Neutrino flavor conversion in random magnetic fields

If massive neutrinos possess magnetic moments, a magnetic field can cause a spin flip. In the case of Dirac neutrinos the spin flip converts an active neutrino into a sterile one and vice versa. By constrast, if neutrinos are Majorana particles, a spin flip converts them to a neutrino of a different flavor. We examine the behavior of neutrinos in a random magnetic field as it occurs, for instance, in certain astronomical objects, such as an active galactic nucleus. Both Dirac and Majorana neutrinos behave ergodically: independently of their initial density matrix, they tend towards an equipartition of the helicity states. As a result, about half of the Dirac neutrinos produced becomes sterile. For Majorana neutrinos, there will be an approximate equipartition of flavors, independently of the production mechanism.     

Oscillation properties of active and sterile neutrinos and neutrino anomalies at short distances

A generalized phenomenological (3 + 2 + 1) model featuring three active and three sterile neutrinos that is intended for calculating oscillation properties of neutrinos for the case of a normal activeneutrino mass hierarchy and a large splitting between the mass of one sterile neutrino and the masses of the other two sterile neutrinos is considered. A new parametrization and a specific form of the general mixing matrix are proposed for active and sterile neutrinos with allowance for possible CP violation in the lepton sector, and test values are chosen for the neutrino masses and mixing parameters. The probabilities for the transitions between different neutrino flavors are calculated, and graphs representing the probabilities for the disappearance of muon neutrinos/antineutrinos and the appearance of electron neutrinos/antineutrinos in a beam of muon neutrinos/antineutrinos versus the distance from the neutrino source for various values of admissible model parameters at neutrino energies not higher than 50 MeV, as well as versus the ratio of this distance to the neutrino energy, are plotted. It is shown that the short-distance accelerator anomaly in neutrino data (LNSD anomaly) can be explained in the case of a specific mixing matrix for active and sterile neutrinos (which belongs to the a₂ type) at the chosen parameter values. The same applies to the short-distance reactor and gallium anomalies. The theoretical results obtained in the present study can be used to interpret and predict the results of ground-based neutrino experiments aimed at searches for sterile neutrinos, as well as to analyze some astrophysical observational data.     

Towards the detection of light and heavy relic neutrinos

The standard Big Bang cosmology predicts that the universe is abundantly populated with neutrinos. As expected there are at least 114 neutrinos per cubic centimeter averaged over the whole space. Like the cosmic background radiation the cosmic neutrinos at present posses a very small kinetic energy due to expansion of the universe. This prediction is one of the cornerstones of modern cosmology. On the other hand the existence of cosmic neutrinos has not yet been confirmed by direct detection experiments. For now we only have a lower limit on the total mass of this free floating ghostly gas of neutrinos, but even so it is roughly equivalent to the total mass of all the visible stars in universe. There could be many more neutrinos at Earth because of condensation of neutrinos, now moving slowly under the gravitational pull of our galaxy. Here we discuss the possibility of detection of relic neutrinos in KATRIN and MARE experiments via neutrino capture on tritium and rhenium, respectively. We also examine single and double relic neutrino capture on double β-decaying nuclei which might be relevant in the context of the new generation double beta decay experiments. Further we explore feasibility of experiments for detection of heavy sterile neutrinos with masses in MeV region, which may have important astrophysical and cosmological implications.     

Decay and decoupling of heavy right-handed Majorana neutrinos in the L–R model

Heavy right-handed neutrinos are of current interest. The interactions and decay of such neutrinos determine their decoupling epoch during the evolution of the universe. This in turn affects various observable features like the energy density, nucleosynthesis, CMBR spectrum, galaxy formation and baryogenesis. Here, we consider reduction of right-handed electron-type Majorana neutrinos, in the left–right symmetric model, by the channel and the channel originating from an anomaly, involving the gauge group, as well as decay of such neutrinos. We study the reduction of these neutrinos for different ranges of left–right model parameters, and find that, if the neutrino mass exceeds the right-handed gauge boson mass, then the neutrinos never decouple for realistic values of the parameters, but, rather, decay in equilibrium. Because there is no out-of-equilibrium decay, no mass bounds can be set for the neutrinos. Received: 1 November 2000 / Published online: 23 February 2001     

Neutrinos in matter and external fields

A brief survey of effects generated by the influence of the environment on neutrinos is presented. The issues considered here include flavor and spin oscillations of neutrinos in matter, in electromagnetic fields of various configurations, and in gravitational fields; the electromagnetic properties of the neutrinos and the environment-induced change in these properties; photoneutrino processes generated by the environment; urca processes in magnetic fields; various mechanisms that may be responsible for the asymmetry of neutrino radiation from neutron stars; quantum states of neutrinos in matter and the spin light of neutrinos in matter and external fields; and astrophysical and cosmological applications of the above processes and phenomena. The method that is employed to describe the effect of the environment on neutrinos (as well as on electrons) and which is based on the application of exact solutions to the corresponding modified Dirac equations for particles in matter is briefly discussed.     

Can scalar neutrinos or massive Dirac neutrinos be the missing mass?

Scalar neutrinos and massive Dirac neutrinos in the mass range 2–20 GeV have been proposed as candidates to provide the dark matter in the halo of our galaxy. If so, the particles are captured inthe Earth with an efficiency of 10⁻¹⁰ − 10⁻⁷. For Dirac neutrinos more massive than about 9 GeV and scalar neutrinos more massive than abour 12 GeV, enough are captured to produce an observable neutrino flux at the surface of the Earth (∼ 10⁻² cm⁻² s⁻¹ for sneutrinos and ∼ 1.4 × 10⁻³ cm⁻² s⁻¹ for Dirac neutrinos), several orders of magnitude above atmospheric background and above what is observed. Hence stable scalar neutrinos of mass 12–20 GeV or Dirac neutrinos of mass 9–20 GeV cannot be the dominant component of the halo.     

Lithium experiment on solar neutrinos to weight the CNO cycle

The measurement of the flux of beryllium neutrinos with an accuracy of about 10% and CNO neutrinos with an accuracy of 20–30% will enable one to find the flux of pp neutrinos in the source with an accuracy better than 1% using the luminosity constraint. The future experiments on νe^? scattering will enable one to measure with very good accuracy the flux of beryllium and pp neutrinos on the Earth. The ratio of the flux of pp neutrinos on the Earth and in the source will enable one to find with very good accuracy a mixing angle θ_⊙. A lithium detector has high sensitivity to CNO neutrinos and can find the contribution of the CNO cycle to the energy generated in the Sun. This will be a stringent test of the theory of stellar evolution and combined with other experiments will provide a precise determination of the flux of pp neutrinos in the source and a mixing angle θ_⊙. The work on the development of the technology of a lithium experiment is now in progress.     

Medium energy neutrinos from solar flares

The production of neutrinos with energies higher than 0.1 GeV in the solar atmosphere during solar flares is discussed. Neutrinos and muons are generated in decays of π^+- mesons produced in nuclear interactions of accelerated solar flare protons with matter of the Sun. Muons themselves decay yielding neutrinos. These neutrinos could come to the Earth and be detected with neutrino telescopes. Estimations of fluxes of such neutrinos are given.     

Sterile neutrinos in string derived models

The MiniBooNE collaboration recently reported further evidence for the existence of sterile neutrinos, implying substantial mixing with the left-handed active neutrinos and at a comparable mass scale. I argue that while sterile neutrinos may arise naturally in large volume string models, they prove more of a challenge in heterotic-string models that replicate the Grand Unified Theory structure of the Standard Model matter states. Sterile neutrinos in heterotic-string models may imply the existence of an additional Abelian gauge symmetry of order 10–100 TeV.     

Three layers of neutrinos

In this Letter we point out that in a class of models for spontaneous R-parity breaking based on gauged B−L$B-L$, the spectrum for neutrinos is quite peculiar. We find that those models generally predict three layers of neutrinos: one heavy sterile neutrino, two massive active neutrinos, and three nearly massless (one active and two sterile) neutrinos.     

Effective potential for highly relativistic neutrinos in a weakly magnetized medium and their oscillation

We have calculated the effective potential experienced by highly relativistic neutrinos in a weakly magnetized electron–positron plasma, where a momentum-dependent finite-width correction to the propagator of W is considered to account for the threshold effect. Magnetars are believed to be sources of TeV–PeV neutrinos which are produced due to photomeson and proton–proton interactions in their atmosphere. We have studied the resonant-oscillation process ν _e ↔ ν _μ,τ of the highly relativistic neutrinos in the atmosphere of SGR 1806-20, which is a magnetar. It is shown that, for high-energy neutrinos propagating within the magnetar atmosphere, the resonance condition can never be satisfied. On the other hand, if GeV neutrinos are produced deep inside the magnetar atmosphere, where the temperature is about 50 keV or more, then these neutrinos can undergo resonant oscillation.     

Monochromatic neutrinos from the annihilation of fourth-generation massive stable neutrinos in the Sun and in the Earth

The accumulation of relic fourth-generation heavy neutrinos (of mass 50 GeV) in the Earth and the Sun, which is followed by their annihilation, is considered. The most conservative estimates of the fluxes of monochromatic electron, muon, and tau neutrinos and antineutrinos of energy 50 GeV from the annihilation of heavy neutrinos are 4.1×10^?6 cm^?2 s^?1 from Earth’s core and 1.1×10^?7 cm^?2 s^?1 from Sun’s core, whence it follows that an analysis of data from underground neutrino observatories may furnish additional information about the existence of fourth-generation neutrinos. It is shown that, because of kinetic equilibrium between the arrival of cosmic neutrinos and their annihilation, the existence of new U(1) gauge interaction of fourth-generation neutrinos has virtually no effect on the estimates of the annihilation fluxes of electron, muon, and tau neutrinos.     

A lithium experiment in the program of solar neutrino research

Experiments sensitive to pp neutrinos from the Sun are very promising for precise measurement of the mixing angle ϑ ₁₂. A νe ⁻ scattering experiment (XMASS) and/or a charged-current experiment (indium detector) can measure the flux of electron pp neutrinos. One can find the total flux of pp neutrinos from a luminosity constraint after the contributions of ⁷Be and CNO neutrinos to the total luminosity of the Sun are measured. A radiochemical experiment utilizing a lithium target has high sensitivity to the CNO neutrinos; thus, it has a good promise for precise measurement of the mixing angle and for a test of the current theory of evolution of the stars. The text was submitted by the authors in English.     

Recent Highlights from IceCube

The ~1 km ³ IceCube neutrino observatory was completed in December, 2010 and is taking data on cosmic-ray muons and neutrinos, extraterrestrial neutrinos, and setting limits on a variety of exotic phenomena. This proceeding will cover recent IceCube results, with an emphasis on cosmic rays and on searches for extraterrestrial neutrinos, with a stress on results presented at the 2013 International Cosmic Ray Conference.     

Dark matter neutrinos must come with degenerate masses

It has been known that there are two schemes in the framework of three flavor neutrinos to accommodate the global features of the hot dark matter neutrinos, the solar neutrino deficit and the atmospheric neutrino anomaly in a manner consistent with terreatrial neutrino experiments, i.e., hierarchical mass neutrinos and almost degenerate neutrinos. We deminstrate that the recent result by the CHOOZ experiment excludes the scheme of hierarchical neutrinos. We also point out in the scheme of almost degenerate neutrinos that if neutrinos are Majorana particles then the double β decay experiments must see positive signals on their way to reach a limit more stringent than the present one by a factor of 5.     

Contribution of gallium experiments to the understanding of solar physics and neutrino physics

The results of gallium measurements of solar neutrino and measurements with artificial sources of neutrinos are presented. Conclusions are drawn from these results, and the potential of the SAGE experiment for studying transitions of active neutrinos to sterile states for Δm ² > 0.5 eV² and a sensitivity of a few percent to the disappearance of electron neutrinos is examined.     

Neutrino Production via e-e+ Collision at Z-Boson Peak

The production of the three normal neutrinos via e^-e⁺ collision at Z-boson peak (neutrino production in a Z-factory) is investigated thoroughly. The differences of v_e-pair production from v_μ-pair and v_τ-pair production are presented in various aspects. Namely the total cross sections, relevant differential cross sections and the forward-backward asymmetry etc. for these neutrinos are presented in terms of figures as well as numerical tables. The restriction on the room for the mixing of the three species of light neutrinos with possible externals (heavy neutral leptons and/or sterile neutrinos) from refined measurements of the invisible width of Z-boson is discussed.     

Resonance amplification and T-violation effects in three-neutrino oscillations in the earth

The matter effects in three-neutrino oscillations in a beam of neutrinos passing through the earth are discussed. An exact analytic expression for the T-violating asymmetry in oscillations involving the three flavour neutrinos in matter with constant density is derived and it is shown that this asymmetry can be amplified considerably by matter. In particular, the T-violating asymmetry in the oscillations of neutrinos which have passed through the earth can be much larger than the analogous asymmetry in the oscillations in vacuum.     

Constraint on the heavy sterile neutrino mixing angles in the SO(10) model with double see-saw mechanism

Constraints on the heavy sterile neutrino mixing angles are studied in the framework of a minimal supersymmetric SO(10) model with the use of the double see-saw mechanism. A new singlet matter in addition to the right-handed neutrinos is introduced to realize the double see-saw mechanism. The light Majorana neutrino mass matrix is, in general, given by a combination of those of the singlet neutrinos and the active neutrinos. The minimal SO(10) model is used to give an example form of the Dirac neutrino mass matrix, which enables us to predict the masses and the mixing angles in the enlarged 9×9 neutrino mass matrix. Mixing angles between the light Majorana neutrinos and the heavy sterile neutrinos are shown to be within the LEP experimental bound on all ranges of the Majorana phases.