Standard Model Neutrinos as Warm Dark Matter
Gian F. Giudice, Edward W. Kolb, Antonio Riotto, Dmitry V. Semikoz, Igor I. Tkachev
TL;DR
This paper challenges the view that Standard Model neutrinos cannot be dark matter by showing that in low-reheat cosmologies with $T_{RH}$ near $1\,\text{MeV}$, neutrino densities are substantially suppressed, allowing $m_\nu$ in the keV range. The authors compute the neutrino abundance by solving the full kinetic equations for the phase-space distribution $f_\nu(p,t)$ during a reheating era driven by a decaying field with decay rate $\Gamma$ linked to $T_{RH}$, rather than relying on chemical equilibrium. They derive simple relations such as $\Omega_\nu h^2 \approx (m_\nu/4\ \text{keV})(T_{RH}/1\ \text{MeV})^3$ and show the final distribution is characterized by an effective chemical potential, indicating keV-scale Standard Model neutrinos could act as warm dark matter compatible with big bang nucleosynthesis and large-scale structure. The results depend on $g_*(T_{RH})$ and improve upon earlier Boltzmann-approximation treatments by explicitly solving the collision integrals for neutrino kinetics. Overall, the work proposes a viable warm dark matter scenario within the Standard Model framework, with potential implications for small-scale structure and cosmological constraints.
Abstract
Standard Model neutrinos are not usually considered plausible dark matter candidates because the usual treatment of their decoupling in the early universe implies that their mass must be sufficiently small to make them ``hot'' dark matter. In this paper we show that decoupling of Standard Model neutrinos in low reheat models may result in neutrino densities very much less than usually assumed, and thus their mass may be in the keV range. Standard Model neutrinos may therefore be warm dark matter candidates.