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

TL;DR

This paper investigates how right-handed Dirac neutrinos with new interactions can contribute to the effective number of relativistic species, focusing on the freeze-in production mechanism and its implications for cosmological observations.

Contribution

It provides a comprehensive, model-independent analysis of right-handed neutrino production via freeze-in, including analytical and numerical calculations across various processes and regimes.

Findings

Calculated the contribution of right-handed neutrinos to N_eff in freeze-in regime.

Derived limits and sensitivities for new neutrino interactions from current and future CMB data.

Estimated the Standard Model Higgs-neutrino interaction contribution to N_eff as approximately 7.5×10^{-12}.

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 $N_{\rm eff}$. 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 $N_{\rm eff}$ 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 $N_{\rm eff}$, 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, $\Delta N_{\rm eff}^{\rm SM} \approx 7.5\times10^{-12}$, assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.