Probing the neutrino mass matrix in next-generation neutrino oscillation experiments

We review the current status of the neutrino mass and mixing parameters needed to reconstruct the neutrino mass matrix. A comparative study of the precision in the measurement of oscillation parameters expected from the next-generation solar, atmospheric, reactor-and accelerator-based neutrino experiments is presented. We discuss the potential of 0νββ experiments in determining the neutrino mass hierarchy and the importance of a better ϑ ₁₂ measurement for it. The text was submitted by the author in English.     

Probing the seesaw mechanism with neutrino data and leptogenesis

We use the current low-energy neutrino data to understand the structure of the neutrino mass matrix. Considering this information and assuming hierarchical neutrino Yukawa couplings, we use the seesaw formula to study the properties of the heavy right-handed neutrinos N_i. We find that successful baryogenesis via leptogenesis requires mass degeneracy and maximal mixing of N₁ and N₂.     

On the possibility to determine neutrino mass hierarchy via supernova neutrinos with short-time characteristics

In this paper, we investigate whether it is possible to determine the neutrino mass hierarchy via a high-statistics and real-time observation of supernova neutrinos with short-time characteristics. The essential idea is to utilize distinct times-of-flight for different neutrino mass eigenstates from a core-collapse supernova to the Earth, which may significantly change the time distribution of neutrino events in the future huge water-Cherenkov and liquid-scintillator detectors. For illustration, we consider two different scenarios. The first case is the neutronization burst of emitted in the first tens of milliseconds of a core-collapse supernova, while the second case is the black hole formation during the accretion phase for which neutrino signals are expected to be abruptly terminated. In the latter scenario, it turns out only when the supernova is at a distance of a few Mpc and the fiducial mass of the detector is at the level of gigaton, might we be able to discriminate between normal and inverted neutrino mass hierarchies. In the former scenario, the probability for such a discrimination is even less due to a poor statistics.     

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.     

The Earth effects on the supernova neutrino spectra

The Earth effects on the energy spectra of supernova neutrinos are studied. We analyze numerically the time-integrated energy spectra of neutrino in a mantle–core–mantle step function model of the Earth's matter density profile. We consider a realistic frame-work in which there are three active neutrinos whose mass squared differences and mixings are constrained by the present understanding of solar and atmospheric neutrinos. We find that the energy spectra change for some allowed mixing parameters. Especially, the expected number of events at SNO shows characteristic behavior with respect to energy, i.e., a great dip and peak. We show that observations of the Earth effect allow us to identify the solar neutrino solution and to probe the mixing angle θ₂.     

Coherent development of neutrino flavor in the supernova environment

We calculate coherent neutrino and antineutrino flavor transformation in the supernova environment, for the first time including self-consistent coupling of intersecting neutrino and antineutrino trajectories. For neutrino mass-squared difference /deltam2/ = 3 x 10(-3) eV2 we find that in the normal (inverted) mass hierarchy the more tangentially-propagating (radially-propagating) neutrinos and antineutrinos can initiate collective, simultaneous medium-enhanced flavor conversion of these particles across broad ranges of energy and propagation direction. Accompanying alterations in neutrino and antineutrino energy spectra and fluxes could affect supernova nucleosynthesis and the expected neutrino signal.     

Inelastic neutrino reactions with light nuclei in a core-collapse supernova

In view of its possible role in a core-collapse supernova and in the subsequent nucleosynthesis, inelastic neutrino scattering with A = 3,4 nuclei is calculated. The predicted cross-sections result from an ab-initio calculation using microscopic two- and three-nucleon interaction and meson-exchange currents.     

Effects of matter oscillations on supernova neutrino flux

If neutrinos have mass it is likely that, as a result of the effect of matter, neutrino flavor eigenstates are converted to mass eigenstates as they emerge from a supernova. In the case of large neutrino mixing angles there can be a significant increase in the mean energy and detected flux of neutrinos from a supernova.     

The Majorana neutrino mass matrix indicated by the current data

The Majorana neutrino mass matrix combines information from the neutrino masses and the leptonic mixing in the flavor basis. Its invariance under some transformation matrices indicates the existence of certain residual symmetry. We offer an intuitive display of the structure of the Majorana neutrino mass matrix, using the whole set of the oscillation data. The structure is revealed depending on the lightest neutrino mass. We find that there are three regions with distinct characteristics of structure. A group effect and the-τ exchange symmetry are observed. Six types of texture non-zeros are shown. Implications for flavor models are discussed.     

Constraints on the electron-neutrino mass from the supernova data A systematic analysis

The energy versus time of arrival pattern of neutrinos from SN1987A is sensitive to a neutrino mass, m_ν, of order a few eV. To disentangle constraints on m_ν from the data, a theory of supernova emission is necessary. We recall the present status of this theory and approximate its predictions in two diffusion models: one designed to reflect the present supernova lore, the other devised to pessimize, within reason, the consequent upper limits on m_ν. We discuss the model dependence and statistical significance of our results, as well as the experimental uncertainties and caveats to which they are subject. We address the question, do the supernova results supercede the present laboratory limits on m_ν?     

Self-induced suppression of collective neutrino oscillations in a supernova

We investigate collective flavor oscillations of supernova neutrinos at late stages of the explosion. We first show that the frequently used single-angle (averaged coupling) approximation predicts oscillations close to, or perhaps even inside, the neutrinosphere, potentially invalidating the basic neutrino transport paradigm. Fortunately, we also find that the single-angle approximation breaks down in this regime; in the full multiangle calculation, the oscillations start safely outside the transport region. The new suppression effect is traced to the interplay between the dispersion in the neutrino-neutrino interactions and the vacuum oscillation term.     

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.     

No collective neutrino flavor conversions during the supernova accretion phase

We perform a dedicated study of the supernova (SN) neutrino flavor evolution during the accretion phase, using results from recent neutrino radiation hydrodynamics simulations. In contrast to what was expected in the presence of only neutrino-neutrino interactions, we find that the multiangle effects associated with the dense ordinary matter suppress collective oscillations. The matter suppression implies that neutrino oscillations will start outside the neutrino decoupling region and therefore will have a negligible impact on the neutrino heating and the explosion dynamics. Furthermore, the possible detection of the next galactic SN neutrino signal from the accretion phase, based on the usual Mikheyev-Smirnov-Wolfenstein effect in the SN mantle and Earth matter effects, can reveal the neutrino mass hierarchy in the case that the mixing angle θ(13) is not very small.     

On the importance of low-energy beta beams for supernova neutrino physics

We argue that isotropization and, consequently, thermalization of the system of gluons and quarks produced in an ultrarelativistic heavy-ion collision does not follow from Feynman diagram analysis to any order in the coupling constant. We conclude that the apparent thermalization of quarks and gluons, leading to success of perfect fluid hydrodynamics in describing heavy-ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects not captured by Feynman diagrams. We proceed by modeling these non-pertrubative thermalization effects using viscous hydrodynamics. We point out that matching Color Glass Condensate inital conditions with viscous hydrodynamics leads to a continuous evolution of all the components of the energy-momentum tensor and, unlike the case of ideal hydrodynamics, does not give rise to a discontinuity in the longitudinal pressure. An important consequence of such a matching is a relationship between the thermalization time and shear viscosity: we observe that small viscosity leads to short thermalization time.