Narrow Results

Recently a positive indication of the neutrinoless double beta decay has been announced. We study the implications of this result taking into consideration earlier results on atmospheric neutrinos and solar neutrinos. We also include in our discussions the recent results from SNO and K2K. We point out that on the confidence level given for the double beta signal, the neutrino mass matrices are now highly constrained. All models predicting Dirac masses are ruled out and leptogenesis becomes a natural choice. Only the degenerate and the inverted hierarchical solutions are allowed for the three-generation Majorana neutrinos. In both cases we find that the radiative corrections destabilize the solutions and the LOW, VO and Just So solutions of the solar neutrinos are ruled out. For the four-generation case only the inverted hierarchical scenario is allowed.

Recently the evidence of the neutrinoless double β (0νββ) decay has been announced. This means that neutrinos are Majorana particles and their mass hierarchy is forced to certain patterns in the diagonal basis of charged lepton mass matrix. We estimate the magnitude of 0νββ decay in the classification of the neutrino mass hierarchy patterns as Type A, m_1,2 ≪ m₃, Type B, m₁ ~ m₂ ≫ m₃, and Type C, m₁ ~ m₂ ~ m₃, where m_i is the ith generation neutrino absolute mass. The data of 0νββ decay experiment suggest that the neutrino mass hierarchy pattern should be Type B or C. Type B predicts a small magnitude of 0νββ decay which is just edge of the allowed region of experimental value in 95% c.l., where Majorana CP phases should be in a certain parameter region. Type C can induce the suitably large amount of 0νββ decay which is consistent with the experimental data, where overall scale of degenerate neutrino mass plays a crucial role, and its large value can induce the large 0νββ decay in any parameter regions of Majorana CP phases.

We consider all possible scalar bilinears, which couple to two fermions of the standard model. The various baryon and lepton number violating couplings allowed by these exotic scalars are studied. We then discuss which are constrained by limits on proton decay (to a lepton and a meson as well as to three leptons), neutron–antineutron oscillations, and neutrinoless double beta decay.

We examine the constraints from the recent HEIDELBERG–MOSCOW double beta decay experiment. It leads us to the almost degenerate or inverse hierarchy neutrino mass scenario. In this scenario, we obtain possible upper bounds for the Majorana CP violating phase in the lepton sector by incorporating the data from the neutrino oscillation, the single beta decay experiments, and from the astrophysical observation. We also predict the neutrino mass that may be measurable in the future beta decay experiments.

We reanalyze the constraints in neutrino masses and MNS lepton mixing parameters using the new data from the terrestrial (KamLAND) and astrophysical (WMAP) observations together with the Heidelberg–Moscow double beta decay experiment. It leads us to the almost degenerate or inverse hierarchy neutrino mass scenario. We discuss the possibility of getting the bound for the Majorana CP-violating phase.

The consequences of a texture zero at the ee entry of neutrino mass matrix in the flavor basis, which also implies a vanishing effective Majorana mass for neutrinoless double beta decay, have been studied for Majorana neutrinos. The neutrino parameter space under this condition has been constrained in the light of all available neutrino data including the CHOOZ bound on .

Neutrinoless double beta decay has been the subject of intensive theoretical work as it represents the only practical approach to discovering whether neutrinos are Majorana particles or not, and whether lepton number is a conserved quantum number. Available calculations of matrix elements and phase-space factors are reviewed from the perspective of a future large-scale experimental search for 0νββ decay. Somewhat unexpectedly, a uniform inverse correlation between phase-space and the square of the nuclear matrix element emerges. As a consequence, no isotope is either favored or disfavored; all have qualitatively the same decay rate per unit mass for any given value of the Majorana mass.

A successful baryogenesis theory requires a baryon-minus-lepton number violation if it works before the electroweak phase transition. The leading dimension-6 baryon number violating interactions conserve baryon-minus-lepton number, which dissociated baryogenesis from baryon number violation. We show that in some models, in which the baryon-minus-lepton number is violated in the proton and neutron decays, the baryogenesis and the nucleon decay could have a common origin. We extend the canonical seesaw model with an isotriplet leptoquark scalar and two isotriplet Higgs scalars and allow the Higgs triplet to have a quartic coupling with three leptoquark triplets and a cubic coupling with two Higgs doublets. The decays of the Higgs triplets can thus generate a baryon-minus-lepton asymmetry. The tiny vacuum expectation values of the Higgs triplets can naturally induce a testable proton decay even if the leptoquark is around the TeV scale. The leptoquark associated with any flavor neutrinos can mediate a neutrinoless double beta decay.

A simple form of neutrino mass matrix which has only two free parameters is proposed from a phenomenological point of view. Using this mass matrix, we succeed to reproduce all the observed values for the Maki–Nakagawa–Sakata (MNS) lepton mixing angles and the neutrino mass squared difference ratio. Our model also predicts δ_ν = 155° for the Dirac CP violating phase in the lepton sector and the effective neutrino mass 〈m 〉 = 6.3×10^-3eV in the neutrinoless double beta decay.

We give a review on neutrino models in which Majorana neutrino masses are generated radiatively through loop diagrams. In particular, we concentrate on the two-loop models which contain extra doubly charged singlet $\Phi$ and triplet $\Delta$ scalars beyond the Standard Model so that the new Yukawa $\Psi {\bar{\ell }}_R^c{\ell }_R$ and effective ${\Psi }^{\pm \pm }{W}^{\mp }{W}^{\mp }$ couplings can be induced. In these two-loop models, we find that the neutrino mass spectrum is a normal hierarchy and the rate of the neutrinoless double beta decay $(0\nu \beta \beta )$ can be large as it is dominated by the short-distance tree contribution. In addition, by using the neutrino oscillation data and comparing with the global fitting result in the literature, we find a unique neutrino mass matrix and predict the Dirac and two Majorana CP phases to be $3(\pi -\eta )∕2$, $\pi +3\eta ∕2$ and $(3\pi -\eta )∕2$ with $\eta =0.07\pi$, respectively.

We review current experimental efforts to search for neutrinoless double beta decay $(0\nu \beta \beta )$. A description of the selected leading experiments is given and the strongest recent results are compared in terms of achieved background indexes (BI) and limits on effective Majorana mass. A combined limit is also shown. The second part of the review covers next generation experiments, highlighting the challenges and new technologies that may be necessary to achieve a justifiable discovery potential. A potential synergy with direct dark matter searches, which could be an especially prudent strategy in case the axial vector coupling constant is quenched in $0\nu \beta \beta$ decay, is emphasized.

Motivated by recent intensive experimental efforts on searching for neutrinoless double beta decays, we present a detailed quantitative analysis on the prospect of resolving neutrino mass ordering in the next generation ${}^{76}$Ge-type experiments.

We examine the possibility of vanishing effective Majorana mass $M_{ee}$ in the presence of one or two sterile neutrinos taking into account the recent data on neutrino masses and mixings, particularly, on ${\theta }_{13}$. Also, within the framework of standard three active neutrinos, we find that effective Majorana mass $M_{ee}$ can be vanishingly small if neutrino masses observe normal hierarchy. However, the same is not valid for inverted hierarchical neutrino masses. The predictions for Majorana phases $\alpha$ and $\beta$ have also been obtained and shown as scatter plots. We also examine the condition $M_{ee}=0$ within the framework wherein fermion sector has been extended by the addition of either one or two sterile neutrinos. The condition of vanishing effective Majorana mass is found to be inconsistent with the recent measurement of ${\theta }_{13}$ in these classes of models except for $1+3$, $2+3$ neutrino mass scheme for small values of the lightest neutrino mass, $m_{\mathrm{\text{light}}}$.

Asymmetric tri-bi-maximal mixing is a recently proposed, grand unified theory (GUT) based, flavor mixing scheme. In it, the charged lepton mixing is fixed by the GUT connection to down-type quarks and a ${{𝒯}}_{13}$ flavor symmetry, while neutrino mixing is assumed to be tri-bi-maximal (TBM) with one additional free phase. Here we show that this additional free phase can be fixed by the residual flavor and CP symmetries of the effective neutrino mass matrix. We discuss how those residual symmetries can be unified with ${{𝒯}}_{13}$ and identify the smallest possible unified flavor symmetries, namely $({\mathbb{Z}}_{13}\times {\mathbb{Z}}_{13})⋊{\mathrm{\text{D}}}_{12}$ and $({\mathbb{Z}}_{13}\times {\mathbb{Z}}_{13})⋊{\mathrm{\text{S}}}_4$. Sharp predictions are obtained for lepton mixing angles, CP violating phases and neutrinoless double beta decay.

In the planned ton-scale neutrinoless double beta $(0\nu \beta \beta )$ decay experiments, the used $Q$ value is the mass-energy difference between the ground states of mother and daughter atoms, without correcting the atomic energy difference due to the alteration in the nuclear charge before and after the decay process. We show that the atomic energy change between the two ground states can potentially reduce the $Q$ value by 5.502, 12.103, and 12.372 keV for the decays from ${}^{76}\mathrm{\text{Ge}}$, ${}^{130}\mathrm{\text{Te}}$, and ${}^{136}\mathrm{\text{Xe}}$, respectively. The available kinetic energy for two electrons can be 2033.51 keV for ${}^{76}\mathrm{\text{Ge}}$ decay, 2515.41 keV for ${}^{130}\mathrm{\text{Te}}$ decay, and 2445.45 keV for ${}^{136}\mathrm{\text{Xe}}$ decay after correcting for atomic effects. This implies that the planned experiments could potentially miss the decay signature if a narrow energy window is designated to reduce background events including $2\nu \beta \beta$ decay events.

We study a neutrino mass model with $A_4$ discrete flavor symmetry using a type-I seesaw mechanism. The inclusion of extra flavons in our model leads to deviations from the exact tribimaximal mixing pattern resulting in a nonzero ${\theta }_{13}$ consistent with the recent experimental results and a sum rule for light neutrino masses is also obtained. In this framework, a connection is established among the neutrino mixing angles-reactor mixing angle (${\theta }_{13}$), solar mixing angle (${\theta }_{12}$), and atmospheric mixing angle (${\theta }_{23}$). This model also allows us a prediction of Dirac CP-phase and Jarlskog parameter $(J)$. The octant of the atmospheric mixing angle ${\theta }_{23}$ occupies the lower octant. Our model prefers Normal Hierarchy (NH) than Inverted Hierarchy (IH). We use the parameter space of our model of neutrino masses to study the neutrinoless double beta decay parameter $m_{\beta \beta }$.

The form of the leptonic mixing matrix emerging from experiment has, in the last few years, generated a lot of interest in the so-called tribimaximal type. This form may be naturally associated with the possibility of a discrete permutation symmetry (S₃) among the three generations. However, trying to implement this attractive symmetry has resulted in some problems and it seems to have fallen out of favor. We suggest an approach in which the S₃ holds to first approximation, somewhat in the manner of the old SU(3) flavor symmetry of the three flavor quark model. It is shown that in the case of the neutrino sector, a presently large experimentally allowed region can be fairly well described in this first approximation. We briefly discuss the nature of the perturbations which are the analogs of the Gell-Mann–Okubo perturbations but confine our attention for the most part to the S₃ invariant model. We postulate that the S₃ invariant mass spectrum consists of nonzero masses for the (τ,b,t) and zero masses for the other charged fermions but approximately degenerate masses for the three neutrinos. The mixing matrices are assumed to be trivial for the charged fermions but of tribimaximal type for the neutrinos in the first approximation. It is shown that this can be implemented by allowing complex entries for the mass matrix and spontaneous breakdown of the S₃ invariance of the Lagrangian.

The phenomenology of 3-neutrino mixing, the current status of our knowledge about the 3-neutrino mixing parameters, including the absolute neutrino mass scale, and of the Dirac and Majorana CP violation in the lepton sector, are reviewed. The problems of CP violation in neutrino oscillations and of determining the nature — Dirac or Majorana — of massive neutrinos, are discussed. The seesaw mechanism of neutrino mass generation and the related leptogenesis scenario of generation of the baryon asymmetry of the universe, are considered. The results showing that the CP violation necessary for the generation of the baryon asymmetry of the universe in leptogenesis can be due exclusively to the Dirac and/or Majorana CP-violating phase(s) in the neutrino mixing matrix U, are briefly reviewed.

If neutrinos are Majorana particles, i.e. fermions that are their own antiparticles, then neutrinoless double-beta (0νββ) decay is possible. In such a process, two neutrons can simultaneously decay into two protons and two electrons without emitting neutrinos. Neutrinos being Majorana particles would explicitly violate lepton number conservation, and might play a role in the matter–antimatter asymmetry in the universe. The MAJORANA DEMONSTRATOR experiment is under construction at the Sanford Underground Research Facility in Lead, SD and will search for the neutrinoless double-beta (0νββ) decay of the ⁷⁶Ge isotope. The goal of the experiment is to demonstrate that it is possible to achieve a sufficiently low background rate in the 4 keV region of interest (ROI) around the 2039 keV Q-value to justify building a tonne-scale experiment. In this paper, we discuss the physics and design of the MAJORANA DEMONSTRATOR, its approach to achieving ultra-low background and the status of the experiment.

With the historic discovery of the Higgs boson our picture of particle physics would have been complete were it not for the neutrino sector and cosmology. I briefly discuss the role of neutrino masses and mixing upon gauge coupling unification, electroweak breaking and the flavor sector. Time is ripe for new discoveries such as leptonic CP violation, charged lepton flavor violation and neutrinoless double beta decay. Neutrinos could also play a role in elucidating the nature of dark matter and cosmic inflation.