upper limit on the damping scale and the model has little room to reduce
the abundances of satellite galaxies. Future observations in the matter power spectrum and the CMB, in
tandem with the impact of self-interactions in galactic halos, makes it possible to measure the gauge
coupling and masses of the dark sector particles even when signals in conventional dark matter searches
are absent.
© 2018 The Author(s). 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
The existence of dark matter (DM) in the universe is inferred
from its gravitational influence on normal matter. Null results in
terrestrial DM searches have put strong constraints on the DM in-
teraction
with the standard model particles, e.g., [1–3]. However,
it does not preclude the possibility that DM may interact strongly
with itself [4,5]. Strong DM self-interactions can change the inner
halo structure, leading to a better agreement with small-scale ob-
servations
than the cold DM (CDM) model (see [6]for a review and
reference therein). In particular, kinetic thermalization due to the
DM self-collisions ties the baryonic and DM distributions in galax-
ies
together [7,8]. It has been shown [9,10] that this can lead to
the observed diversity in rotation curves of low and high surface
brightness galaxies [11,12].
A
generic feature of self-interacting DM models is the existence
of a light force carrier for mediating strong DM self-interactions
in galactic halos. A mass hierarchy between the mediator and the
DM particle is required to get a self-scattering cross section that
decreases for velocities of O (1000 km/s) (cluster scale) [5]. This
mediator must decay to avoid the over-closing the universe [13,14],
*
Corresponding author.
E-mail
address: ran.huo@ucr.edu (R. Huo).
unless it is (almost) massless (see, e.g., [15,16]). The minimal mod-
els
where the mediator decays to standard model particles have
been strongly constrained by DM direct detection experiments [14,
17,18],
since the DM-nucleus scattering cross section is enhanced
due to the smallness of the mediator mass. In addition, the s-wave
DM annihilation can be boosted, resulting in strong constraints
from indirect detection experiments [19–21].
A
simple solution is to introduce a massless particle species ( f )
in the dark sector, in addition to the DM particle (χ ) and the me-
diator
(φ). In the early universe, φ can be in thermal equilibrium
with f , so that its number density becomes Boltzmann suppressed
when the temperature is below its mass, avoiding the over-closure
problem. Since φ is not necessary to couple to the standard model
in this case, conventional DM signals can be absent. Aside from
usual χ–χ self-scattering, φ also mediates χ– f collisions in the
early thermal bath. A tight coupling between matter and radia-
tion
in the early universe will lead to a cutoff in the linear matter
power spectrum [15,22–33]. This model was invoked previously to
simultaneously flatten the density profiles of dwarfs and reduce
their abundances [27,34,35].
In
this Letter, we use this model to explicitly demonstrate how
astrophysical observations can pin down the particle physics pa-
rameter
space. After constraining it to explain the dark matter
halos inferred for dwarf galaxies and galaxy clusters, we explore
https://doi.org/10.1016/j.physletb.2018.06.024
0370-2693/
© 2018 The Author(s). 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
.