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Dirac neutrinos and $N_{{\rm eff}}$
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abstract
If neutrinos are Dirac particles the existence of light right-handed neutrinos $\nu_{R}$ is implied. Those would contribute to the effective number of relativistic neutrino species $N_{{\rm eff}}$ in the early Universe. With pure standard model interactions, the contribution is negligibly small. In the presence of new interactions, however, the contribution could be significantly enhanced. We consider the most general effective four-fermion interactions for neutrinos (scalar, pseudo-scalar, vector, axial-vector and tensor), and compute the contribution of right-handed neutrinos to $N_{{\rm eff}}$. Taking the Planck 2018 measurement of $N_{{\rm eff}}$, strong constraints on the effective four-fermion coupling are obtained, corresponding to interaction strengths of $10^{-5}\sim10^{-3}$ in units of the Fermi constant. This translates in new physics scales of up to 43 TeV and higher. Future experiments such as CMB-S4 can probe or exclude the existence of effective 4-neutrino operators for Dirac neutrinos. Ways to avoid this conclusion are discussed.
Forward citations
Cited by 11 Pith papers
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Singlet-Doublet fermion origin of dark matter, neutrino mass and inverse first-order electroweak phase transition
New singlet and doublet fermions, a scalar singlet, and right-handed neutrinos can simultaneously explain Dirac neutrino masses, dark matter, and a two-step inverse electroweak phase transition with observable gravita...
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Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics
CMB-S4 forecasts: the two-site design reaches sigma(Neff)=0.029 in seven years, the Chile-only revised design reaches 0.030 in nine, and a cosmic-variance-limited survey would reach 0.0073.
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Dark Photons in the Early Universe: From Thermal Production to Cosmological Constraints
Dark photon thermal production is computed analytically, yielding a fixed 4πe/27 ≈ 0.14 resonance-to-inverse-decay ratio and new cosmological limits down to ε~10^-12 over 0.1–6 MeV.
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If one neutrino species in the cosmic neutrino background is still relativistic today, UHE neutrinos scattering off it can resonantly disappear over a broad energy range, and GRAND could detect that dip to probe neutr...
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Improved cosmological limits on $Z^\prime$ models with light right-handed neutrinos
The ACT DR6 bound on extra radiation yields the strongest exclusion limits yet on U(1)_{B-L} Z' bosons coupled to light right-handed neutrinos, beating colliders across most of the mass range.
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$Z^\prime$ Portal Dark Matter with Observable $\Delta N_{\rm eff}$
Dirac right-handed neutrinos in a U(1)_{B-L} Z' portal model produce observable ΔN_eff that, together with direct/indirect detection and collider bounds, carves out testable WIMP and FIMP dark-matter regions.
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A Gauge Model for Quasi-Dirac Neutrinos
A U(1) gauge model with six chiral singlet fermions produces naturally small Dirac neutrino masses plus smaller Majorana masses, giving a quasi-Dirac spectrum and a TeV-scale new boson.
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Effective theory of light Dirac neutrino portal dark matter with observable ${\Delta N_{\rm eff}}$
A dark matter candidate interacting only with right-handed neutrinos is shown to produce ΔNeff ≥ 0.21, testable by future CMB experiments.
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Type-II Seesaw Mechanism for Dirac Neutrinos and its Implications on $N_{\text{eff}}$ and Lepton Flavor Violation in a 3-3-1 model
A scalar sextet generates eV-scale Dirac neutrino masses in a 3-3-1 model, and the associated right-handed neutrino population constrains the Z' boson mass to exceed 4.4 TeV.
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Probing Long-Range Forces Between Neutrinos with Cosmic Structures
Neutrino long-range forces stronger than gravity would make the cosmic neutrino background collapse into bound states, and existing matter power spectrum and reionization data now exclude a band of couplings for force...
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A Z4-symmetric Dirac seesaw makes the imaginary part of a seesaw scalar a stable dark matter candidate and links its phenomenology to gravitational wave and CMB observables.
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