Effective Number Of Neutrinos Research Articles

ALP-assisted strong first-order electroweak phase transition and baryogenesis

Axion-like particles (ALPs) can be naturally lighter than the electroweak scale. We consider an ALP that couples to the Standard Model Higgs to achieve the strong first-order electroweak phase transition. We discuss the two-field dynamics of the phase transition and the associated computation in detail and identify the viable parameter space. The ALP mass can be from the MeV to GeV scale. Baryon asymmetry can be explained by local baryogenesis without violating the current electron and atom electric dipole moment bound in most of the viable parameter space. The viable parameter space can be probed through Higgs exotic decay, rare kaon decay, the electron and atomic electric dipole moment, and the effective number of neutrinos in the cosmic microwave background in the future. The gravitational-wave signal is too weak to be detected.

A cosmological sandwiched window for lepton-number breaking scale

A singlet majoron can arise from the seesaw framework as a pseudo-Goldstone boson when the heavy Majorana neutrinos acquire masses via the spontaneous breaking of global U(1) L symmetry. The resulting cosmological impacts are usually derived from the effective majoron-neutrino interaction, and the majoron abundance is accumulated through the freeze-in neutrino coalescence. However, a primordial majoron abundance can be predicted in a minimal setup and lead to distinctive cosmological effects. In this work, we consider such a primordial majoron abundance from relativistic freeze-out and calculate the modification to the effective neutrino number N eff. We demonstrate that the measurements of N eff will constrain the parameter space from a primordial majoron abundance in an opposite direction to that from neutrino coalescence. When the contributions from both the primordial abundance and the freeze-in production coexist, the U(1) L -breaking scale (seesaw scale) f will be pushed into a “sandwiched window”. Remarkably, for majoron masses below 1 MeV and above the eV scale, the future CMB-S4 experiment will completely close such a low-scale seesaw window for f ∈ [1,105] GeV. We highlight that any new light particle with a primordial abundance that couples to Standard Model particles may lead to a similar sandwiched window, and such a general phenomenon deserves careful investigation.

Thermal production of massless dark photons

A dark photon is predicted by several well-motivated Standard Model extensions and UV completions. Here the most general effective field theory up to dimension-six operatorsdescribing the interactions of a massless dark photon with all Standard Model particles is considered. This captures the predictions of a generic model featuring this type of vector boson at sufficiently low energies. In such framework the thermal production rate of dark photonsis computed at leading order, including the contributions of all SM particles. The corresponding cosmological yield of the dark photon and its contribution to the effective number of neutrinos are also calculated. These predictions satisfy the current observational bounds and will be tested by future measurements.

Asymmetric dark matter with a spontaneously broken U(1)′ : Self-interaction and gravitational waves

Motivated by the collisionless cold dark matter small scale structure problem, we propose an asymmetric dark matter model where dark matter particle interact with each other via a massive dark gauge boson. This model easily avoid the strong limits from cosmic microwave background (CMB) observation, and have a large parameter space to be consistent with small scale structure data. We focus on a special scenario where portals between dark sector and visible sector are too weak to be detected by traditional methods. We find that this scenario can increase the effective number of neutrinos ($N_{\text{eff}}$).In addition, the spontaneous $U(1)'$ symmetry breaking process, which makes dark gauge boson massive, can generate stochastic gravitational waves with peak frequency around $10^{-6} - 10^{-7} \text{ Hz}$.

Non-unitary three-neutrino mixing in the early Universe

Deviations from unitarity in the three-neutrino mixing canonical picture are expected in many physics scenarios beyond the Standard Model. The mixing of new heavy neutral leptons with the three light neutrinos would in principle modify the strength and flavour structure of charged-current and neutral-current interactions with matter. Non-unitarity effects would therefore have an impact on the neutrino decoupling processes in the early Universe and on the value of the effective number of neutrinos, N eff. We calculate the cosmological energy density in the form of radiation with a non-unitary neutrino mixing matrix, addressing the possible interplay between parameters. Highly accurate measurements of N eff fromforthcoming cosmological observations can provide independent and complementary limits on thedepartures from unitarity. For completeness, we relate the scenario of small deviations fromunitarity to non-standard neutrino interactions and compare the forecasted constraints to otherexisting limits in the literature.

Neutrino magnetic moments meet precision Neff measurements

In the early universe, Dirac neutrino magnetic moments due to their chirality-flipping nature could lead to thermal production of right-handed neutrinos, which would make a significant contribution to the effective neutrino number, Neff. We present in this paper a dedicated computation of the neutrino chirality-flipping rate in the thermal plasma. With a careful and consistent treatment of soft scattering and the plasmon effect in finite temperature field theories, we find that neutrino magnetic moments above 2.7 × 10−12μB have been excluded by current CMB and BBN measurements of Neff, assuming flavor-universal and diagonal magnetic moments for all three generation of neutrinos. This limit is stronger than the latest bounds from XENONnT and LUX-ZEPLIN experiments and comparable with those from stellar cooling considerations.

Cosmological imprints of Dirac neutrinos in a keV-vacuum 2HDM* *Supported by the National Natural Science Foundation of China (12135006, 12075097, 12047527, 11775092), as well as by the Fundamental Research Funds for the Central Universities (CCNU20TS007, CCNU19TD012, CCNU22LJ004)

The Dirac neutrino masses could be simply generated by a neutrinophilic scalar doublet with a vacuum being dramatically different from the electroweak one. While the case with an eV-scale vacuum has been widely explored previously, we exploit in this work the desert where the scalar vacuum is of scale. In this regime, there would be rare hope to probe the keV-vacuum neutrinophilic scalar model via the lepton-flavor-violating processes, which makes it distinguishable from the widely considered eV-scale vacuum. Although such a keV-vacuum scenario is inert in the low-energy flavor physics, we show that the baryogenesis realized via the lightest Dirac neutrino can be a natural candidate in explaining the baryon asymmetry of the Universe. Furthermore, the Dirac neutrinos with a keV-vacuum scalar can generate a shift of the effective neutrino number within the range , which can be probed by the future Simons Observatory experiments. In particular, the model with a minimal value can already be falsified by the future CMB Stage-IV and Large Scale Structure surveys, providing consequently striking exploratory avenues in the cosmological regime for such a keV-vacuum scenario.

Linking anomalies to Hubble tension via a single right-handed neutrino* *Supported by the National Natural Science Foundation of China (12135006, 12075097, 11675061, 11775092), and the Fundamental Research Funds for the Central Universities (CCNU20TS007, CCNU19TD012, CCNU22LJ004)

Updated measurements from the LHCb and SH0ES collaborations have respectively strengthened the deviations of the ratio in rare semi-leptonic -meson decays and the present-day Hubble parameter in the Universe, implying tantalizing hints of new physics beyond the standard model. In this paper, we consider a simple flavor-specific two-Higgs-doublet model, where long-standing anomalies can be addressed by a one-flavor right-handed neutrino. An intriguing prediction resulting from the parameter space for the resolution under flavor- and collider-physics constraints points toward a shift in the effective neutrino number, , as a favored way to ease the tension. Depending on whether the neutrino is of the Dirac or Majorana type, we show that the resulting shift is for the former case and for the latter case. While the Dirac case is disfavored by the CMB polarization measurements, the Majorana solution is consistent with recent studies using a combined dataset from various sources. Consequently, such a simple flavor-specific two-Higgs-doublet model provides a link between anomalies and tension, which in turn can be readily verified or disproved by upcoming measurements.

A Review of Neutrino Decoupling from the Early Universe to the Current Universe

We review the distortions of spectra of relic neutrinos due to the interactions with electrons, positrons, and neutrinos in the early universe. We solve integro-differential kinetic equations for the neutrino density matrix, including vacuum three-flavor neutrino oscillations, oscillations in electron and positron background, a collision term and finite temperature corrections to electron mass and electromagnetic plasma up to the next-to-leading order O(e3). After that, we estimate the effects of the spectral distortions in neutrino decoupling on the number density and energy density of the Cosmic Neutrino Background (CνB) in the current universe, and discuss the implications of these effects on the capture rates in direct detection of the CνB on tritium, with emphasis on the PTOLEMY-type experiment. In addition, we find a precise value of the effective number of neutrinos, Neff=3.044. However, QED corrections to weak interaction rates at order O(e2GF2) and forward scattering of neutrinos via their self-interactions have not been precisely taken into account in the whole literature so far. Recent studies suggest that these neglections might induce uncertainties of ±(10−3–10−4) in Neff.

Simple estimate of BBN sensitivity to light freeze-in dark matter

We provide a simple analysis of the big-bang nucleosynthesis (BBN) sensitivity to the light dark matter (DM) generated by the thermal freeze-in mechanism. It is shown that the ratio of the effective neutrino number shift $\Delta N_{\nu}$ over the DM relic density $\omega\equiv \Omega h^2$, denoted by $R_\chi\equiv\Delta N_\nu/\omega$, cancels the decaying particle mass and the feeble coupling, rendering therefore a simple visualization of $\Delta N_{\nu}$ at the BBN epoch in terms of the DM mass. This property drives one to conclude that the shift with a sensitivity of $\Delta N_{\nu}\simeq \mathcal{O}(0.1)$ cannot originate from a single warm DM under the Lyman-$\alpha$ forest constraints. For the cold-plus-warm DM scenarios where the Lyman-$\alpha$ constraints are diluted, the ratio $R_\chi$ can be potentially used to test the thermal freeze-in mechanism in generating a small warm component of DM and a possible excess at the level of $\Delta N_{\nu}\simeq \mathcal{O}(0.01)$.

Cosmological radiation density with non-standard neutrino-electron interactions

Non-standard interactions (NSI) between neutrinos and electrons can significantly modify the decoupling of neutrinos from the plasma. These interactions have two effects on the overall picture: (i) they alter neutrino oscillations though matter effects and (ii) they modify the scattering and annihilation processes involving neutrinos and electrons and positrons. We study the role of non-universal and flavour-changing NSI in the decoupling and how they impact the determination of the effective number of neutrinos, N eff. We examine the degeneracies between NSI parameters and we compare the expected sensitivity from future cosmological surveys with the current limits from terrestrial experiments. We outline the complementarity between both approaches.

Cosmological radiation density with non-standard neutrino-electron interactions

Neutrino non-standard interactions (NSI) with electrons are known to alter the picture of neutrino decoupling from the cosmic plasma. NSI modify both flavour oscillations through matter effects, and the annihilation and scattering between neutrinos and electrons and positrons in the thermal plasma. In view of the forthcoming cosmological observations, we perform a precision study of the impact of non-universal and flavour-changing NSI on the effective number of neutrinos, Neff. We present the variation of Neff arising from the different NSI parameters and discuss the existing degeneracies among them, from cosmology alone and in relation to the current bounds from terrestrial experiments. Even though cosmology is generally less sensitive to NSI than these experiments, we find that future cosmological data would provide competitive and complementary constraints for some of the couplings and their combinations.

Towards a precision calculation of the effective number of neutrinos \U0001d5ad_\U0001d5be\U0001d5bf\U0001d5bf in the Standard Model. Part II. Neutrino decoupling in the presence of flavour oscillations and finite-temperature QED

We present in this work a new calculation of the standard-model benchmark value for the effective number of neutrinos, Neff 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 O(e3), 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 Neff SM value, through testing the value's 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 Neff SM 3.0440 ±0.0002, where the nominal uncertainty is attributed predominantly to errors incurred in the numerical solution procedure (|δ Neff| ∼10-4), augmented by measurement errors in the solar mixing angle sin2θ 12 (|δ Neff| ∼10-4).

Self-interacting dark matter with scalar dilepton mediator

The cold dark matter (CDM) candidate with weakly interacting massive particles can successfully explain the observed dark matter relic density in cosmic scale and the large-scale structure of the Universe. However, a number of observations at the satellite galaxy scale seem to be inconsistent with CDM simulation. This is known as the small-scale problem of CDM. In recent years, it has been demonstrated that self-interacting dark matter (SIDM) with a light mediator offers a reasonable explanation for the small-scale problem. We adopt a simple model with SIDM and focus on the effects of Sommerfeld enhancement. In this model, the dark matter candidate is a leptonic scalar particle with a light mediator. We have found several regions of the parameter space with proper masses and coupling strength generating a relic density that is consistent with the observed CDM relic density. Furthermore, this model satisfies the constraints of recent direct searches and indirect detection for dark matter as well as the effective number of neutrinos and the observed small-scale structure of the Universe. In addition, this model with the favored parameters can resolve the discrepancies between astrophysical observations and $N$-body simulations.

Neutrino decoupling including flavour oscillations and primordial nucleosynthesis

We revisit the decoupling of neutrinos in the early universe with flavour oscillations. We rederive the quantum kinetic equations which determine the neutrino evolution based on a BBGKY-like hierarchy, and include for the first time the full collision term, with both on- and off-diagonal terms for all relevant reactions. We focus on the case of zero chemical potential and solve these equations numerically. We also develop an approximate scheme based on the adiabatic evolution in the matter basis. In fact, the large difference between the oscillations and cosmological time scales allows to consider averaged flavour oscillations which can speed up the numerical integration by two orders of magnitude, when combined with a direct computation of the differential system Jacobian. The approximate numerical scheme is also useful to gain more insight into the physics of neutrino decoupling. Including the most recent results on plasma thermodynamics QED corrections, we update the effective number of neutrinos to Neff = 3.0440. Finally we study the impact of flavour oscillations during neutrino decoupling on the subsequent primordial nucleosynthesis.

Cosmic imprints of XENON1T axions

The recent electron recoil excess observed by XENON1T has a possible interpretation in terms of solar axions coupled to electrons. If such axions are still relativistic at recombination they would also leave a cosmic imprint in the form of an additional radiation component, parameterized by an effective neutrino number Δ Neff. We explore minimal scenarios with a detectable signal in future CMB surveys: axions coupled democratically to all fermions, axion-electron coupling generated radiatively, the DFSZ framework for the QCD axion. The predicted Δ Neff is larger than 0.03–0.04 for all cases, close to the 2σ forecasted sensitivity of CMB-S4 experiments. This opens the possibility of testing with cosmological observations the solar axion interpretation of the XENON1T excess.

A precision calculation of relic neutrino decoupling

We study the distortions of equilibrium spectra of relic neutrinos due to the interactions with electrons, positrons, and neutrinos in the early Universe. We solve the integro-differential kinetic equations for the neutrino density matrix, including three-flavor oscillations and finite temperature corrections from QED up to the next-to-leading order O(e3) for the first time. In addition, the equivalent kinetic equations in the mass basis of neutrinos are directly solved, and we numerically evaluate the distortions of the neutrino spectra in the mass basis as well, which can be easily extrapolated into those for non-relativistic neutrinos in the current Universe. In both bases, we find the same value of the effective number of neutrinos, Neff=3.044, which parameterizes the total neutrino energy density. The estimated error for the value of Neff due to the numerical calculations and the choice of neutrino mixing parameters would be at most 0.0005.

Big-Bang Nucleosynthesis after Planck

We assess the status of big-bang nucleosynthesis (BBN) in light of the final Planck data release and other recent developments, and in anticipation of future measurements. Planck data from the recombination era fix the cosmic baryon density to 0.9% precision, and now damping tail measurements determine the helium abundance and effective number of neutrinos with precision approaching that of astronomical and BBN determinations respectively. All three parameters are related by BBN . In addition, new high-redshift measurements give D/H to better precision than theoretical predictions, and new Li/H data reconfirm the lithium problem. We present new 7Be(n,p)7Li rates using new neutron capture measurements; we have also examined the effect of proposed changes in the d(p,γ)3He rates. Using these results we perform a series of likelihood analyses. We assess BBN/CMB consistency, with attention to how our results depend on the choice of Planck data, as well as how the results depend on the choice of non-BBN, non-Planck data sets. Most importantly the lithium problem remains, and indeed is more acute given the very tight D/H observational constraints; new neutron capture data reveals systematics that somewhat increases uncertainty and thus slightly reduces but does not essentially change the problem. We confirm that d(p,γ)3He theoretical rates brings D/H out of agreement and slightly increases 7Li new experimental data are needed at BBN energies. Setting the lithium problem aside, we find the effective number of neutrino species at BBN is Nν = 2.86 ± 0.15. Future CMB Stage\ obreakdash-4 measurements promise substantial improvements in BBN parameters: helium abundance determinations will be competitive with the best astronomical determinations, and Neff will approach sensitivities capable of detecting the effects of Standard Model neutrino heating of the primordial plasma.

Towards a SM prediction of N eff, the effective number of neutrinos

In this work, we demonstrate how (e (3)), where e is the elementary electric charge, corrections to the QED partition function, previously deemed to be too small to be considered, will affect the effective number of neutrinos in the universe N eff. Working in the instantaneous decoupling approximation, we find that their inclusion results in δN eff ≃ –0.001. We use this result to argue that (e (3)) corrections should be included in a full calculation of N eff that would include a complete treatment of neutrino decoupling, and accounting for neutrino oscillations. We predict that relative to N eff = 3.044 computed in the most recent such calculation, this should give N eff = 3.043.

Active-sterile neutrino oscillations in the early Universe with the complete mixing matrix

In the framework of a 3+1 scheme with an additional inert state, we consider the thermalisation of sterile neutrinos in the early Universe taking into account the full 4 × 4 mixing matrix. The evolution of the neutrino energy distributions is found solving the momentum-dependent kinetic equations with full diagonal collision terms. The degree of thermalisation of the sterile state is shown in terms of the effective number of neutrinos, N eff, and its dependence on the three additional mixing angles (θ 14, θ 24, θ 34) and on the squared mass difference ∆m 2 41 is discussed. Our results are relevant for fixing the contribution of a fourth light neutrino species to the cosmological energy density, whose value is very well constrained by the final Planck analysis. For the preferred region of active-sterile mixing parameters from short-baseline neutrino experiments, we find that the fourth state is fully thermalised (N eff ≃ 4).