N eff constraints on light mediators coupled to neutrinos: the dilution-resistant effect

Abstract

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 eff. If the light particles are produced from decoupled neutrinos, N 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 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 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 eff constraints on the couplings will be improved by many orders of magnitude.

Keywords

• Early Universe Particle Physics
• Non-Standard Neutrino Properties