Inverted Neutrino Mass Hierarchy Research Articles

Lepton flavor violating decays as probes of neutrino mass spectra and heavy Majorana neutrino masses

We investigate the lepton flavor violating (LFV) rare decays in the supersymmetric minimal seesaw model in which the Frampton-Glashow-Yanagida ansatz is incorporated. The branching ratio of mu -> e gamma is calculated in terms of the snowmass points and slopes (SPS). We find that the inverted mass hierarchy is disfavored by all SPS points. In addition, once the ratio of BR(tau ->mu gamma) to BR(mu -> e gamma) is measured, one may distinguish the normal and inverted neutrino mass hierarchies, and confirm the masses of heavy right-handed Majorana neutrinos by means of the LFV processes and the thermal leptogenesis mechanism. It is worthwhile to stress that this conclusion is independent of the supersymmetric parameters.

Collective neutrino flavor transformation in supernovae

We examine coherent active-active channel neutrino flavor evolution in environments where neutrino-neutrino forward scattering can engender large-scale collective flavor transformation. We introduce the concept of neutrino flavor isospin which treats neutrinos and antineutrinos on an equal footing, and which facilitates the analysis of neutrino systems in terms of the spin precession analogy. We point out a key quantity, the ``total effective energy,'' which is conserved in several important regimes. Using this concept, we analyze collective neutrino and antineutrino flavor oscillation in the synchronized mode and what we term the bi-polar mode. We thereby are able to explain why large collective flavor mixing can develop on short time scales even when vacuum mixing angles are small in, e.g., a dense gas of initially pure ${\ensuremath{\nu}}_{e}$ and ${\overline{\ensuremath{\nu}}}_{e}$ with an inverted neutrino mass hierarchy (an example of bi-polar oscillation). In the context of the spin precession analogy, we find that the corotating frame provides insights into more general systems, where either the synchronized or bi-polar mode could arise. For example, we use the corotating frame to demonstrate how large flavor mixing in the bi-polar mode can occur in the presence of a large and dominant matter background. We use the adiabatic condition to derive a simple criterion for determining whether the synchronized or bi-polar mode will occur. Based on this criterion, we predict that neutrinos and antineutrinos emitted from a protoneutron star in a core-collapse supernova event can experience synchronized and bi-polar flavor transformations in sequence before conventional Mikhyev-Smirnov-Wolfenstein flavor evolution takes over. This certainly will affect the analyses of future supernova neutrino signals, and might affect the treatment of shock reheating rates and nucleosynthesis depending on the depth at which collective transformation arises.

Double beta decay, nuclear structure and physics beyond the standard model

After the discovery of neutrino oscillations, the search for the 0 ν β β -decay became one of the most fundamental problems of neutrino physics. In this contribution the recent developments in the field of nuclear double beta decay are reviewed. The light Majorana neutrino mass mechanism of the neutrinoless double beta decay ( 0 ν β β -decay) is discussed in the context of recent neutrino oscillation data. The expected half-lives of the 0 ν β β -decay are calculated by assuming the normal and inverted hierarchy of neutrino masses. The problem of reliable determination of the 0 ν β β -decay nuclear matrix elements is addressed. A sterile neutrino ν h contribution to the 0 ν β β -decay is investigated. From the lower bound on the 0 ν β β -decay half-life stringent constraints on the ν h − ν e mixing in a wide region of sterile neutrino masses are derived.

Evaluation of techniques for [formula omitted]-decay experiments, and prospects for direct observation

The data from neutrino oscillations yield values for neutrino mixing angles and squares of the differences of neutrino mass eigenvalues. These imply that if neutrinos are Majorana particles, the half-lives for neutrinoless double-beta 0 ν β β decay should be within the sensitivity of well-designed experiments. In this paper, some of the proposed experimental techniques will be analyzed using criteria that allow comparison of their relative merits. The target sensitivity of the physics reach of a one-ton scale experiment is defined as the ability to cover the inverted neutrino mass hierarchy.

Neutrino mass in GUT constrained supersymmetry withR−parityviolation in light of neutrino oscillations

The neutrino masses are generated in grand unified theory (GUT) constrained supersymmetric model with R-parity violation. The neutrinos acquire masses via tree-level neutrino-neutralino mixing as well as via one-loop radiative corrections. The theoretical mass matrix is compared with the phenomenological one, which is reconstructed by using neutrino oscillation and neutrinoless double beta decay data. This procedure allows to obtain significantly stronger constraints on R-parity breaking parameters than those existing in the literature. The implication of normal and inverted neutrino mass hierarchy on the sneutrino expectation values, lepton-Higgs bilinear and trilinear R-parity breaking couplings is also discussed.

Collider signature of bulk neutrinos in large extra dimensions

We consider the collider signature of right-handed neutrinos propagating in $\ensuremath{\delta}$ (large) extra dimensions, and interacting with standard model fields only through a Yukawa coupling to the left-handed neutrino and the Higgs boson. These theories are attractive as they can explain the smallness of the neutrino mass, as has already been shown. We show that if $\ensuremath{\delta}$ is bigger than two, it can result in an enhancement in the production rate of the Higgs boson, decaying either invisibly or to a $b$ anti-$b$ quark pair, associated with an isolated high ${p}_{T}$ charged lepton and missing transverse energy at future hadron colliders, such as the LHC. The enhancement is due to the large number of Kaluza-Klein neutrinos produced in the final state. The observation of the signal event would provide an opportunity to distinguish between the normal and inverted neutrino mass hierarchies, and to determine the absolute scale of neutrino masses by measuring the asymmetry of the observed event numbers in the electron and muon channels.

Normal vs. inverted hierarchy in type I seesaw models

We demonstrate that, for every grand unified model based on a conventional type I seesaw mechanism leading to a normal light neutrino mass hierarchy, one can easily generate a corresponding model with an inverted hierarchy which yields the same neutrino oscillation parameters. However, the latter type model has several unattractive instabilities which will disfavor any grand unified type I seesaw model, if an inverted neutrino mass hierarchy is observed experimentally. This should be contrasted with the softly-broken Le–Lμ–Lτ flavor symmetry models which are eliminated, if the data favors a normal mass hierarchy.

Constructing the large mixing angle MNS matrix in see-saw models with right-handed neutrino dominance

Recent SNO results strongly favour the large mixing angle (LMA) MSW solar solution. We argue that there are only two technically natural low energy neutrino mass matrix structures consistent with the LMA MSW solution, corresponding to either a hierarchy or an inverted hierarchy with pseudo-Dirac neutrinos. We first present a model-independent analysis in which we diagonalise each of these two mass matrix structures to leading order in $\theta_{13}$ and extract the neutrino masses, mixing angles and phases. In this analysis we express the MNS matrix to leading order in the small angle $\theta_{13}$ including the neutrino {\em and} charged lepton mixing angles and phases, the latter playing a crucial r\^{o}le for allowing the inverted hierarchy case to be consistent with the LMA MSW solution. We then consider the see-saw mechanism with right-handed neutrino dominance and show how the successful neutrino mass matrix structures may be constructed with no tuning and with small radiative corrections, leading to a full, partial or inverted neutrino mass hierarchy. In each case we derive approximate analytic relations between the input see-saw parameters and the resulting neutrino masses, mixing angles and phases, which will provide a useful guide for unified model building. For the hierarchical cases the LMA MSW solution gives a soft lower bound $|U_{e3}|\simgt 0.1$, just below the current CHOOZ limit. Both hierarchical and inverted hierarchical cases predict small $\beta \beta_{0\nu}$ with $|m_{ee}|\sim 0.01$ eV within the sensitivity of future proposals such as GENIUS. Successful leptogenesis is possible if the dominant right-handed neutrino is the heaviest one, but the leptogenesis phase is unrelated to the MNS phases.

The HLMA project: determination of high Δ m2 LMA mixing parameters and constraint on | Ue3| with a new reactor neutrino experiment

In the forthcoming months, the KamLAND experiment will probe the parameter space of the solar large mixing angle MSW solution as the origin of the solar neutrino deficit with ν ̄ e ’s from distant nuclear reactors. If however the solution realized in nature is such that Δ m 2 sol≳2×10 −4 eV 2 (thereafter named the HLMA region), KamLAND will only observe a rate suppression but no spectral distortion and hence it will not have the optimal sensitivity to measure the mixing parameters. In this case, we propose a new medium baseline reactor experiment located at Heilbronn (Germany) to pin down the precise value of the solar mixing parameters. In this paper, we present the Heilbronn detector site, we calculate the ν ̄ e interaction rate and the positron spectrum expected from the surrounding nuclear power plants. We also discuss the sensitivity of such an experiment to | U e3| in both normal and inverted neutrino mass hierarchy scenarios. We then outline the detector design, estimate background signals induced by natural radioactivity as well as by in situ cosmic ray muon interaction, and discuss a strategy to detect the anti-neutrino signal ‘free of background’.

Constraints on | Ue3 | 2 from a three-neutrino oscillation analysis of the CHOOZ data

Results of a new analysis of the CHOOZ data, performed in the framework of three-neutrino mixing, are presented. Both the cases of normal and inverted neutrino mass hierarchy are considered. The parameters characterizing the solar neutrino oscillations, Δm 2 ⊙ and tan 2 θ ⊙, are assumed to lie in the region of the large mixing angle (LMA) MSW solution of the solar neutrino problem, which is favored by the current solar neutrino data. At Δm 2 ⊙≲10 −4 eV 2 the new CHOOZ exclusion curve in the Δm 2 31–| U e3 | 2 plane practically coincides with the exclusion curve obtained in the two-neutrino mixing analysis. For Δm 2 ⊙≳2×10 −4 eV 2, the constraints of the CHOOZ data on | U e3 | 2 are more stringent. For, e.g., sin 2 θ ⊙=0.50 and Δm 2 ⊙=6×10 −4 eV 2, and at Δm 2 31=2.5×10 −3 eV 2 (the Super-Kamiokande best fit-point) we find that (at 90% C.L.) | U e3 | 2<1.7×10 −2, which is by more than a factor of 2 smaller than the upper bound obtained in the original two-neutrino mixing analysis of the CHOOZ data (| U e3 | 2<3.7×10 −2).

Bilarge leptonic mixing from Abelian horizontal symmetries

We construct and present a model for leptonic mixing based on higher-dimensional operators, using the Froggatt–Nielsen mechanism, and Abelian horizontal symmetries (flavor symmetries) of continuous and discrete type. Our model naturally yields bilarge leptonic mixing, coming from both the charged leptons and the neutrinos, and an inverted neutrino mass hierarchy spectrum. The obtained values of the parameters, i.e., the leptonic mixing parameters and the neutrino mass squared differences, are all consistent with the atmospheric neutrino data and the Mikheyev–Smirnov–Wolfenstein large mixing angle solution for the solar neutrino problem.

A 2-3 symmetry in neutrino oscillations

Maximum mixing in atmospheric neutrino oscillation, as well as vanishing of the MNS matrix element U e3 , are consequences of a 2-3 symmetry, under which the neutrino mass matrix is invariant under the interchange of second and third generation neutrinos. These predictions of the 2-3 symmetry are consistent with the results of Super-Kamiokande, K2K, and CHOOZ experiments. If the symmetry is exact at a high-energy scale set by right-handed neutrinos, a deviation from these predictions generated by renormalization-group corrections will occur at experimental energies. With an MSSM dynamics, the result can be made to agree with a global fit of the neutrino data, if normal hierarchy is assumed on the neutrino mass spectrum and if the mass of the electron-neutrino is at least about 0.025 eV. The presence of this mass lower bound is a novel and interesting feature of the symmetry that can be falsified by future experiments. Of the three viable solar neutrino solutions, only LMA gives a sizable MNS matrix element U e3 that can hopefully be detected in future reactor experiments. Inverted neutrino mass hierarchy is not permitted by this symmetry.

Do experiments and astrophysical considerations suggest an inverted neutrino mass hierarchy?

The recent results from the Los Alamos neutrino oscillation experiment, together with assumptions of neutrino oscillation solutions for the solar and atmospheric neutrino deficit problems, may place powerful constraints on any putative scheme for neutrino masses and mixings. Assuming the validity of these experiments and assumptions, we argue that a nearly unique spectrum of neutrino masses emerges as a fit, if two additional astrophysical arguments are adopted: (1) the sum of the light neutrino masses is \ensuremath{\sim}5 eV, as large scale structure simulations with mixed cold plus hot dark matter seem to suggest; and (2) r-process nucleosynthesis originates in neutrino-heated ejecta from type II supernovae. In this fit, the masses of the neutrinos must satisfy ${\mathit{m}}_{\ensuremath{\nu}\mathit{e}}$\ensuremath{\approxeq}${\mathit{m}}_{\ensuremath{\nu}\mathit{s}}$\ensuremath{\approxeq}2.7 eV (where ${\ensuremath{\nu}}_{\mathit{e}}$ is split from a sterile species ${\ensuremath{\nu}}_{\mathit{s}}$ by \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}6}$ eV) and ${\mathit{m}}_{\ensuremath{\nu}\mathrm{\ensuremath{\tau}}}$\ensuremath{\approxeq}${\mathit{m}}_{\ensuremath{\nu}\mathrm{\ensuremath{\mu}}}$\ensuremath{\approxeq}1.1 eV (where these species are split by \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}2}$ eV). We discuss alternative neutrino mass spectra that are allowed if we decline to adopt certain experiments or astrophysical models.

An inverted neutrino mass hierarchy for hot dark matter and the solar neutrino deficit

The cosmological model in which 20% of the dark matter is shared by two nearly equal mass neutrinos fits the structure of the universe on all scales. This has been motivated by a ν μ → ν τ oscillation explanation of the deficit of atmospheric ν μ ' s. If the observed atmospheric ν μ ν e ratio has an alternative explanation, the cosmological model can be retained if the deficit of solar neutrinos is explained by ν e → ν τ oscillation. In this case, an inverted neutrino mass hierarchy is required with m ν μ ⪡ m ν e ≈ m ν τ ≈ 2.4 eV. We show that if there exists an L e − L τ symmetry in nature, then both the near mass degeneracy of ν e and ν τ , as well as the consistency of the above values for neutrino masses with the negative results of the neutrinoless double beta decay search experiments are easily understood. We show that this symmetry implemented in the context of a high-scale left-right symmetric model with the see-saw mechanism can lead to a simple theoretical understanding of the desired form of the mass matrix.