Sterile neutrino dark matter: impact of active-neutrino opacities
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Springer
Received: May 13, 2020 Accepted: July 6, 2020 Published: July 29, 2020
Dietrich B¨ odeker and Alexander Klaus Fakult¨ at f¨ ur Physik, Universit¨ at Bielefeld, 33501 Bielefeld, Germany
E-mail: [email protected], [email protected] Abstract: The resonant production of keV sterile-neutrino dark matter mainly takes place during the QCD epoch of the early universe. It has been argued that it could be strongly affected by the opacities (or damping rates) of active neutrinos, which receive nonperturbative QCD-contributions. We find that for lepton asymmetries nLα /s below 10−6 the opacities significantly affect the sterile-neutrino yield, but that for larger asymmetries, which are necessary for producing a significant fraction of the dark matter, the yield is insensitive to changes of the opacities. Thus non-perturbative QCD contributions to the opacities at temperatures around 160 MeV will not affect this dark matter scenario. We obtain larger sterile-neutrino yields than previous studies, and thus weaker lower limits on the active-sterile mixing angle from Big Bang Nucleosynthesis. Keywords: Cosmology of Theories beyond the SM, Thermal Field Theory ArXiv ePrint: 2005.03039
c The Authors. Open Access, Article funded by SCOAP3 .
https://doi.org/10.1007/JHEP07(2020)218
JHEP07(2020)218
Sterile neutrino dark matter: impact of active-neutrino opacities
Contents 1 Introduction
1
2 Non-equilibrium evolution equations 2.1 Setup 2.2 Equations of motion 2.3 Active-neutrino self-energy
2 2 3 5 7 7 10
4 Summary and conclusions
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A Emergence of resonances with increasing lepton asymmetries
12
1
Introduction
Sterile neutrinos with mass in the keV range have long been discussed a dark matter candidate [1]. Through their mixing with active neutrinos they can be produced from the Standard Model (SM) plasma in the early universe. They are warm dark matter, giving rise to less structure on small scales than cold dark matter, which may solve some problems of the standard ΛCDM model. The detection of small scale structure in the Lyman-α forest results in lower bounds on the mass of the sterile neutrinos [2]. The sterile neutrinos can decay into an active neutrino and a photon, giving rise to monochromatic X-rays. Non-observations of X-ray lines imposes upper limits to the active-sterile mixing angle which depend on the sterile-neutrino mass. The original production scenario proposed by Dodelson and Widrow [1], has already been ruled out as the sole source of dark matter production by combining Lyman-α and X-ray constraints [3–5]. One way to circumvent these constraints was suggested by Shi and Fuller [6]: They assume lepton asymmetries much larger than the observed baryon asymmetry. They lead to resonant production, resulting generally in a non-thermal spectrum, which can be colder than in the DodelsonWidrow scenario (see, however, [7]), and so evade Lyman-α constraints. Furthermore, resonant enhancement makes the production much more efficient, requiring smaller mixing angles and thus
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