Neutrino Asymmetry Research Articles

Tunneling away the relic neutrino asymmetry

The Earth acts as a matter potential for relic neutrinos which modifies their index of refraction from vacuum by δ∼10−8. It has been argued that the refractive effects from this potential should lead to a large O(δ) neutrino-antineutrino asymmetry at the surface of the Earth. This result was computed by treating the Earth as flat. In this work, we revisit this calculation in the context of a perfectly spherical Earth. We demonstrate, both numerically and through analytic arguments, that the flat-Earth result is only recovered under the condition δ3/2kR≫1, where k is the typical momentum of the relic neutrinos and R is the radius of the Earth. This condition is required to prevent antineutrinos from tunneling into classically inaccessible trajectories below the Earth’s surface and washing away the large asymmetry. As the physical parameters of the Earth do not satisfy this condition, we find that the asymmetry at the surface should only be O(δ). While the asphericity of the Earth may serve as a loophole to our conclusions, we argue that it is still difficult to generate a large asymmetry even in the presence of local terrain. Published by the American Physical Society 2024

Measuring neutrino mass and asymmetry with matter pairwise velocities

ABSTRACT Neutrinos are believed to be the most abundant fermions in the Universe, but their masses are unknown, except for being non-zero but much smaller than other fermions. Cosmological relic neutrinos could also have non-zero chemical potentials (or asymmetries). Using neutrino-involved N-body simulations, we investigate the neutrino effects on the matter pairwise velocity, which itself is an interesting probe of cosmology. We find that for light-halo ([1011, 1013] M⊙) mean pairwise velocity, in the transition range ([4, 15] Mpc), the effects of neutrino masses overwhelm the effects of neutrino asymmetries, while in the two-halo-group range ([25, 50] Mpc), for both light and heavy haloes ([1013, 1015] M⊙), the effects of neutrino asymmetries dominate, making it possible to disentangle the two effects. We provide fitting formulae to quantify the effects of neutrino mass and asymmetry on halo–halo pairwise velocities.

Large neutrino asymmetry from TeV scale leptogenesis

Large neutrino asymmetry from TeV scale leptogenesis

Effects of neutrino masses and asymmetries on dark matter halo assembly

Massive cosmological neutrinos suppress the Large-Scale Structure (LSS) in the Universe by smoothing the cosmic over-densities, and hence structure formation is delayed relative to that in the standard Lambda-Cold Dark Matter (ΛCDM) model.We characterize the merger and mass accretion history of dark matter halos with the halo formation time a 1/2, tree entropy s and halo leaf function ℓ(X) and measure them using neutrino-involved N-body simulations. We show that a non-zero sum of neutrino masses Mν delays the a 1/2 for halos with virial mass between 1013 M ⊙ and 3 × 1013 M ⊙, whereas a non-zero neutrino asymmetry parameter η 2 has the opposite effect. While the mean tree entropy s̅ does not depend significantly on either Mν or η 2, the halo leaf function does. Furthermore, the dependencies of ℓ on Mν and η 2 have significant evolution in redshift z, with the relative contributions of Mν and η 2 showing a sigmoid-like transition as a function of z around z ≈ 0.6. Together with the matter power spectrum, these halo parameters allow us to break the parameter degeneracy between Mν and η 2 so that they can both be constrained in principle.

Primordial neutrino asymmetry evolution with full mean-field effects and collisions

Neutrino oscillations and mean-field effects considerably enrich the phenomenology of neutrino evolution in the early Universe. Taking into account these effects, most notably the neutrino self-interaction mean-field contribution, we revisit the problem of the evolution of primordial neutrino asymmetries including for the first time the complete expression for collisions, which describe scattering and annihilations with electrons/positrons and reactions among (anti)neutrinos. We show that a generalisation of the adiabatic transfer of averaged oscillations (ATAO) scheme, a numerical method previously developed without neutrino degeneracy and based on the large separation of time scales in this problem, is sufficient to reach the same accuracy as the full quantum kinetic equation integration, but is notably faster. This approximation highlights the physics of synchronous oscillations at play in the evolution of neutrino chemical potentials, especially in the particular case with only two-neutrino mixing. In particular, it allows to understand what controls the beginning and the amplitude of oscillations, but also why there is a subsequent regime of collective oscillations with larger frequencies. We also find that it is very important to use the full collision term instead of relying on damping-like approximations, in order not to overestimate how collisions reduce these synchronous oscillations. Finally we study qualitatively how mixing parameters affect the final neutrino configuration, and in particular we show that the CP-violating Dirac phase cannot substantially affect the final N eff nor the final electronic (anti)-neutrino spectrum, and thus should not affect cosmological observables.

Can magnetogenesis driven by dynamo instabilities in chiral fermion plasmas, favor Einstein–Cartan AdS cosmology?

In this letter we show that the magnetic field (MF) bounds induced by Einstein–Cartan–de Sitter (EC) with AdS anti-de Sitter spacetime geometry and fermionic torsion with chiral dynamo plasmas instabilities, with inhomogeneous uniform chiral chemical potential, indicates that there is a possibility, as far as magnetogenesis is concerned, that we may obtain more stringent results within ECAdS gravity, compared with general relativity (GR) limits obtained by Tsagas using torsionless magnetogenesis. Spinorial fermionic source with torsion induces a chiral plasmas oscillation. The time dependent chiral chemical potential, sometimes associated formally with torsion, (de Andrade, 2018), is obtained from chiral flipping equation. The MFs obtained from the present universe ECdS chiral dynamo model, meet the criteria of galactic seed fields with better limits within Tsagas range (Tsagas, 2015) seed galactic magnetic fields for dynamo mechanisms, than the bound obtained by chiral dynamos in torsionless AdS cosmology. Majorana neutrinos are used as spatial components of axial spin-torsion source of dynamo instability from neutrino asymmetry.

Magnetic fields in a hot dense neutrino plasma and the gravitational waves

In the present work, we have studied the spectrum of the primordial gravitational waves due to magnetic instability in the presence of neutrino asymmetry. The magnetic instability generates a helical magnetic field on a large scale. The anisotropic stress generated by the magnetic field shown to be a source of primordial gravitational waves (GWs) at the time of matter-neutrino decoupling. We expect that the theoretically predicted GWs by this mechanism may be detected by Square Kilometer Array (SKA) or pulsar time array (PTA) observations. We also compare our findings with the results obtained by the earlier work where the effect of magnetic instability was not considered.

On the Possibility of Measuring the Ratio GA/GV by means of Polarized Ultracold Neutrons

The possibility of experimentally determining the ratio λ of the axial weak-interaction constant GA to the vector weak-interaction constant GV by simultaneously measuring the electron and neutrino asymmetries at the same setup is discussed. The proposed measurement and data-processing procedures are described in detail. The determination of λ by the method in question permits disregarding the possible contribution of the Fierz interference term and dispensing with an accurate measurement of the neutron polarization. It is shown that this method makes it possible to measure λ to a precision at a level of 10−4.

Time-varying neutrino mass from a supercooled phase transition: Current cosmological constraints and impact on the Ωm−σ8 plane

In this paper we investigate a time-varying neutrino mass model, motivated by the mild tension between cosmic microwave background (CMB) measurements of the matter fluctuations and those obtained from low-redshift data. We modify the minimal case of the model proposed by Dvali and Funcke (2016) that predicts late neutrino mass generation in a post-recombination cosmic phase transition, by assuming that neutrino asymmetries allow for the presence of relic neutrinos in the late-time Universe. We show that, if the transition is supercooled, current cosmological data (including CMB temperature, polarization and lensing, baryon acoustic oscillations, and Type Ia supernovae) prefer the scale factor $a_s$ of the phase transition to be very large, peaking at $a_s\sim 1$, and therefore supporting a cosmological scenario in which neutrinos are almost massless until very recent times. We find that in this scenario the cosmological bound on the total sum of the neutrino masses today is significantly weakened compared to the standard case of constant-mass neutrinos, with $\sum m_\nu<4.8$~eV at 95\% confidence, and in agreement with the model predictions. The main reason for this weaker bound is a large correlation arising between the dark energy and neutrino components in the presence of false vacuum energy that converts into the non-zero neutrino masses after the transition. This result provides new targets for the coming KATRIN and PTOLEMY experiments. We also show that the time-varying neutrino mass model considered here does not provide a clear explanation to the existing cosmological $\Omega_m$-$\sigma_8$ discrepancies.

ACORN: Measuring the electron-antineutrino correlation in neutron beta decay

The aCORN experiment uses a novel asymmetry method to measure the electron-antineutrino correlation (a-coefficient) in free neutron decay that does not require precision proton spectroscopy. aCORN completed two physics runs at the NIST Center for Neutron Research. The first run on the NG-6 beam line obtained the result a = 0.1090 +/- 0.0030 (stat) +/- 0.0028 (sys), the most precise to date. The second run on the new NG-C high flux beam line promises an improvement in precision to ¡ 2%. In addition we show that an improved measurement of the neutrino asymmetry (B-coefficient) can be made using the aCORN apparatus on a highly polarized neutron beam.

Generation of the relic neutrino asymmetry in a hot plasma of the early universe

The neutrino asymmetry in the early universe plasma, [Formula: see text], is calculated both before and after the electroweak phase transition (EWPT). In the Standard Model, before EWPT, the leptogenesis is well known to be driven by the abelian anomaly in a massless hypercharge field. The generation of the neutrino asymmetry in the Higgs phase after EWPT, in its turn, has not been considered previously because of the absence of any quantum anomaly in an external electromagnetic field for such electroneutral particles as neutrino, unlike the Adler–Bell–Jackiw anomaly for charged left and right polarized massless electrons in the same electromagnetic field. Using the neutrino Boltzmann equation, modified by the Berry curvature term in the momentum space, we establish the violation of the macroscopic neutrino current in plasma after EWPT and exactly reproduce the nonconservation of the lepton current in the symmetric phase before EWPT arising in quantum field theory due to the nonzero lepton hypercharge and corresponding triangle anomaly in an external hypercharge field. In the last case, the nonconservation of the lepton current is derived through the kinetic approach without a computation of corresponding Feynman diagrams. Then, the new kinetic equation is applied for the calculation of the neutrino asymmetry accounting for the Berry curvature and the electroweak interaction with background fermions in the Higgs phase. Such an interaction generates a neutrino asymmetry through the electroweak coupling of neutrino currents with electromagnetic fields in plasma, which is [Formula: see text]. It turns out that this effect is especially efficient for maximally helical magnetic fields.

Cosmological bounds on neutrino degeneracy and the Dirac neutrino magnetic moment

The amplification of a seed cosmological magnetic field (CMF) in a hot electroweak plasma of early Universe driven by neutrino degeneracy (asymmetry) is provided by a lower bound on such asymmetries that is in agreement with the known upper (BBN) bound on the electron neutrino asymmetry. Independently of a mechanism for CMF generation one predicts a stringent upper bound on the Dirac neutrino magnetic moment using the lower bound on CMF amplitude found from the Fermi satellite experiment.

Flavor versus mass eigenstates in neutrino asymmetries: implications for cosmology

We show that, if they exist, lepton number asymmetries (L_alpha ) of neutrino flavors should be distinguished from the ones (L_i) of mass eigenstates, since Big Bang Nucleosynthesis (BBN) bounds on the flavor eigenstates cannot be directly applied to the mass eigenstates. Similarly, Cosmic Microwave Background (CMB) constraints on the mass eigenstates do not directly constrain flavor asymmetries. Due to the difference of mass and flavor eigenstates, the cosmological constraint on the asymmetries of neutrino flavors can be much stronger than the conventional expectation, but they are not uniquely determined unless at least the asymmetry of the heaviest neutrino is well constrained. The cosmological constraint on L_i for a specific case is presented as an illustration.

Neutrino induced vorticity, Alfv\xe9n waves and the normal modes

We consider a plasma consisting of electrons and ions in the presence of a background neutrino gas and develop the magnetohydrodynamic equations for the system. We show that the electron neutrino interaction can induce vorticity in the plasma even in the absence of any electromagnetic perturbations if the background neutrino density is left–right asymmetric. This induced vorticity supports a new kind of Alfvén wave whose velocity depends on both the external magnetic field and on the neutrino asymmetry. The normal mode analysis show that in the presence of neutrino background the Alfvén waves can have different velocities. We also discuss our results in the context of dense astrophysical plasma such as magnetars and show that the difference in the Alfvén velocities can be used to explain the observed pulsar kick. We discuss also the relativistic generalisation of the electron fluid in presence of an asymmetric neutrino background.

Nonconservation of lepton current and asymmetry of relic neutrinos

The neutrino asymmetry in the early universe plasma, $n_\nu - n_{\bar \nu}$, is calculated both before and after the electroweak phase transition (EWPT). The leptogenesis before EWPT within the standard model is well known to be driven by the abelian anomaly in a massless hypercharge field. The generation of the neutrino asymmetry in the Higgs phase after EWPT, in its turn, has not been considered previously because of the absence of any quantum anomaly in an external electromagnetic field for such electroneutral particles as neutrino, unlike the Adler anomaly for charged left and right polarized massless electrons in the same electromagnetic field. Using the neutrino Boltzmann equation, modified by the Berry curvature term in the momentum space, we establish the violation of the macroscopic neutrino current in plasma after EWPT and exactly reproduce the nonconservation of the lepton current in the symmetric phase before EWPT arising in quantum field theory due to the non-zero lepton hypercharge and corresponding triangle anomaly in an external hypercharge field. In the last case the violation of the lepton current is derived through the kinetic approach without the computation of the corresponding Feynman diagrams. Then the new kinetic equation is applied for the calculation of the neutrino asymmetry accounting for the the Berry curvature and the electroweak interaction with background fermions in the Higgs phase, including the stage after the neutrino decoupling at the absence of neutrino collisions in plasma. This asymmetry is found to be rather small to be observed. Thus, a difference of the relic neutrino and antineutrino densities, if exists, should be acquired mainly in the symmetric phase before EWPT.

Generation of cosmic magnetic fields in electroweak plasma

We study the generation of strong magnetic fields in magnetars and in the early universe. For this purpose we calculate the antisymmetric contribution to the photon polarization tensor in a medium consisting of an electron-positron plasma and a gas of neutrinos and antineutrinos, interacting within the Standard Model. Such a contribution exactly takes into account the temperature and the chemical potential of plasma as well as the photon dispersion law in this background matter. It is shown that a nonvanishing Chern-Simons parameter, which appears if there is a nonzero asymmetry between neutrinos and antineutrinos, leads to the instability of a magnetic field resulting to its growth. We apply our result to the description of the magnetic field amplification in the first second of a supernova explosion. It is suggested that this mechanism can explain strong magnetic fields of magnetars. Then we use our approach to study the cosmological magnetic field evolution. We find a lower bound on the neutrino asymmetries consistent with the well-known Big Bang nucleosynthesis bound in a hot universe plasma. Finally we examine the issue of whether a magnetic field can be amplified in a background matter consisting of self-interacting electrons and positrons.

Instability of magnetic fields in electroweak plasma driven by neutrino asymmetries

The magnetohydrodynamics (MHD) is modified to incorporate the parity violation in the Standard Modelleading to a new instability of magnetic fields in the electroweak plasma in the presence ofnonzero neutrino asymmetries. The main ingredient for such a modified MHD is the antisymmetric part ofthe photon polarization tensor in plasma, where the parity violating neutrino interaction with charged leptonsis present. We calculate this contribution to the polarizationtensor connected with the Chern-Simons term in effective Lagrangian ofthe electromagnetic field. The general expression for such a contribution which depends onthe temperature and the chemical potential of plasma as well as on the photon's momentum is derived.The instability of a magnetic field driven by the electron neutrino asymmetry for the ν-burst during the first second of a supernova explosion can amplify a seed magnetic field of a protostar, and, perhaps, can explain the generation of strongest magnetic fields in magnetars.The growth of a cosmological magnetic field driven by the neutrino asymmetry density Δnν = nν−nν̄≠0 is provided by a lower bound on |ξνe| = |μνe|/T which is consistent with the well-known Big Bang nucleosynthesis (upper) bound on neutrino asymmetries in a hot universe plasma.

The strongest bounds on active-sterile neutrino mixing after Planck data

Light sterile neutrinos can be excited by oscillations with active neutrinos in the early universe. Their properties can be constrained by their contribution as extra-radiation, parameterized in terms of the effective number of neutrino species Neff, and to the universe energy density today Ωνh2. Both these parameters have been measured to quite a good precision by the Planck satellite experiment. We use this result to update the bounds on the parameter space of (3+1) sterile neutrino scenarios, with an active-sterile neutrino mass squared splitting in the range (10−5–102) eV2. We consider both normal and inverted mass orderings for the active and sterile states. For the first time we take into account the possibility of two non-vanishing active-sterile mixing angles. We find that the bounds are more stringent than those obtained in laboratory experiments. This leads to a strong tension with the short-baseline hints of light sterile neutrinos. In order to relieve this disagreement, modifications of the standard cosmological scenario, e.g. large primordial neutrino asymmetries, are required.

Multimomentum and multiflavor active-sterile neutrino oscillations in the early universe: Role of neutrino asymmetries and effects on nucleosynthesis

We perform a study of the flavour evolution in the early universe of a multi-flavour active-sterile neutrino system with parameters inspired by the short-baseline neutrino anomalies. In a neutrino-symmetric bath a "thermal" population of the sterile state would quickly grow, but in the presence of primordial neutrino asymmetries a self-suppression as well as a resonant sterile neutrino production can take place, depending on temperature and chosen parameters. In order to characterize these effects, we go beyond the usual average momentum and single mixing approximations and consider a multi-momentum and multi-flavour treatment of the kinetic equations. We find that the enhancement obtained in this case with respect to the average momentum approximation is significant, up to \sim 20 % of a degree of freedom. Such detailed and computationally demanding treatment further raises the asymmetry values required to significantly suppress the sterile neutrino production, up to large and preferentially net asymmetries |L_{\nu}| > O(10^{-2}). For such asymmetries, however, the active-sterile flavour conversions happen so late that significant distortions are produced in the electron (anti)neutrino spectra. The larger |L_{\nu}|, the more the impact of these distortions takes over as dominant cosmological effect, notably increasing the 4 He abundance in primordial nucleosynthesis (BBN). The standard expression of the primordial yields in terms of the effective number of neutrinos and asymmetries is also greatly altered. We numerically estimate the magnitude of such effects for a few representative cases and comment on possible implications for forthcoming cosmological measurements.

Active-sterile neutrino oscillations in the early universe with dynamical neutrino asymmetries

In the last recent years different anomalies observed in short-baseline neutrino oscillation experiments seem to point towards the existence of light sterile neutrinos. These sterile neutrinos can also be produced in the early universe by oscillations of the active neutrinos and can affect different cosmological observables. In order to quantify the abundance of sterile neutrinos, we perform a detailed study of the flavor evolution in (3+1) and (2+1) oscillation schemes, in presence of dynamical primordial neutrino asymmetries L. We find that for |L|≲10−4eV sterile neutrinos would be completely thermalized creating a tension with the cosmological data. An asymmetry of |L|≳10−3 is then required in order to suppress the sterile production and to reconcile them with cosmology.