Neutrino Asymmetry Research Articles

Large neutrino asymmetries from neutrino oscillations.

We re-examine neutrino oscillations in the early universe. Contrary to previous studies, we show that large neutrino asymmetries can arise due to oscillations between ordinary neutrinos and sterile neutrinos. This means that the Big Bang Nucleosynthesis (BBN) bounds on the mass and mixing of ordinary neutrinos with sterile neutrinos can be evaded. Also, it is possible that the neutrino asymmetries can be large (i.e. $\stackrel{>}{\sim} 10\%$), and hence have a significant effect on BBN through nuclear reaction rates.

Reconciling sterile neutrinos with Big Bang nucleosynthesis.

We re-examine the big bang nucleosynthesis (BBN) bounds on the mixing of neutrinos with sterile species. These bounds depend on the assumption that the relic neutrino asymmetry $L_{\nu}$ is very small. We show that for $L_{\nu}$ large enough (greater than about $10^{-5}$) the standard BBN bounds do not apply. We apply this result to the sterile neutrino solution to the atmospheric neutrino anomaly and show that for $L_{\nu} > 7 \times 10^{-5}$ it is consistent with BBN. The BBN bounds on sterile neutrinos mixing with electron neutrinos can also be weakened considerably.

ELECTROMAGNETIC FORWARD NEUTRINO-NEUTRINO SCATTERING IN DENSE MATTER

Using covariant methods we calculate the coherent forward amplitude for the exchange of a virtual photon between neutrinos having medium-induced electromagnetic vertices. Such a process can affect the important Mikheyev-Smirnov-Wolfenstein (MSW) mechanism in media with large neutrino asymmetries. It can be considered as a finite temperature and density (FTD) version of a two-loop contribution which gives rise to non-universal (νι-dependent) radiative corrections to the forward scattering νν → νν. Assuming no inert neutrino species, we discuss the possible importance of this contribution to the MSW effect in a supernova core and in the early Universe with large but equal neutrino asymmetries.

Neutrino asymmetry and oscillations in the early universe

We consider oscillations of the electron neutrino v e into an inert neutrino species v x in the heat bath of the early universe. There thermal corrections dominate the oscillation patterns and give rise to resonant regions for a wide range in the parameter space. We show that the electron neutrino asymmetry will dynamically be adjusted to zero, provided the initial asymmetry is not very large. If B- L is conserved, neutrino oscillations cannot give rise to a large neutrino asymmetry during the onset of nucleosynthesis.

Angular correlations in the decay of the free neutron

The sensitivity of a 2×2 π solid-angle neutron beta-decay spectrometer to various beta-decay correlation coefficients is calculated. A spectrometer similar to one (PERKEO) recently used to measure the neutron beta-decay asymmetry, but incorporating both electron and proton detection, would provide a practical method for measuring the neutrino asymmetry and the electron-neutrino correlation.

Gravitational coupling of neutrinos to matter vorticity. II. Microscopic asymmetries in angular momentum modes.

Microscopic asymmetries in neutrino physics are examined, which are due to the presence of a vorticity field of matter. We extend the results of an earlier paper to a complete basis of invariant angular momentum and energy modes. These modes provide a representation basis for the algebra of the total angular momentum of the system. The angular momentum space appears to be polarized along the direction determined by the local vorticity field \ensuremath{\Omega}\ensuremath{\rightarrow}. The neutrino asymmetry of currents is calculated for these modes.

Majorana neutrinos, nucleosynthesis, and the lepton asymmetry of the Universe

The lepton asymmetry $L\ensuremath{\simeq}\frac{({n}_{\ensuremath{\nu}}\ensuremath{-}{n}_{\overline{\ensuremath{\nu}}})}{{n}_{\ensuremath{\gamma}}}$ of the Universe is one of the main uncertainties in the standard hot big-bang cosmological model. Direct limits on $L$ from the total energy density of the Universe are extremely weak ($|L|<4\ifmmode\times\else\texttimes\fi{}{10}^{4}$), and Harvey and Kolb have recently shown that it is possible to construct grand unified theories in which $L$ is much larger than the baryon asymmetry $B\ensuremath{\sim}{10}^{\ensuremath{-}10}$. On the other hand, many authors have pointed out that the standard scenario for nucleosynthesis would be significantly altered if $|L|$ were of order unity. In this paper we consider the role of the lepton-number violation associated with Majorana neutrino masses in the 10-eV range in reducing an initial large lepton asymmetry. We find that in most models with explicit hard lepton-number violation any initial lepton asymmetry would be reduced to an insignificant level long before nucleosynthesis. For models in which lepton number is violated spontaneously (or relies on S${\mathrm{U}}_{2}$\ifmmode\times\else\texttimes\fi{}${\mathrm{U}}_{1}$ breaking), we utilize an argument due to Linde that a large initial lepton asymmetry would prevent the restoration of S${\mathrm{U}}_{2}$\ifmmode\times\else\texttimes\fi{}${\mathrm{U}}_{1}$ (and of lepton number) in the early Universe. For a large class of models and initial conditions, the asymmetry of the lightest neutrino species would be reduced to a fixed-point value of order unity, prior to nucleosynthesis, while the asymmetries in the more massive neutrinos would be driven to negligible levels. This is precisely the range for which the neutrino asymmetries would significantly affect nucleosynthesis.

First-order radiative corrections to asymmetry coefficients in neutron decay

Electromagnetic radiative corrections to the electron-neutrino, electron, and neutrino asymmetry coefficients in neutron $\ensuremath{\beta}$ decay are calculated. Strong-interaction effects are retained as suggested by Sirlin.