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

• Published in: arXiv.org 2021-03
• Main Authors: Luo, Xuheng, Rodejohann, Werner, Xun-Jie Xu
• Format: Article
• Language: English
• Subjects: Couplings; Neutrinos
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2011.13059

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.