Physics Letters B 765 (2017) 127–131
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Physics Letters B
www.elsevier.com/locate/physletb
Polarization of photons emitted by decaying dark matter
W. Bonivento
a
, D. Gorbunov
b,c
, M. Shaposhnikov
d
, A. Tokareva
b,d,∗
a
Sezione INFN di Cagliari, Cagliari, Italy
b
Institute for Nuclear Research of Russian Academy of Sciences, 117312 Moscow, Russia
c
Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Russia
d
Ecole Polytechnique Fédérale de Lausanne, CH-1015, Lausanne, Switzerland
a r t i c l e i n f o a b s t r a c t
Article history:
Received
24 October 2016
Received
in revised form 17 November 2016
Accepted
23 November 2016
Available
online 30 November 2016
Editor:
A. Ringwald
Radiatively decaying dark matter may be searched through investigating the photon spectrum of galaxies
and galaxy clusters. We explore whether the properties of dark matter can be constrained through the
study of a polarization state of emitted photons. Starting from the basic principles of quantum mechanics
we show that the models of symmetric dark matter are indiscernible by the photon polarization.
However, we find that the asymmetric dark matter consisted of Dirac fermions is a source of circularly
polarized photons, calling for the experimental determination of the photon state.
© 2016 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
Dark matter (DM) particles can be unstable with a life-time ex-
ceeding
that of the Universe. If these particles have a radiative
decay mode, the emission of an almost monochromatic photon
is a specific signature allowing to search for them in astrophysi-
cal
observations. Similar imprint can also come from two-photon
annihilation of DM states. The examples of DM particles produc-
ing
photon(s) include sterile neutrino with the mass in the keV
range [1–5] (see also [6,7] for reviews), axions and axionlike parti-
cles
[8–10], sgoldstinos [11], majorons [12], axinos [13], gravitinos
[14], self-annihilating particles [15] and transitions between two
dark matter states [16]. Some (controversial) indications in favour
of these types of DM particles came recently from the analysis of
photon spectra emitted by dif ferent astrophysical objects in X-ray
region (an unidentified 3.5 keV line) and were reported in [17,18].
1
This unidentified X-ray line may have no connection with the
dark sector and come from some atomic transition, see the discus-
sion
[21]. Namely, there are several atomic lines near the 3.5 keV
feature: potassium K XVIII lines at 3.48 and 3.52 keV, and charge
exchange induced line of sulfur, S XVI at 3.47 keV. The situation
remains unclear because it is hard to estimate accurately the flux
of photons coming from these transitions, given the lack of pre-
cise
knowledge of the chemical composition and temperature of
the cosmic plasma. However, future observations with enhanced
*
Corresponding author.
E-mail
address: tokareva@ms2.inr.ac.ru (A. Tokareva).
1
For the current status of this line and future prospects see [7,19,20].
spectral resolution should be able to distinguish dark matter and
atomic lines.
But
one can address a question: even if the dark matter na-
ture
of the line would be proved, how can we decide what type
of dark matter particle produces this line? For example, can we
distinguish between a boson and a fermion? Of course, if we had
a possibility to catch and study all decay products of hypothetical
dark matter particle then we would be able to determine the spin
and parity of the original particle. Unfortunately, we can register
only one particle—photon—because another decay product flies in
the opposite direction.
Clearly,
besides the energy photons may carry another infor-
mation
encoded in their polarization state. The aim of this paper
is to address the question whether the polarization measurements
can help to constrain the properties of the dark matter.
2
We will
show that the quantum mechanical state of each arriving individ-
ual
photon may be different for the different types of dark matter
particles. Still, we will demonstrate that it is in principle not pos-
sible
to make such measurements that could determine the spin of
dark matter particle if it is symmetric (i.e. contains equal numbers
of particles and antiparticles).
We
will show, however, that the case of asymmetric dark mat-
ter
consisting of Dirac fermions is different. Namely, it provides
a circularly polarized photons when decaying to the photon and
the fermion (e.g. neutrino). This circular polarization, in principle,
2
For discussion of photon polarizations in collider experiments and in indirect
searches see [22] and [23,24], respectively.
http://dx.doi.org/10.1016/j.physletb.2016.11.048
0370-2693/
© 2016 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
.