The Solar Neutrino Problem: Neither Astrophysics Nor Oscillations?

There is no consistent solar model which can describe all experimental data on the solar neutrinos. The problem can be formulated essentially in a model independent way. The key points are the comparison of the Homestake and the Kamiokande data as well as the comparison of the GALLEX and SAGE results with minimal signal estimated from the solar luminosity. It is argued than in such a comparison one should use the Homestake-II data (only after 1986) with caution. The results of the model independent analysis show strong suppression of the beryllium neutrino flux. The data can be well described by the resonant flavor conversion. For the ``low flux model" which can accommodate the Kamiokande signal, a consistent solution can be found for the neutrino mass squared difference $\Delta m^2 = (0.3 - 1.0)\cdot 10^{-5}$ eV$^2$ and values of mixing angle $\sin^2 2\theta > 5 \cdot 10^{-4}$ (``very small mixing solution").Comment: (Talk given at the International Workshop ``Solar Neutrino Problem: Astrophysics or Oscillations", Gran Sasso, Italy, February 28 - March 1, 1994), 18 pages (7 figures available upon request), LaTeX, DOE/ER/40561-136-INT94-13-0

Theory of neutrino masses and mixing

In spite of enormous experimental progress in determination of the neutrino parameters, theory of neutrino mass and mixing is still on the cross-roads. Guidelines could be (i) the connection between zero neutrino charges (and therefore a possibility to be Majorana particle), smallness of the neutrino mass and large lepton mixing, (ii) joint description of leptons and quarks, (iii) existence of the right handed (RH) neutrinos without special quantum numbers. Properties of the RH neutrinos and the UV completion of the seesaw may turn out to be the key to understand the neutrino mass and mixing. In view of the LHC results minimalistic scenarios like $\nu$MSM look rather plausible. Still the GUT's with additional hidden sector, QLC, high scale flavor symmetries are appealing. Concerning mixing, the main issue is "symmetry or no symmetry" behind the observed pattern. The symmetry group condition is useful tool to study consequences of symmetries and to perform "symmetry building". Sterile neutrinos are challenge but also opportunity for the present theoretical constructions.Comment: 13 pages, Talk given at the "Pontecorvo 100" Symposium, Pisa, Italy, September 18 - 20, 201

Alternatives to the seesaw mechanism

The observed pattern of lepton mixing does not give an evidence of the seesaw. It is easier to disprove seesaw showing that one of the alternatives gives dominant contribution to the neutrino mass. We consider alternative mechanisms based on (i) small (tree level) effective couplings, (ii) small VEV, (iii) radiative generation of masses, (iv) protection by SUSY breaking scale or by $\mu$-term, (v) small overlap of wave functions of the left and right handed neutrino components in extra dimensions. Seesaw can be the mechanism of suppression of the Dirac mass terms and not dominant mechanism of the neutrino mass giving just a sub-leading contribution.Comment: 16 pages, latex, 5 figures. Talk given at the Conference ``Seesaw Mechanism and Neutrino Masses: 25 Years Later'', 10-11 June 2004, Paris. References added, statement in sec. 6 correcte

Neutrino Physics after KamLAND

The neutrino anomalies were driving force of the developments in neutrino physics during the last 30 - 35 years. I will consider status of the anomalies after the first KamLAND result. The main questions are "What is left?" and "What is the next?" In the new phase, the phenomenological objectives of neutrino physics consist of accomplishing the program of reconstruction of the neutrino mass and flavor spectrum and searches for physics beyond the ``standard'' picture. The latter includes searches for new (sterile) neutrino states, new neutrino interactions, effects of violation of the fundamental symmetries in the neutrino sector.Comment: 21 pages, latex, 6 figures. Invited talk given at the International workshop NOON2003, February 10 - 14, 2003, Kanazawa, Japa

Neutrino mass hierarchy and matter effects

Matter effects modify the mixing and the effective masses of neutrinos in a way which depends on the neutrino mass hierarchy. Consequently, for normal and inverted hierarchies the oscillations and flavor conversion results are different. Sensitivity to the mass hierarchy appears whenever the matter effects on the 1-3 mixing and mass splitting become substantial. This happens in supernovae in wide energy range and in the matter of the Earth. The Earth density profile is a multi-layer medium where the resonance and parametric enhancements of oscillations occur. The enhancement is realized in neutrino (antineutrino) channels for the normal (inverted) mass hierarchy. Multi-megaton scale under ice (water) atmospheric neutrino detectors with low energy threshold can establish mass hierarchy with $(3 - 10) \sigma$ confidence level in few years. The main challenges of these experiments are discussed and various ways to improve sensitivity are outlined. In particular, inelasticity measurements will allow to increase significance of the hierarchy identification by $20 - 50 \%$ .Comment: 15 pages, 19 figures, Talk given at XV Workshop on Neutrino Telescopes, 11-15 March 2013, Venice, Ital

Low Scale Left-Right Symmetry and Naturally Small Neutrino Mass

We consider the low scale ($10$ - $100$ TeV) left-right symmetric model with "naturally" small neutrino masses generated through the inverse seesaw mechanism. The Dirac neutrino mass terms are taken to be similar to the masses of charged leptons and quarks in order to satisfy the quark-lepton similarity condition. The inverse seesaw implies the existence of fermion singlets $S$ with Majorana mass terms as well as the "left" and "right" Higgs doublets. These doublets provide the portal for $S$ and break the left-right symmetry. The inverse seesaw allows to realize a scenario in which the large lepton mixing originates from the Majorana mass matrix of $S$ fields which has certain symmetry. The model contains heavy pseudo-Dirac fermions, formed by $S$ and the right-handed neutrinos, which have masses in the $1$ GeV - $100$ TeV range and can be searched for at current and future colliders such as LHC and FCC-ee as well as in SHiP and DUNE experiments. Their contribution to neutrinoless double beta decay is unobservable. The radiative corrections to the mass of the Higgs boson and the possibility for generating the baryon asymmetry of the Universe are discussed. Modification of the model with two singlets ($S_L$ and $S_R$) per generation can provide a viable keV-scale dark matter candidate.Comment: 22 pages, 3 figures; comments and references adde

Improving the neutrino mass hierarchy identification with inelasticity measurement in PINGU and ORCA

Multi-megaton scale under ice and underwater detectors of atmospheric neutrinos with few GeV's energy threshold (PINGU, ORCA) open up new possibilities in the determination of neutrino properties, and in particular the neutrino mass hierarchy. With a dense array of optical modules it will be possible to determine the inelasticity, $y$, of the charged current $\nu_\mu$ events in addition to the neutrino energy $E_\nu$ and the muon zenith angle $\theta_\mu$. The discovery potential of the detectors will substantially increase with the measurement of $y$. It will enable (i) a partial separation of the neutrino and antineutrino signals; (ii) a better reconstruction of the neutrino direction; (iii) the reduction of the neutrino parameters degeneracy; (iv) a better control of systematic uncertainties; (v) a better identification of the $\nu_\mu$ events. It will improve the sensitivity to the CP-violation phase. The three dimensional, $(E_\nu, \theta_\mu, y)$, $\nu_\mu-$oscillograms with the kinematical as well as the experimental smearing are computed. We present the asymmetry distributions in the $E_\nu - \theta_\mu$ plane for different intervals of $y$ and study their properties. We show that the inelasticity information reduces the effect of degeneracy of parameters by 30%. With the inelasticity, the total significance of establishing mass hierarchy may increase by 20% - 50%, thus effectively increasing the volume of the detector by factor 1.5 - 2.Comment: 19 pages, 10 figure

Leptonic CP Violation Phases, Quark-Lepton Similarity and Seesaw Mechanism

We explore generic features of the leptonic CP violation in the framework of the seesaw type I mechanism with similarity of the Dirac lepton and quarks mass matrices $m_D$. For this, we elaborate on the standard parametrization conditions which allow to simultaneously obtain the Dirac and Majorana phases. If the only origin of CP violation is the left-handed (LH) transformation which diagonalizes $m_D$ (similar to quarks), the leptonic CP violation is suppressed and the Dirac phase is close to $\pi$ or to $0$ with $\sin \delta_{CP} \approx (\sin \theta_{13}^q /\sin \theta_{13}) \cos \theta_{23} \sin \delta_q \sim \lambda^2 \sin \delta_q$. Here $\lambda \sim \theta_C$, is the Cabibbo mixing angle, and $\theta_{13}^q$ and $\theta_{13}$ are the 1-3 mixing angles of quarks and leptons respectively. The Majorana phases $\beta_1$ and $\beta_2$ are suppressed as $\lambda^3\sin\delta_q$. For Majorana neutrinos implied by seesaw, the right-handed (RH) transformations are important. We explore the simplest extension inspired by Left-Right (L-R) symmetry with small CKM-type CP violation. In this case, seesaw enhancement of the CP violation occurs due to strong hierarchy of the eigenvalues of $m_D$ leading to $\delta_{CP} \sim 1$. The enhancement is absent under the phase factorization conditions which require certain relations between parameters of the Majorana mass matrix of RH neutrinos.Comment: 30 pages. v3(typos fixed, matches version published in Nucl. Phys. B