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We investigate the impact of new light particles, carrying significant energyin the early universe after neutrino decoupling, on the cosmological effectiverelativistic neutrino species, $N_{{\rm eff}}$. If the light particles areproduced from decoupled neutrinos, $N_{{\rm eff}}$ is predominantly modifiedthrough the dilution-resistant effect. This effect arises because the energystored in the mass of new particles is less diluted than the photon andneutrino energy as the universe expands. Our study comprehensively exploresthis effect, deriving $N_{{\rm eff}}$ constraints on the couplings of lightmediators with neutrinos, encompassing both scalar and vector mediators. Wefind that the dilution-resistant effect can increase $N_{{\rm eff}}$ by 0.118and 0.242 for scalar and vector mediators, respectively. These values can bereadily reached by forthcoming CMB experiments. Upon reaching these levels,future $N_{{\rm eff}}$ constraints on the couplings will be improved by manyorders of magnitude.

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