Neutrino Mass Pattern Research Articles

Neutrino oscillations due to nonuniversal gauge interactions in the weak sector

We discuss the structure of the neutrino mass matrix as derived from a nonuniversal electroweak gauge interaction model. We discuss two interesting patterns of neutrino masses. The first pattern is hierarchal which fits the large mixing angle or low probability, low mass solution of solar neutrino data. The second pattern gives rise to an inverted mass spectrum which fits the currently not preferred small mixing angle solution. The mechanism for generating a light sterile neutrino mass is interesting and is briefly discussed.

Quark and lepton mass patterns and the absolute neutrino mass scale

We investigate what could be learned about the absolute scale of neutrino masses from comparisons among the patterns within quark and lepton mass hierarchies. First, we observe that the existing information on neutrino masses fits quite well to the unexplained, but apparently present regularities in the quark and charged lepton sectors. Second, we discuss several possible mass patterns, pointing out that this quite generally leads towards hierarchical neutrino mass patterns especially disfavoring the vacuum solution.

A hidden hierarchy of neutrino masses

The Dirac masses of neutrinos can exhibit a strong hierarchy even if the Majorana masses of the right-handed neutrinos are degenerate and the hierarchy of the mass scales governing the oscillations of solar and atmospheric neutrinos is rather small. This phenomenon results from the see-saw mechanism and the algebraic structure of the effective mass operator for the active neutrinos. The large hierarchy of the Dirac masses is drastically modified by a symmetric unitary matrix R acting in the flavor space. A realistic pattern of neutrino masses and mixing is obtained. Maximal mixing for atmospheric neutrinos is attributed to the charged lepton sector. Large mixing in solar neutrino oscillations is due to the neutrino sector. Small U e3 is a natural consequence of the model. The masses of the active neutrinos are given by μ 3≈ Δm @ 2 and μ 1/ μ 2≈tan 2 θ ⊙.

Constraining four neutrino mass patterns from neutrinoless double beta decay

All existing data on neutrino oscillations (including those from the LSND experiment) imply a four neutrino scheme with six different allowed mass patterns. Some of the latter are shown to be disfavored by using a conservative upper bound on the ββ0 ν nuclear decay rate, if neutrinos are assumed to be Majorana particles. Comparisons are also made with restrictions from tritium β-decay and cosmology.

CONFRONTING HEAVY TAU NEUTRINOS WITH NEUTRINO OSCILLATIONS

If the tau neutrino is as heavy as 10 MeV which may have certain astrophysical implications, the neutrino mass pattern is studied so as to accommodate the new oscillation observations. It predicts that the electron neutrino has Marjorana mass around 0.05 eV. A supersymmetric model is described to realize the above scenario.

Can precision measurements of slepton masses probe right handed neutrinos?

In a supersymmetric model, the presence of a right handed neutrino with a large Yukawa coupling $f_{\nu}$ would affect slepton masses via its contribution to the renormalization group evolution between the grand unification and weak scales. Assuming a hierarchichal pattern of neutrino masses, these effects are large for only the third generation of sleptons. We construct mass combinations to isolate the effect of $f_{\nu}$ from mass corrections already expected from tau Yukawa couplings. We then analyze the size of these effects, assuming that the Super-Kamiokande data constrain 0.033 eV $\alt m_{\nu_{\tau}} \alt 0.1$ eV and that neutrino masses arise via a see-saw mechanism. We also explore whether these effects might be detectable in experiments at future $e^+e^-$ linear colliders. We find that $m_{\tnu_{\tau}}$ needs to be measured with a precision of about 2-3% to measure the effect of $f_{\nu}$ if the neutrino and top Yukawa couplings unify at the grand unification scale. In a simple case study, we find a precision of only 6-10% might be attainable after several years of operation. If the neutrino Yukawa coupling is larger, or in more complicated models of neutrino masses, a determination of $\ttau_1$ and $\tnu_{\tau}$ masses might provide a signal of a Yukawa interaction of neutrinos.

Atmospheric neutrino anomaly without maximal mixing?

We consider a pattern of neutrino masses in which there is an approximate mass degeneracy between the two mass eigenstates most coupled to the $\nu_\mu$ and $\nu_\tau$ flavour eigenstates. Earth-matter effects can lift this degeneracy and induce an effectively maximal mixing between these two generations. This occurs if $\nu_e$'s contain comparable admixtures of the degenerate eigenstates, even rather small ones. This provides an explanation of the atmospheric neutrino anomaly in which the {\it ab initio} introduction of a large mixing angle is not required. To test this possibility we perform a novel and detailed analysis of the 52 kiloton-year SuperKamiokande data, and we find that in a large region of parameter space the corresponding confidence levels are excellent. The most recent results from the Chooz reactor experiment, however, severely curtail this region, so that the conventional scenario with nearly maximal mixing angles --which we also analyse in detail-- is supported by the data.

Model of quark and lepton masses: The neutrino sector

If neutrinos have masses, why are they so tiny? Are these masses of the Dirac type or of the Majorana type? We are already familiar with the mechanism of how to obtain a tiny Majorana neutrino mass by the famous see-saw mechanism. The question is: Can one build a model in which a tiny Dirac neutrino mass arises in a more or less "natural" way? What would be the phenomenological consequences of such a scenario, other than just merely reproducing the neutrino mass patterns for the oscillation data? In this article, a systematic and detailed analysis of a model is presented, with, as key components, the introduction of a family symmetry as well as a new SU(2) symmetry for the right-handed neutrinos. In particular, in addition to the calculations of light neutrino Dirac masses, interesting phenomenological implications of the model will be presented.

Are messages of R-parity violating supersymmetry hidden within top quark signals?

In an R-parity nonconserving supersymmetric theory, the lighter stop can dominantly decay into b&mgr; and btau if R-parity breaking has to explain the neutrino mass and mixing pattern suggested by the data on atmospheric muon neutrinos. This should give rise to dilepton + dijet and single - lepton + jets, signals identical with those of the top quark at the Fermilab Tevatron. One can thus constrain the stop parameter space using the current top search data and can similarly look for the first signals of supersymmetry at the upgraded runs of the Tevatron.

Determination of the pattern of neutrino masses at a neutrino factory

We study the precision with which the sign of δm 32 2 can be determined at a neutrino factory, as a function of stored muon energies and baselines. This is done by simultaneously fitting the channels ν μ → ν μ , ν ̄ e→ ν ̄ μ from μ − decays and the channels ν ̄ μ→ ν ̄ μ , ν e → ν μ from μ + decays. For a 20 GeV muon storage ring we investigate as a function of the baseline length and the magnitude of δm 32 2, the minimum value of the parameter sin 22 θ 13 for which the sign of δm 32 2 can be determined to at least 3 standard deviations. We find that for baselines longer than ∼3000 km, the sign of δm 2 32 can be determined for sin 22 θ 13 down to the 10 −3 level with 10 20 decays of 20 GeV muons.

Neutrino masses in the supersymmetric standard model with right-handed neutrinos and spontaneousR-parity violation

We propose an extension of the supersymmetric standard model with right-handed neutrinos and a singlet Higgs field, and study the neutrino masses in this model. The Majorana masses for the right-handed neutrinos are generated around the supersymmetry breaking scale through the vacuum expectation value of the singlet Higgs field. This model may induce spontaneous R-parity violation via the vacuum expectation value of the right-handed sneutrino. In the case, the effective theory is similar to a bilinear R-parity violating model. There are two sources for the neutrino masses: one is this bilinear R-parity breaking effect, and the other is the ordinary seesaw effect between left- and right-handed neutrinos. Combining these two effects, the hierarchical neutrino mass pattern arises even when the neutrino Yukawa matrices are not hierarchical. We acquire appropriate masses and mixings to explain both the solar and atmospheric neutrino oscillations.

Long baseline neutrino oscillations projects

The ν μ disappearance has been seen in atmospheric neutrino experiments. This has led to the “evidence for neutrino oscillations”. The main problem in neutrino physics is to perform the right experiment(s) to elucidate in a comprehensive way the pattern of neutrino masses and mixing. The long baseline experiements will play a fundamental role at settling definitively the question of flavor oscillation. The long baseline neutrino oscillations project are summarized, with an emphasis on the proposed projects in Europe. A second generation of very long baseline experiments (VLBL) is also briefly discussed.

Four light neutrinos in the singular seesaw mechanism with Abelian flavor symmetry

The four light neutrino scenario, which explains the atmosphere, solar and LSND neutrino experiments, is studied in the framework of the seesaw mechanism. By taking both the Dirac and Majorana mass matrices of neutrinos to be singular, the four neutrino mass spectrum consisting of two almost degenerate pairs separated by a mass gap $\ensuremath{\sim}1 \mathrm{eV}$ is naturally generated. Moreover, the right-handed neutrino Majorana mass can be at the $\ensuremath{\sim}{10}^{14} \mathrm{GeV}\mathrm{}$ scale unlike in the usual singular seesaw mechanism. Abelian flavor symmetry is used to produce the required neutrino mass pattern. A specific example of the flavor charge assignment is provided to show that maximal mixings between the ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and ${\ensuremath{\nu}}_{e}\ensuremath{-}{\ensuremath{\nu}}_{s}$ are respectively attributed to the atmosphere and solar neutrino anomalies while small mixing between two pairs to the LSND results. The implications to the other fermion masses are also discussed.

Neutrino mass patterns, R-parity violating supersymmetry and associated phenomenology

Motivated by the recent SuperKamiokande results on atmospheric neutrinos, we incorporate massive neutrinos, with large angle oscillation between the second and third generations, in a theory with R-parity violating supersymmetry. The general features of such a theory are briefly reviewed. We emphasize its testability through the observation of comparable numbers of muons and taus, produced together with the W-boson, in decays of the lightest neutralino. A distinctly measurable decay gap is another remarkable feature of such a scenario.

Neutrino textures in light of Super-Kamiokande data and a realistic string model

Motivated by the Super-Kamiokande atmospheric neutrino data, we discuss possible textures for Majorana and Dirac neutrino masses within the see-saw framework. There are two main purposes of this paper: first, to gain intuition into this area from a purely phenomenological analysis, and second, to explore to what extent it may be realized in a specific model. We comment initially on the simplified two-generation case, emphasizing that large mixing is not incompatible with a large hierarchy of mass eigenvalues. We also emphasize that renormalization-group effects may amplify neutrino mixing, and we present semi-analytic expressions for estimating this amplification. Several examples are then given of three-family neutrino mass textures, which may also accommodate the persistent solar neutrino deficit, with different assumptions for the neutrino Dirac mass matrices. We comment on a few features of neutrino mass textures arising in models with a U(1) flavour symmetry. Finally, we discuss the possible pattern of neutrino masses in a “realistic” flipped SU(5) model derived from string theory, illustrating how a desirable pattern of mixing may emerge. Both small- or large-angle MSW solutions are possible, while a hierarchy of neutrino masses appears more natural than near-degeneracy. This model contains some unanticipated features that may be relevant in other models also: The neutrino Dirac matrices may not be related closely to the quark mass matrices, and the heavy Majorana states may include extra gauge-singlet fields.

Particle physics implications of neutrinoless double beta decay

Neutrinoless double beta decay is a sensitive probe of the patterns of neutrino masses and mixings if the neutrinos are Majorana particles as well as other new physics scenarios beyond the standard model. In this talk, the present experimental lower bound on the lifetime for ββ 0 ν is used to constrain the neutrino mixings and set limits on the parameters of the new physics scenarios such as the left-right symmetric models, R-parity violating SUSY models etc which lead to neutrinoless double beta decay. We then discuss proposed high precision searches for ββ 0 ν decay that can provide extremely valuable insight not only into the nature of neutrino mixings and masses but also put constraints on (or even rule out) new physics scenarios.

Oscillation searches with laboratory neutrinos: Theoretical aspects

Neutrino oscillations searches with laboratory beams from accelerators or reactors explore several flavor transition channels, and thus cannot be globally analysed in a simplistic two-flavor approach. A three-neutrino spectrum with two widely different mass gaps provides a simple and natural scheme to analyse both positive and negative oscillation searches. In this scheme, the possible patterns of neutrino masses and mixing are strongly constrained by the recent results of the Chooz reactor experiment. The “surviving” scenarios predict distinct signatures that can be probed in future short or long baseline laboratory experiments.

Pseudo-Dirac neutrinos as a potential complete solution to the neutrino oscillation puzzle

A solution for the neutrino mass and mixing pattern is proposed which is compatible with all available experimental data on neutrino oscillations. This solution involves Majorana neutrinos of the pseudo-Dirac type, i.e. m Majorana≪ m Dirac. The solar and atmospheric neutrino observations are mainly explained as ν e − ν e S and ν μ − ν μ S oscillations, where S indicates the sterile (“righthanded”) partner of each neutrino generation, while the LSND result is interpreted in terms of standard ν μ − ν e oscillations. The resulting constraints on ν μ − ν τ and ν τ − ν τ S oscillations are also discussed. This solution leaves room for a hierarchical mass and mixing scheme with a ν τ mass in the few eV range, as favoured by some dark matter scenarios. The apparent conflict with standard Big Bang nucleosynthesis is addressed and the implications for current and future experiments are discussed. It is argued that both short and long baseline accelerator neutrino experiments are needed in order to decide between this solution and other oscillation scenarios.

THE STATUS OF NEUTRINO MASS

New experimental results, if correct, require at least one light sterile neutrino, in addition to the three active ones, to accommodate the mass differences required to explain the solar νe deficit, the anomalous μ/e ratio produced by atmospheric neutrinos, and either the candidate events for νμ→ νe (or [Formula: see text]) from the LSND experiment, or the possible need for a hot component of dark matter. This neutrino mass pattern can not only accommodate all these four requirements, but also provide a robust solution to a problem presently making heavy-element synthesis by supernovae impossible and resolve a possible discrepancy between big bang nucleosynthesis theory and observations.

Neutrino Mass Texture with Large Mixing

We propose a simple texture for the right-handed Majorana mass matrix to give a large $\nu_\mu-\nu_\tau$ mixing angle and hierarchical left-handed neutrino mass pattern. Consistently with the Dirac mass texture of the quark sector realizing the CKM mixing, this naturally explains the recent experimental results on both the atmospheric neutrino anomaly observed by the Superkamiokande collaboration and the solar neutrino problem. In this texture the right-handed Majorana mass of the third generation is of the order of GUT scale, which is favorable for reproducing the observed bottom-tau mass ratio.