Papers › Dirac neutrinos and N_(eff) II: the freeze-in case
Dirac neutrinos and N_(eff) II: the freeze-in case
Xuheng Luo, Werner Rodejohann, Xun-Jie Xu
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We discuss Dirac neutrinos whose right-handed component ν_R has new interactions that may lead to a measurable contribution to the effective number of relativistic neutrino species N_(eff). We aim at a model-independent and comprehensive study on a variety of possibilities. Processes for ν_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 ν_R to N_(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_(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, ΔN_(eff)^(SM) ≈7.5×10⁻¹², assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.
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