Dirac neutrinos and Neff. Part II. The freeze-in case

Abstract

Abstract We discuss Dirac neutrinos whose right-handed component νR has new interac­tions that may lead to a measurable contribution to the effective number of relativistic neutrino species Neff. We aim at a model-independent and comprehensive study on a variety of possibilities. Processes for νR-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 νR to Neff 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 Neff, 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, Δ Neff SM ≃ 7.5 × 10-12, assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.