
Physics Letters B 749 (2015) 283–288
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Physics Letters B
www.elsevier.com/locate/physletb
Production of Sterile Neutrino dark matter and the 3.5 keV line
Alexander Merle
a,b
, Aurel Schneider
c,d,∗
a
Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Föhringer Ring 6, 80805 München, Germany
b
Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, United Kingdom
c
Institute for Computational Science, University of Zürich, Winterthurerstrasse 190, CH-8057, Zürich, Switzerland
d
Department of Physics & Astronomy, University of Sussex, Brighton, BN1 9QH, United Kingdom
a r t i c l e i n f o a b s t r a c t
Article history:
Received
12 May 2015
Accepted
31 July 2015
Available
online 4 August 2015
Editor:
S. Dodelson
The recent observation of an X-ray line at an energy of 3.5 keV mainly from galaxy clusters has initiated
a discussion about whether we may have seen a possible dark matter signal. If confirmed, this signal
could stem from a decaying sterile neutrino of a mass of 7.1 keV. Such a particle could make up all the
dark matter, but it is not clear how it was produced in the early Universe. In this letter we show that
it is possible to discriminate between different production mechanisms with present-day astronomical
data. The most stringent constraint comes from the Lyman-α forest and seems to disfavor all but one of
the main production mechanisms proposed in the literature, which is the production via decay of heavy
scalar singlets. Pinning down the production mechanism will help to decide whether the X-ray signal
indeed comprises an indirect detection of dark matter.
© 2015 The Authors. 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
A dream of particle physicists, cosmologists, and astrophysicists
is to discover the true nature of dark matter (DM), which makes
up more than 80% of the matter in the Universe [1]. The generic
candidate is a Weakly Interacting Massive Particle (WIMP), i.e. a
heavy particle which interacts as weakly as neutrinos. However,
the many recent attempts to directly detect such a particle [2] or
to produce it at colliders [3], as well as the hunts for its annihila-
tion
products [4], have so far not found a clear indication. In this
situation, the detection of an X-ray line in several galaxy clusters
and in the Andromeda galaxy [5,6] has attracted the attention of
the community. This line, if stemming from DM decay, could be a
smoking gun signal for a very different type of DM particle: an ex-
tremely
weakly interacting (“sterile”) neutrino with a mass smaller
than that of a WIMP by about seven orders of magnitude. WIMPs
are produced by thermal freeze-out [7] which means that they de-
couple
from the primordial thermal plasma as soon as the Hubble
expansion becomes larger than the interaction rate. Sterile neutri-
nos
with keV-masses cannot be produced in this way because their
interactions are too weak. However, even very feebly interacting
particles can be gradually produced in the early Universe [8]. For
*
Corresponding author.
E-mail
addresses: amerle@mpp.mpg.de (A. Merle), aurel@physik.uzh.ch
(A. Schneider).
sterile neutrinos this can be achieved by their tiny admixtures θ
to active neutrinos, the so-called Dodelson–Widrow (DW) mecha-
nism [9],
but this is known to produce a too hot spectrum [10,11],
i.e., too fast DM particles. However, active-sterile neutrino tran-
sitions
could be resonantly enhanced if the background medium
carries a net lepton number. This production proposed by Shi
and Fuller [12] seems in a better shape when confronted with
data [13], and it has been recently advocated to be able to pro-
duce
DM in agreement with the line signal [14,15].
What
is the status of the 3.5 keV line? Refs. [5,6] have in-
dependently
reported evidence in samples by the XMM-Newton
and Chandra satellites from nearby clusters and the Andromeda
galaxy (stating > 4σ significance for the stacked signal). These
findings were criticised by Refs. [16,17], who state that Chandra
should see a line signal from the center of the Milky way and that
other chemical lines are able to explain the signal. However, these
remarks were again criticised in Refs. [18–21], arguing that the
centre of the Milky Way is too noisy for a clear signal. Ref. [19]
questions
the range of validity of the background model assumed
in [17]. Finally, Ref. [22] has found no signal in stacked XMM-
Newton
data from dwarf galaxies, which they claim should provide
a clean signal, although the constraint provided is not significantly
more stringent than previous ones [23]. Obviously the situation is
not clear at the moment and more data is required. On the other
hand, the technical development of satellites proceeds slower than
one would like, so that we cannot expect to see a very bright sig-
nal
where none had been seen before. Ultimately, we should take
http://dx.doi.org/10.1016/j.physletb.2015.07.080
0370-2693/
© 2015 The Authors. 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
.