Sterile neutrino dark matter as a consequence of nuMSM-induced lepton asymmetry (0804.4543v2)

Abstract: It has been pointed out in ref.[1] that in the nuMSM (Standard Model extended by three right-handed neutrinos with masses smaller than the electroweak scale), there is a corner in the parameter space where CP-violating resonant oscillations among the two heaviest right-handed neutrinos continue to operate below the freeze-out temperature of sphaleron transitions, leading to a lepton asymmetry which is considerably larger than the baryon asymmetry. Consequently, the lightest right-handed (``sterile'') neutrinos, which may serve as dark matter, are generated through an efficient resonant mechanism proposed by Shi and Fuller [2]. We re-compute the dark matter relic density and non-equilibrium momentum distribution function in this situation with quantum field theoretic methods and, confronting the results with existing astrophysical data, derive bounds on the properties of the lightest right-handed neutrinos. Our spectra can be used as an input for structure formation simulations in warm dark matter cosmologies, for a Lyman-alpha analysis of the dark matter distribution on small scales, and for studying the properties of haloes of dwarf spheroidal galaxies.

• The paper demonstrates that a significant lepton asymmetry induced by resonant νMSM oscillations can produce sterile neutrinos that account for the observed dark matter relic density.
• The paper employs quantum field theory to compute the momentum distribution of keV-scale sterile neutrinos, matching theoretical predictions with astrophysical data.
• The paper highlights that robust lepton asymmetry helps bypass X-ray and Lyman-alpha constraints, establishing sterile neutrinos as viable warm dark matter candidates.

Sterile Neutrino Dark Matter and Lepton Asymmetry in the νMSM

The paper authored by Mikko Laine and Mikhail Shaposhnikov provides an exploration into the thermodynamic and cosmic implications of predicting sterile neutrino dark matter using the framework of the Neutrino Minimal Standard Model (νMSM). The work explores the mechanism of dark matter production through the lepton asymmetry induced by the νMSM and explores the astrophysical consequences of sterile neutrino dark matter.

Model and Theoretical Foundations

The νMSM extends the Standard Model by incorporating three right-handed neutrinos with masses below the electroweak scale. This extension allows the νMSM to potentially explain several cosmological phenomena that the Standard Model cannot, such as dark matter, baryon asymmetry, and neutrino oscillations. The paper focuses on sterile neutrinos, particularly the lightest right-handed neutrinos in the keV mass range, as a viable dark matter candidate.

In this model, CP-violating resonant oscillations among the heavier right-handed neutrinos produce a significant lepton asymmetry, which persists below the sphaleron freeze-out temperature. The lepton asymmetry enhances the production of the lightest right-handed sterile neutrinos through a resonant mechanism, originally suggested by Shi and Fuller. The authors employ quantum field theory methods to calculate the relic density and momentum distribution of these sterile neutrinos under significant lepton asymmetry conditions.

Numerical Results and Boundaries

The paper presents a recalculation of dark matter relic density and sterile neutrino distribution, matching the predictions against existing astrophysical data to derive constraints on their properties. The paper highlights that mechanisms which depend heavily on non-equilibrated, resonant processes could feasibly produce enough sterile neutrinos to account for observed dark matter without conflicting existing observational constraints.

One of the key results is that a sufficiently large lepton asymmetry, in the order of $n_{\nu_e}/s \sim 0.8 \times 10^{-5}$, can ensure that sterile neutrinos constitute the full dark matter abundance, corroborated with scenarios demonstrating a range of viable sterile neutrino masses. These findings are contingent on the assumption that a substantial lepton asymmetry not only circumvents X-ray constraints from decaying particles but also meets structural formation limitations posed by Lyman-alpha forest studies.

Implications and Conclusion

The research illustrates that sterile neutrinos, under the νMSM framework with substantial lepton asymmetry, can satisfy both cosmic and structural constraints necessary for them to be considered as viable warm dark matter candidates. The conclusions address the interplay between theoretical particle physics models and cosmological observations, reinforcing the need for specific parameter spaces in the νMSM to explain dark matter entirely through sterile neutrinos.

Unless structure formation simulations become more accurate in integrating non-equilibrium distributions, reliance on simplified interpretations such as those currently used will persist. Nonetheless, the opportunity to experimentally detect sterile neutrinos remains challenging, primarily due to their small coupling constants. The possibility of detecting diffuse X-ray emissions from dark matter decays present the most realistic chance within the foreseeable future. Further research must continue exploring these neutrinos via both experimental detection strategies and more refined simulations of cosmic structure formation.