$N_{{\rm eff}}$ constraints on light mediators coupled to neutrinos: the dilution-resistant effect

TL;DR

This paper explores how new light particles coupled to neutrinos affect the cosmological parameter $N_{\rm eff}$, revealing a dilution-resistant effect that can significantly increase $N_{\rm eff}$ and improve constraints with future CMB experiments.

Contribution

It provides a comprehensive analysis of the dilution-resistant effect on $N_{\rm eff}$ due to light mediators, deriving new constraints on their couplings with neutrinos.

Findings

Dilution-resistant effect can increase $N_{\rm eff}$ by up to 0.242 for vector mediators.

Future CMB experiments can detect these $N_{\rm eff}$ increases.

Constraints on mediator couplings will improve by many orders of magnitude.

Abstract

We investigate the impact of new light particles, carrying significant energy in the early universe after neutrino decoupling, on the cosmological effective relativistic neutrino species, $N_{{\rm eff}}$. If the light particles are produced from decoupled neutrinos, $N_{{\rm eff}}$ is predominantly modified through the dilution-resistant effect. This effect arises because the energy stored in the mass of new particles is less diluted than the photon and neutrino energy as the universe expands. Our study comprehensively explores this effect, deriving $N_{{\rm eff}}$ constraints on the couplings of light mediators with neutrinos, encompassing both scalar and vector mediators. We find that the dilution-resistant effect can increase $N_{{\rm eff}}$ by 0.118 and 0.242 for scalar and vector mediators, respectively. These values can be readily reached by forthcoming CMB experiments. Upon reaching these levels, future $N_{{\rm eff}}$ constraints on the couplings will be improved by many orders of magnitude.