keV sterile neutrino dark matter in gauge extensions of the standard model (0912.4415v2)

Abstract: It is known that a keV scale sterile neutrino is a good warm dark matter candidate. We study how this possibility could be realized in the context of gauge extensions of the standard model. The na\"ive expectation leads to large thermal overproduction of sterile neutrinos in this setup. However, we find that it is possible to use out-of-equilibrium decay of the other right-handed neutrinos of the model to dilute the present density of the keV sterile neutrinos and achieve the observed dark matter density. We present the universal requirements that should be satisfied by the gauge extensions of the standard model, containing right-handed neutrinos, to be viable models of warm dark matter, and provide a simple example in the context of the left-right symmetric model.

Analysis of keV Sterile Neutrino Dark Matter Models in Gauge Extensions of the Standard Model

The paper in question investigates the intriguing possibility of keV-scale sterile neutrinos serving as viable warm dark matter (WDM) candidates, specifically within the framework of gauge extensions of the Standard Model (SM). The motivation for considering sterile neutrinos in this mass range, often referenced as "keV-DM", is their potential to address certain astrophysical observations and structure formation issues that other dark matter candidates struggle to explain.

Key Findings and Hypotheses

The authors begin by acknowledging the well-substantiated evidence for dark matter constituting approximately 20% of the universe's energy density. They position sterile neutrinos as WDM candidates, which implies a velocity distribution sufficient to mitigate issues of over-dense cores in cosmic structures—problems often attributed to cold dark matter models.

A significant challenge presented in incorporating keV sterile neutrinos into these models is their overproduction during the Universe's thermal history. The naive expectation based on their thermal relic nature suggests an abundance that would result in the Universe being overclosed. However, the authors propose a mechanism whereby additional right-handed neutrinos present in gauge-extended versions of the SM undergo out-of-equilibrium decay processes. These processes effectively dilute the density of keV sterile neutrinos, thereby aligning their abundance with observational limits.

Theoretical Framework and Model Constraints

The paper outlines universal requirements for any gauge extension of the SM to successfully incorporate keV sterile neutrinos as WDM. Central to these requirements is the necessity of non-equilibrium decay processes, which the paper illustrates using an example from the left-right symmetric model, a popular candidate for a gauge extension.

The authors provide detailed constraints arising from cosmological considerations, such as the mass range $M_1$ for the DM candidate, which must be sufficiently large (on the order of keV) to avoid over-suppressing structure formation on small scales—addressed quantitatively by the Lyman-alpha bounds. Additionally, the mixing angles $\theta_I$ between sterile and active neutrinos must remain small to prevent excessive decay rates, as current x-ray observations stringently limit the decay widths associated with radiative processes.

Model Implementations

The paper explores two primary models for accommodating keV sterile neutrinos: type I seesaw and type II seesaw mechanisms. The authors demonstrate that a low-scale type I seesaw does not satisfactorily resolve issues surrounding overproduction due to inherent mixing angle constraints. However, they find success in employing a type II seesaw mechanism within the left-right symmetric model. This approach leverages an additional scalar sector to generate neutrino masses while maintaining compatibility with cosmological bounds and observational data.

Implications and Future Directions

The implications of successfully integrating keV sterile neutrinos into gauge extensions are multifold. Practically, this approach provides a potential pathway to reconcile certain discrepancies between simulation data and astronomical observations. Theoretically, it demands careful consideration of entropy generation mechanisms through the decay of heavy neutrinos to achieve present-day dark matter densities.

Future research could explore further variations of gauge extensions, potentially incorporating other symmetries or fields that may naturally generate satisfactory seesaw scales. Additionally, the framework established here can be extended or refined to incorporate new observational data, particularly from cosmological surveys probing small-scale structures and x-ray emissions.

In conclusion, the paper effectively navigates the complex interplay between model constraints and observational requirements, providing a comprehensive framework to further analyze keV-scale sterile neutrinos as promising candidates for dark matter. As experimental techniques advance, they may offer empirical validation or necessary revisions to the theoretical landscape explored in this work.