When reading about why SM neutrinos are not a candidate for DM, it was stated:
"Neutrinos are involved in weak interactions that keep them in thermal equilibrium in the early Universe down to the temperatures of few MeV."
A similar statement can be found here.
I don't understand how these interactions could maintain a thermal equilibrium. Could someone give me an example of such an interaction?
My "guess" is that in a weak interaction, such as $\mu^- + W^+ \to \nu_\mu$ this would be exothermal and release heat to compensate the clear mass difference between the neutrino (lighter) and the other SM particles.
But to keep an equilibrium wouldn't there need to be isothermic interactions? How could this be done if neutrinos don't decay?