Dirac neutrinos and $N_{\mathrm{eff}}$ II: the freeze-in case

Abstract

We discuss Dirac neutrinos whose right-handed component ${\nu }_R$ has new interactions that may lead to a measurable contribution to the effective number of relativistic neutrino species $N_{\mathrm{eff}}$. We aim at a model-independent and comprehensive study on a variety of possibilities. Processes for ${\nu }_R$-genesis from decay or scattering of thermal species, with spin-0, spin-1/2, or spin-1 initial or final states are all covered. We calculate numerically and analytically the contribution of ${\nu }_R$ to $N_{\mathrm{eff}}$ primarily in the freeze-in regime, since the freeze-out regime has been studied before. While our approximate analytical results apply only to freeze-in, our numerical calculations work for freeze-out as well, including the transition between the two regimes. Using current and future constraints on $N_{\mathrm{eff}}$, we obtain limits and sensitivities of CMB experiments on masses and couplings of the new interactions. As a by-product, we obtain the contribution of Higgs-neutrino interactions, $\Delta N_{\mathrm{eff}}^{\mathrm{SM}}\approx 7.5\times 10^{-12}$, assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.

Comments: 29 pages, 6 figures, comments on non-thermal distributions added, JCAP published version