Sterile Neutrinos as Dark Matter: Alternative Production Mechanisms in the Early Universe

Abstract

We study various production mechanisms of sterile neutrinos in the early universe beyond and within the standard model. We obtain the quantum kinetic equations for production and the distribution function of sterile-like neutrinos at freeze-out, from which we obtain free streaming lengths, equations of state and coarse grained phase space densities. In a simple extension beyond the standard model, in which neutrinos are Yukawa coupled to a Higgs-like scalar, we derive and solve the quantum kinetic equation for sterile production and analyze the freeze-out conditions and clustering properties of this dark matter constituent. We argue that in the mass basis, standard model processes that produce active neutrinos also yield sterile-like neutrinos, leading to various possible production channels. Hence, the final distribution function of sterile-like neutrinos is a result of the various kinematically allowed production processes in the early universe. As an explicit example, we consider production of light sterile neutrinos from pion decay after the QCD phase transition, obtaining the quantum kinetic equation and the distribution function at freeze-out. A sterile-like neutrino with a mass in the keV range produced by this process is a suitable warm dark matter candidate with a free-streaming length of the order of few kpc consistent with cores in dwarf galaxies.

Study of a Viscous ΛWDM Model: Near-Equilibrium Condition, Entropy Production, and Cosmological Constraints

“…We have shown in this work that, previously to any constraining from the cosmological data, the study of thermodynamics consistences required by the Eckart's approach, such as the near-equilibrium condition and entropy production, leads to important constraints on the cosmological parameters, such as the given one by Equation (1), which implies the necessity of WDM, in agreement with some previous results found in [105][106][107]. On the other hand, the constraint (12) tells us that Big-Rip singularities are avoided at late times if the near-equilibrium condition is preserved, even though the exact solution explored behaves very similarly to the standard model, and opens the possibility of a more realistic fluid description of the DM containing dissipation processes within the Eckart's framework, giving us physically important clues about the EoS of this component (γ) and the size of dissipation involved (Ω ξ 0 ).…”

Warm Dark Matter in Simulations

“…Bridging theory with observations, numerical simulations represent an important tool in studying and testing the hypothesized dark matter models, and in making predictions that can be then tested against astronomical observations. Several warm dark matter (WDM) models have been theorized and proposed as viable dark matter candidates, e.g., [1][2][3][4][5][6]. Given the challenges that the cold dark matter (CDM) model faces, these models, especially those with particles in the keV range, are gaining a lot of traction as feasible alternatives [7][8][9][10][11][12][13][14][15].…”