Neutrino mixing and the mw/mz mass ratio

A comparison of neutral and charged current data according to the standard model provides bounds on neutrino mixing parameters, independently of the number of fermions. The mixing may also affect the determination of sin² θ and m_w/m_z.     

Neutrino mass hierarchy and stepwise spectral swapping of supernova neutrino flavors

We examine a phenomenon recently predicted by numerical simulations of supernova neutrino flavor evolution: the swapping of supernova nu(e) and nu(mu,tau) energy spectra below (above) energy E(C) for the normal (inverted) neutrino mass hierarchy. We present the results of large-scale numerical calculations which show that in the normal neutrino mass hierarchy case, E(C) decreases as the assumed effective 2x2 vacuum nu(e)<==>nu(mu,tau) mixing angle (approximately theta13) is decreased. In contrast, these calculations indicate that E(C) is essentially independent of the vacuum mixing angle in the inverted neutrino mass hierarchy case. With a good neutrino signal from a future galactic supernova, the above results could be used to determine the neutrino mass hierarchy even if theta13 is too small to be measured by terrestrial neutrino oscillation experiments.     

Neutrino absorption cross sections in the supernova environment

We study charged-current neutrino cross sections on neutron-rich nuclei in the mass A∼60 region. Special attention is paid to environmental effects, i.e., finite temperature and density, on the cross sections. As these effects are largest for small neutrino energies, it is sufficient to study only the Gamow–Teller (GT) contributions to the cross sections. The relevant GT strength distributions are derived from large-scale shell model calculations. We find that the low-energy cross sections are enhanced at finite temperatures. However, for (ν_e,e⁻) reactions Pauli blocking of the electrons in the final state makes the cross sections for low-energy neutrinos much smaller than for the competing inelastic scattering on electrons at moderate and large densities. Absorption cross sections for low-energy antineutrinos are strongly enhanced at finite temperatures.     

Neutrino mixing and see-saw mechanism

Models of neutrino masses are discussed capable of explaining in a natural way the maximal mixing between ν_μ and ν_τ observed by the Super-Kamiokande Collaboration. For three generations of leptons two classes of such models are found implying: a) Δm₂₃²Δm₁₂²≈Δm₁₃² and a small mixing between ν_e and the other two neutrinos, b) Δm₁₂²Δm₁₃²≈Δm₂₃² and a nearly maximal mixing for solar neutrino oscillations in vacuum.     

Total energy of the neutrino burst from the supernova 1987A and the mass of the neutron star just born

From the data of the neutrino burst detected by the Kamiokande-II and IMB groups we calculated the total neutrino energy carefully taking into account the efficiency of their water Čerenkov counters. By using this result, we discussed the neutron star mass which was formed by this supernova explosion. We conjectured that the mass is 1.0–1.7M_⊙ and a black hole was not formed, assuming that the distance to the supernova is 50 kpc. If the distance is 56 kpc, the mass range turns into 1.0–1.8M_⊙ and the possibility of black hole formation is also ruled out.     

Neutrino masses and flavor mixing

We study a model for the mass matrices of the leptons, based on texture zero elements. We are able to relate the mass eigenvalues of the charged leptons and of the neutrinos to the mixing angles, and can predict the masses of the neutrinos. We find a normal hierarchy—the masses are 0.005 eV, 0.01 eV and 0.05 eV. Predictions for the double beta decay and the reactor neutrino experiments are made.     

Neutrino mixing in the seesaw model

In the seesaw model with hierarchical Dirac masses, the neutrino mixing angle exhibits the behavior of a narrow resonance. In general, the angle is strongly suppressed, but it can be maximal for special parameter values. We delineate the small regions in which this happens for the two-flavor problem. On the other hand, the physical neutrino masses are hierarchical, in general, except in a large part of the region in which the mixing angle is sizable, where they are nearly degenerate. Our general analysis is also applicable to the RGE of the neutrino mass matrix, where we find analytic solutions for the running of the physical parameters, in addition to a complex RGE invariant relating them. It is also shown that, if one mixing angle is small, the three-neutrino problem reduces to two two-flavor problems. Received: 16 March 2001 / Revised version: 17 May 2001 / Published online: 19 July 2001     

Neutrino mass and mixing implied by underground deficit of low energy muon-neutrino events

Recent observations of a deficit of cosmic ray muon-neutrino interactions in underground detectors suggest that the muon neutrinos may have oscillated to another state. We examine possible neutrino mass and mixing patterns, and their implications for vacuum and matter effects on solar neutrinos, on neutrinos passing through the earth, and on terrastrial neutrino beams. By invoking the see-saw mechanism of neutrino mass generation, we draw inferences on closure of the universe with neutrino masses, on the number of generations, on t-quark and fourth generation masses, and on the Peccei-Quinn symmetry breaking scale. Testable predictions are suggested.     

Neutrino masses, mixing and hierarchy

We build a model to describe neutrinos based on strict hierarchy, incorporating as much as possible, the latest known data, for Δ_sol and Δ_atm, and for the mixing angles determined from neutrino oscillation experiments, including that from KamLAND. Since the hierarchy assumption is a statement about mass ratios, it lets us obtain all three neutrino masses. We obtain a mass matrix, M_ν and a mixing matrix, U, where both M_ν and U are given in terms of powers of Λ, the analog of the Cabibbo angle λ in the Wolfenstein representation, and two parameters, ρ and κ, each of order one. The expansion parameter, Λ, is defined by

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, and ρ expresses our ignorance of the lightest neutrino mass m₁, (m₁=ρΛ⁴m₃), while κ scales s₁₃ to the experimental upper limit, s₁₃=κΛ²≈0.16κ. These matrices are similar in structure to those for the quark and lepton families, but with Λ about 1.6 times larger than the λ for the quarks and charged leptons. The upper limit for the effective neutrino mass in double β-decay experiments is 4×10⁻³ eV if s₁₃=0 and 6×10⁻³ eV if s₁₃ is maximal. The model, which is fairly unique, given the hierarchy assumption and the data, is compared to supersymmetric extension and texture zero models of mass generation.     

Neutrino magnetic moment and shock-wave revival in a supernova explosion

The ν _L → ν _R → ν _L double conversion of the Dirac neutrino helicity is analyzed under supernova conditions, in which case the first stage is due to the interaction of the neutrino magnetic moment with plasma electrons and protons in the supernova core, while the second stage is caused by a resonance neutrino-spin flip in the magnetic field of the supernova envelope. It is shown that, if the neutrino has a magnetic moment in the range 10^?13 µ_B < µ _ν < 10^?12 µ_B and if a magnetic field of strength 10¹³ G exists between the neutrinosphere and the region of shock-wave stagnation, an additional energy on the order of 10⁵¹ erg, which is sufficient for stimulating a damped attenuated shock wave, can be injected in this region within the stagnation time.