Dynamical framework for KeV Dirac neutrino warm dark matter

Abstract

If the source of the reported 3.5 keV x-ray line is a sterile neutrino, comprising an $\mathcal{O}(1)$ fraction of the dark matter (DM), then it exhibits the property that its mass times mixing angle is $\ensuremath{\sim}\text{ }\text{few}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\text{ }\text{ }\mathrm{eV}$, a plausible mass scale for the active neutrinos. This property is a common feature of Dirac neutrino mixing. We present a framework that dynamically produces light active and keV sterile Dirac neutrinos, with appropriate mixing angles to be the x-ray line source. The central idea is that the right-handed active neutrino is a composite state, while elementary sterile neutrinos gain keV masses similarly to the quarks in extended technicolor. The entire framework is fixed by just two dynamical scales and may automatically exhibit a warm dark matter (WDM) production mechanism---dilution of thermal relics from late decays of a heavy composite neutrino---such that the keV neutrinos may comprise an $\mathcal{O}(1)$ fraction of the DM. In this framework, the WDM is typically quite cool and within structure formation bounds, with temperature $\ensuremath{\sim}\text{ }\text{few}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{T}_{\ensuremath{\nu}}$ and free-streaming length $\ensuremath{\sim}\text{ }\mathrm{few}\text{ }\text{ }\mathrm{kpc}$. A toy model that exhibits the central features of the framework is also presented.