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Register a new userDirac neutrinos and $N_{{rm eff}}$ II: the freeze-in case
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We discuss Dirac neutrinos whose right-handed component $ u_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 $ u_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 $ u_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, $Delta N_{rm eff}^{rm SM} approx 7.5times10^{-12}$, assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.
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If neutrinos are Dirac particles the existence of light right-handed neutrinos $ u_{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.
We evaluate the contribution to $N_{rm eff}$ of the extra sterile states in low-scale Type I seesaw models (with three extra sterile states). We explore the full parameter space and find that at least two of the heavy states always reach thermalisation in the Early Universe, while the third one might not thermalise provided the lightest neutrino mass is below ${mathcal O}(10^{-3}$eV). Constraints from cosmology therefore severely restrict the spectra of heavy states in the range 1eV- 100 MeV. The implications for neutrinoless double beta decay are also discussed.
We present in this work a new calculation of the standard-model benchmark value for the effective number of neutrinos, $N_{rm eff}^{rm SM}$, that quantifies the cosmological neutrino-to-photon energy densities. The calculation takes into account neutrino flavour oscillations, finite-temperature effects in the quantum electrodynamics plasma to ${cal O}(e^3)$, where $e$ is the elementary electric charge, and a full evaluation of the neutrino--neutrino collision integral. We provide furthermore a detailed assessment of the uncertainties in the benchmark $N_{rm eff}^{rm SM}$ value, through testing the values dependence on (i)~optional approximate modelling of the weak collision integrals, (ii)~measurement errors in the physical parameters of the weak sector, and (iii)~numerical convergence, particularly in relation to momentum discretisation. Our new, recommended standard-model benchmark is $N_{rm eff}^{rm SM} = 3.0440 pm 0.0002$, where the nominal uncertainty is attributed predominantly to errors incurred in the numerical solution procedure ($|delta N_{rm eff}| sim10^{-4}$), augmented by measurement errors in the solar mixing angle $sin^2theta_{12}$ ($|delta N_{rm eff}| sim10^{-4}$).
A new U(1) gauge symmetry is the simplest extension of the Standard Model and has various theoretical and phenomenological motivations. In this paper, we study the cosmological constraint on the MeV scale dark photon. After the neutrino decoupling era at $T = mathcal{O}(1),$MeV, the decay and annihilation of the dark photon heats up the electron and photon plasma and accordingly decreases the effective number of neutrino $N_{mathrm{eff}}$ in the recombination era. We derive a conservative lower-limit of the dark photon mass around 8.5 MeV from the current Planck data if the mixing between the dark photon and ordinary photon is larger than $mathcal{O}(10^{-9})$. We also find that the future CMB stage-$rm I! V$ experiments can probe up to 17 MeV dark photon.
Gravitinos are a fundamental prediction of supergravity, their mass ($m_{G}$) is informative of the value of the SUSY breaking scale, and, if produced during reheating, their number density is a function of the reheating temperature ($T_{text{rh}}$). As a result, constraining their parameter space provides in turn significant constraints on particles physics and cosmology. We have previously shown that for gravitinos decaying into photons or charged particles during the ($mu$ and $y$) distortion eras, upcoming CMB spectral distortions bounds are highly effective in constraining the $T_{text{rh}}-m_{G}$ space. For heavier gravitinos (with lifetimes shorter than a few $times10^6$ sec), distortions are quickly thermalized and energy injections cause a temperature rise for the CMB bath. If the decay occurs after neutrino decoupling, its overall effect is a suppression of the effective number of relativistic degrees of freedom ($N_{text{eff}}$). In this paper, we utilize the observational bounds on $N_{text{eff}}$ to constrain gravitino decays, and hence provide new constaints on gravitinos and reheating. For gravitino masses less than $approx 10^5$ GeV, current observations give an upper limit on the reheating scale in the range of $approx 5 times 10^{10}- 5 times 10^{11}$GeV. For masses greater than $approx 4 times 10^3$ GeV they are more stringent than previous bounds from BBN constraints, coming from photodissociation of deuterium, by almost 2 orders of magnitude.
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