Fugacity and Reheating of Primordial Neutrinos

TL;DR

This paper investigates how different freeze-out temperatures affect primordial neutrino properties, emphasizing the importance of neutrino fugacity as a key parameter in cosmological models and CMB analysis.

Contribution

It establishes a quantitative link between neutrino fugacity, freeze-out temperatures, and reheating, highlighting the need to include fugacity as a standard cosmological parameter.

Findings

Neutrino fugacity $5$<1 indicates chemical nonequilibrium.

Reheating relation connects $T_ u(5)$ to $T_5$ and $T_5$.

Fugacity measurement can inform on kinetic freeze-out temperature $T_k$.

Abstract

We clarify in a quantitative way the impact that distinct chemical $T_c$ and kinetic $T_k$ freeze-out temperatures have on the reduction of the neutrino fugacity $\Upsilon_\nu$ below equilibrium, i.e. $\Upsilon_\nu<1$, and the increase of the neutrino temperature $T_\nu$ via partial reheating. We establish the connection between $\Upsilon_\nu$ and $T_k$ via the modified reheating relation $T_\nu(\Upsilon_\nu)/T_\gamma$, where $T_\gamma$ is the temperature of the background radiation. Our results demonstrate that one must introduce the chemical nonequilibrium parameter, i.e., the fugacity, $\Upsilon_\nu$, as an additional standard cosmological model parameter in the evaluation of CMB fluctuations as its value allows measurement of $T_k$.