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.

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

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Cited by in corpus (30)

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• Freeze-in Dark Matter via Light Dirac Neutrino Portal
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• Affleck-Dine Cogenesis of Baryon and Dark Matter
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• Imprints of MeV Scale Hidden Dark Sector at Planck Data
• Linking the $R_{K^{(\ast )}}$ anomalies to the Hubble tension via a single right-handed neutrino
• Primordial Black Holes Evaporating before Big Bang Nucleosynthesis
• Signatures of Bulk Neutrinos in the Early Universe
• Probing Long-Range Forces Between Neutrinos with Cosmic Structures
• Phenomenology of Dirac neutrino EFTs up to dimension six
• Probing the muon $g-2$ with future beam dump experiments
• Cosmic Collider Gravitational Waves sourced by Right-handed Neutrino production from Bubbles: Testing Seesaw, Leptogenesis and Dark Matter
• Sterile neutrino Dark Matter in the minimal Dirac Seesaw