Neutrino oscillations in the early Universe

We discuss the oscillations effects on neutrinos in the early Universe and update the cosmological constraints on the oscillation parameters. It is shown that sterile LOW solution to the solar neutrino problem is almost completely excluded from cosmological nucleosynthesis considerations. Two possibilities for the relaxation of this constraint are discussed: high primordial helium-4 yield and a relic lepton asymmetry present at nucleosynthesis epoch. The numerical analysis proved that Y_p=0.25 only relaxes the constraint on LOW solution, while L > 10^{-5} is capable to remove it.Comment: 4 pages, 3 eps figures included, espcrc2.sty, talk at NOW2000 Workshop, 11-16 September 2000, Otranto, Italy, to be published in Nucl. Phys. B Supp

BBN with Late Electron-Sterile Neutrino Oscillations - The Finest Leptometer

A relic lepton asymmetry orders of magnitude bigger than the baryon one may hide in the relic neutrino background. No direct theoretical or experimental limitations on its magnitude and sign are known. Indirect cosmological constraints exist ranging from $|L|<0.01$ to $L<10$. We discuss a BBN model with late electron-sterile neutrino oscillations which is a fine leptometer - it is capable of feeling extremely small relic lepton asymmetry - $|L|>10^{-8}$. This sensitivity is achieved through the influence of such small $L$ on the neutrino oscillations, suppressing or enhancing them, and thus changing the primordially produced $^4He$. The influence of $L$ on nucleons freezing in pre-BBN epoch is numerically analyzed in the full range of the oscillation parameters of the model and for $L \ge 10^{-10}$. The case of oscillations generated asymmetry by late electron-sterile oscillations and its effect on primordial $^4He$ is also briefly discussed.Comment: 13 pages, 6 fig

Big Bang Nucleosynthesis and Cosmological Constraints on Neutrino Oscillation Parameters

We present a review of cosmological nucleosynthesis (CN) with neutrino oscillations, discussing the different effects of oscillations on CN, namely: increase of the effective degrees of freedom during CN, spectrum distortion of the oscillating neutrinos, neutrino number density depletion, and growth of neutrino-antineutrino asymmetry due to active-sterile oscillations. We discuss the importance of these effects for the primordial yield of helium-4. Primordially produced He-4 value is obtained in a selfconsistent study of the nucleons and the oscillating neutrinos. The effects of spectrum distortion, depletion and neutrino-antineutrino asymmetry growth on helium-4 production are explicitly calculated. An update of the cosmological constraints on active-sterile neutrino oscillations parameters is presented, giving the values: delta m^2 sin^8 (2 theta) 0, and |delta m^2| < 8.2 x 10^{-10} eV^2 at large mixing angles for delta m^2 < 0. According to these constraints, besides the active-sterile LMA solution, also the active-sterile LOW solution to the solar neutrino problem is almost totally excluded.Comment: Extended and updated version of the invited talk, presented at the International Conference "Hot Points in Astrophysics", held in JINR, Dubna, Russia, August 22-26, 2000. An update of the cosmological constraints on active-sterile neutrino oscillation parameters is presented (Fig.2 added

Neutrino oscillations in the early universe: How large lepton asymmetry can be generated?

The lepton asymmetry that could be generated in the early universe through oscillations of active to sterile neutrinos is calculated (almost) analytically for small mixing angles, sin 2\theta < 10^{-2}. It is shown that for a mass squared difference, \delta m^2=-1 eV^2 it may rise at most by 6 orders of magnitude from the initial ``normal'' value of 10^{-10}, since the back-reaction from the refraction index terminates this rise while the asymmetry is still small. Only for very large mass differences, \delta m^2 about 10^9 eV^2, the lepton asymmetry could reach a significant magnitude exceeding 0.1.Comment: 25 pages, LaTeX (5 graphs

Neutrino Spectrum Distortion Due to Oscillations and its BBN Effect

We study the distortion of electron neutrino energy spectrum due to oscillations with the sterile neutrino nu_e nu_s, for different initial populations of the sterile state delta N_s at the onset of oscillations. The influence of this spectrum distortion on Big Bang Nucleosynthesis is analyzed. Only the case of an initially empty sterile state was studied in previous publications. The primordial abundance of He-4 is calculated for all possible delta N_s: 0<= delta N_s<= 1 in the model of oscillations, effective after electron neutrino decoupling, for which the spectrum distortion effects on the neutron--proton transitions are the strongest. It is found that the spectrum distortion effect may be the dominant one not only in the case of small delta N_s, but also in the case of big initial population of nu_s. For example, in the resonant case it may play a considerable role even for very big delta N_s ~ 0.8.Comment: 13 pages, 2 figures, a revised shortened version, main results unchanged, references updated, typos correcte