Sterile neutrino

Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that are believed to interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term sterile neutrino is used to distinguish them from the known, ordinary active neutrinos in the Standard Model, which carry an isospin charge of ±1/± and engage in the weak interaction. The term typically refers to neutrinos with right-handed chirality (see right-handed neutrino), which may be inserted into the Standard Model. Particles that possess the quantum numbers of sterile neutrinos and masses great enough such that they do not interfere with the current theory of Big Bang nucleosynthesis are often called neutral heavy leptons (NHLs) or heavy neutral leptons (HNLs). The existence of right-handed neutrinos is theoretically well-motivated, because the known active neutrinos are left-handed and all other known fermions have been observed with b

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Freeze-in and freeze-out generation of lepton asymmetries after baryogenesis in the nu MSM

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The nu MSM an extension of the Standard Model by three relatively light singlet Majorana fermions N-1,N-2,N-3 allows for the generation of lepton asymmetry which is several orders of magnitude larger than the observed baryon asymmetry of the Universe. The lepton asymmetry is produced in interactions of N-2,N-3 (with masses in the GeV region) at temperatures below the sphaleron freeze out T less than or similar to 130 GeV and can enhance the cosmological production of dark matter (DM) sterile neutrinos N-1 (with the mass of the keV scale) happening at T similar to 200 MeV due to active-sterile neutrino mixing. This asymmetry can be generated in freeze-in, freeze-out, or later in decays of heavy neutral leptons. In this work, we address the question of the magnitude of the late-time asymmetry (LTA) generated by the heavy neutral leptons N-2,N-3 during their freeze-in and freeze-out, leaving the decays for later work. We study how much of this asymmetry can survive down to the lower temperatures relevant for the sterile neutrino DM creation. We find that this LTA could result in the production of a sizeable fraction of dark matter. We also examine a role played by magnetic fields and the Abelian chiral anomaly in the generation of LTA, not accounted for in the previous studies. We argue that the production of LTA can be increased significantly and make an estimate of the influence of this effect.

IOP Publishing Ltd2022

On neutrinoless double beta decay in the nu MSM

Shintaro Eijima

We consider the neutrinoless double beta (0 nu beta beta) decay in the nu MSM, in which three right-handed neutrinos with masses below the electroweak scale are additionally introduced to the Standard Model. In this model there appear three heavy neutral leptons N-1, N-2, and N-3 corresponding to right-handed neutrinos. It has been known that the lightest one N-1 with keV mass, which is a candidate for dark matter, gives a negligible contribution to the 0 nu beta beta decay. By contrast, the heavier ones N-2 and N-3, which are responsible to the seesaw mechanism of neutrino masses and baryogenesis, give the destructive contribution (compared with one from active neutrinos). This is because their mass degeneracy at high precision has been assumed, which is expected by analytical studies of baryogenesis. In this analysis, we find that the effective mass of the 0 nu beta beta decay becomes larger than one from active neutrinos due to the N-2 and N-3 constructive contribution when the mass difference becomes larger and the mass ordering of active neutrinos is inverted. Such a possibility will be explored by the current and near future experiments of the 0 nu beta beta decay. (C) 2016 The Author(s). Published by Elsevier B.V.

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Sterile Neutrinos as the Origin of Dark and Baryonic Matter

Laurent Canetti, Marco Drewes

We demonstrate for the first time that three sterile neutrinos alone can simultaneously explain neutrino oscillations, the observed dark matter, and the baryon asymmetry of the Universe without new physics above the Fermi scale. The key new point of our analysis is leptogenesis after sphaleron freeze-out, which leads to resonant dark matter production, evading thus the constraints on sterile neutrino dark matter from structure formation and x-ray searches. We identify the range of sterile neutrino properties that is consistent with all known constraints. We find a domain of parameters where the new particles can be found with present day experimental techniques, using upgrades to existing experimental facilities. DOI: 10.1103/PhysRevLett.110.061801

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