可见的无菌中微子是最早的宇宙学遗迹探针
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JID:PLB AID:135113 /SCO Doctopic: Astrophysics and Cosmology [m5Gv1.3; v1.261; Prn:28/11/2019; 12:32] P.1 (1-6)
Physics Letters B ••• (••••) ••••••
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Visible sterile neutrinos as the earliest relic probes of cosmology
Graciela B. Gelmini, Philip Lu, Volodymyr Takhistov
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
22 September 2019
Received
in revised form 20 November 2019
Accepted
20 November 2019
Available
online xxxx
Editor:
H. Peiris
A laboratory detection of a sterile neutrino could provide the first indication of the evolution of the
Universe before Big-Bang Nucleosynthesis (BBN), an epoch yet untested. Such “visible” sterile neutrinos
are observable in upcoming experiments such as KATRIN/TRISTAN and HUNTER in the keV mass range
and PTOLEMY and others in the eV mass range. A set of standard assumptions is typically made
about cosmology before the temperature of the Universe was 5 MeV. However, non-standard pre-BBN
cosmologies based on alternative assumptions could arise in motivated theoretical models and are
equally in agreement with all existing data. We revisit the production of sterile neutrinos of mass
0.01 eV to 1 MeV in two examples of such models: scalar-tensor and low reheating temperature pre-
BBN
cosmologies. In both of them, the putative 3.5 keV X-ray signal line corresponds to a sterile
neutrino with a mixing large enough to be tested in upcoming laboratory experiments. Additionally, the
cosmological/astrophysical upper limits on active-sterile neutrino mixings are significantly weaker than
in the standard pre-BBN cosmology, which shows that these limits are not robust. For example, in the
scalar-tensor case the potential signal regions implied by the LSND and MiniBooNE short-baseline as well
as the DANSS and NEOS reactor experiments are entirely not bound by cosmological restrictions. Our
work highlights that sterile neutrinos may constitute a sensitive probe of the pre-BBN epoch.
© 2019 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
.
The cosmological evolution of the Universe before its temper-
ature
was T = 5MeV is unknown because we have not detected
so far any remnant from it. The earliest cosmological remnants are
so far the light nuclei produced during Big Bang Nucleosynthesis
(BBN) and if the highest temperature of the radiation-domination
epoch in which BBN happened was just 5 MeV, BBN and all the
subsequent evolution of the Universe would be unchanged [1–6].
Assumptions are made about cosmology to compute the relic
abundance and momentum distributions of dark matter (DM) par-
ticles
which are produced in this pre-BBN epoch. The standard
assumptions are that the Universe was radiation-dominated, that
only Standard Model (SM) particles are present and that no extra
entropy in matter and radiation is produced. We call this set of
assumptions the standard pre-BBN cosmology, which is an exten-
sion
at higher temperatures of the standard cosmology we know
at lower temperatures, T < 5MeV. However, cosmologies based
on alternative assumptions are equally in agreement with all ex-
isting
data. The pre-BBN cosmological evolution could drastically
differ from standard in some well motivated theoretical models,
e.g. some based on moduli, extra dimensions or quintessence. A
E-mail addresses: gelmini@physics.ucla.edu (G.B. Gelmini), philiplu11@gmail.com
(P. Lu),
vtakhist@physics.ucla.edu (V. Takhistov).
non-standard cosmological evolution, consistent with all existing
bounds, could drastically affect the properties of any relics pro-
duced
before the temperature of the Universe was 5 MeV. Detect-
ing
any relics sensitive to the pre-BBN cosmological history will
open a new window into this yet unexplored epoch. The SM pre-
dicts
that the three active neutrinos ν
α
, α = e, μ, τ , coupled to the
W and Z weak gauge bosons are massless. The discovery of neu-
trino
oscillations [7] confirmed that neutrinos are massive and led
to detailed studies of neutrino scenarios beyond the three-flavor
paradigm of the SM. A promising explanation of the non-zero ac-
tive
neutrino masses involves minimally extending the SM to in-
clude
one or more additional “sterile” neutrinos ν
s
which do not
interact weakly and in minimal scenarios only mix with the ν
α
.
For simplicity, here we assume a ν
s
that has a mixing sin θ with
only one ν
α
, and we take α = e in our figures. Sterile neutrinos
with mass of O (keV) and a spectrum close to thermal constitute
a viable Warm DM (WDM) candidate (see e.g. [8]).
Sterile
neutrinos without additional interactions beyond the SM
are produced in the early Universe through active-sterile flavor
oscillations and collisional processes. In the absence of a large
lepton asymmetry the oscillations are non-resonant, and the an-
alytic
solution for the relic number density of sterile neutrinos
produced through this mechanism was first obtained by Dodelson
https://doi.org/10.1016/j.physletb.2019.135113
0370-2693/
© 2019 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
.
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