Such a model is undoubtedly economical and very predictive, by reason of the small number of degrees of freedom it contains, but it is also strongly constrained. Thus, one may argue that no new physics is needed above the electroweak scale to explain neutrino masses, baryogenesis, and dark matter, defining the so-called “ ν minimal standard model” ( νMSM). Light active neutrino masses are easily generated, via the seesaw mechanism, provided the sterile neutrino masses are significantly larger than about 1 eV. The minimal model addressing these three issues requires one sterile neutrino N 1 at the keV scale as dark matter candidate, and two additional sterile neutrinos N 2, 3 for leptogenesis, which is induced either by N-decays, for sterile neutrino masses above the TeV scale, or by N-oscillations, for sterile neutrino masses at the GeV scale. Thus, sterile neutrinos can provide a simple solution to the three open problems of the SM: neutrino masses, the baryon asymmetry, and the dark matter. In that case, they are naturally long-lived so that, unlike dark matter candidates at the electroweak scale, no additional symmetries are required to stabilize them. On the phenomenological side, they provide a non-vanishing mass to the active SM neutrinos, they allow to realize baryogenesis via leptogenesis, and they are also a viable candidate for dark matter, as long as their mass lies in the keV range. On the theoretical side, they are a prediction of left-right symmetric theories, they allow to gauge B - L by removing its anomaly and they are necessary in S O ( 10 ) grand unification. Sterile neutrinos, that is, fermions singlet under the S U ( 3 ) × S U ( 2 ) × U ( 1 ) gauge symmetry, are a very well-motivated extension of the standard model (SM). This model thus describes a minimal, testable scenario for neutrino masses, the baryon asymmetry, and dark matter. Experimental signatures of this scenario include an X-ray line from dark matter decays, and the direct production of δ + at the LHC. The lepton asymmetry is generated either by N 2, 3-decays for masses M 2, 3 ≳ TeV, or by N 2, 3-oscillations for M 2, 3 ∼ GeV. In addition, we demonstrate that, thanks to the couplings between the heavier sterile neutrinos N 2, 3 and δ +, baryogenesis via leptogenesis can be realized close to the electroweak scale. Instead, N 1 is produced, via freeze-in, by the decays of δ + while it is in equilibrium in the early Universe. Specifically, we propose a new production mechanism for the dark matter particle-a multi-keV sterile neutrino, N 1-that does not depend on the active-sterile mixing angle and does not rely on a large primordial lepton asymmetry. We show that novel paths to dark matter generation and baryogenesis are open when the standard model is extended with three sterile neutrinos N i and a charged scalar δ +.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |