Degenerate Neutrinos Research Articles

Radiative Seesaw Mechanism and Degenerate Neutrinos

The radiative seesaw mechanism of Witten generates the right-handed neutrino masses in SO(10) with the spinorial 16(H) Higgs field. We study here analytically the 2nd and 3rd generations for the minimal Yukawa structure containing and Higgs representations. In the approximation of small 2nd generation masses and gauge loop domination we find the following results: (1) beta-tau unification, (2) natural coexistence between large theta(l) and small theta(q), and (3) degenerate neutrinos.

On leptogenesis with two degenerate right-chiral neutrinos

We analyze the predictions of the most general version of the supersymmetric minimal seesaw model with only two right-chiral neutrinos, which are degenerate in masses at the scale of Grand Unification. We apply the renormalization group technique to the running of the masses of the right-chiral neutrinos and the neutrino Yukawa couplings and find that thermal leptogenesis can account for the observed baryon asymmetry of the Universe even with a low reheating temperature, allowing to overcome the gravitino problem.

Single and pair production of doubly charged Higgs bosons at hadron colliders

Current searches for doubly charged Higgs bosons (H^{\pm\pm}) at the Fermilab Tevatron are sensitive to single production of H^{\pm\pm}, although the pair production mechanism q\bar q\to H^{++}H^{--} is assumed to be dominant. In the context of a Higgs Triplet Model we study the mechanism q'\bar q\to H^{\pm\pm}H^{\mp} at the Tevatron and CERN Large Hadron Collider, and show that its inclusion can significantly improve the search potential for H^{\pm\pm}. Moreover, assuming that the neutrino mass is generated solely by the triplet field Yukawa coupling to leptons, we compare the branching ratios of H^{\pm\pm}\to l^\pm l^\pm and H^{\pm\pm}\to H^\pm W^* for the cases of a normal hierarchical, inverted hierarchical and degenerate neutrino mass spectrum.

Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology

We calculate and display the primordial light-element abundances as a function of a neutrino degeneracy parameter $\ensuremath{\xi}$ common to all flavors. It is the only unknown parameter characterizing the thermal medium at the primordial nucleosynthesis epoch. The observed primordial helium abundance ${Y}_{\mathrm{p}}$ is the most sensitive cosmic ``leptometer.'' Adopting the conservative ${Y}_{\mathrm{p}}$ error analysis of Olive and Skillman implies $\ensuremath{-}0.04\ensuremath{\lesssim}\ensuremath{\xi}\ensuremath{\lesssim}0.07$ whereas the errors stated by Izotov and Thuan imply $\ensuremath{\xi}=0.0245\ifmmode\pm\else\textpm\fi{}0.0092$ ($1\ensuremath{\sigma}$). Improved determinations of the baryon abundance have no significant impact on this situation. A determination of ${Y}_{\mathrm{p}}$ that reliably distinguishes between a vanishing or nonvanishing $\ensuremath{\xi}$ is a crucial test of the cosmological standard assumption that sphaleron effects equilibrate the cosmic lepton and baryon asymmetries.

Constraint on [formula omitted] cosmic string from leptogenesis with degenerate neutrinos

In the early universe, as a consequence of U(1)B–L gauge symmetry-breaking, the so-called B–L cosmic strings are expected to be produced at the breaking scale ηB–L according to the Kibble mechanism. The decaying, collapsing closed loops of these strings can release the right-handed neutrinos, whose subsequent decay can contribute to the baryon asymmetry of the universe (BAU), through the “slow death” (SD) process and/or the “quick death” (QD) process. In this Letter, we assume that the decay of the lightest heavy Majorana neutrinos released from the B–L cosmic string loops can produce a baryon asymmetry consistent with the cosmic microwave background (CMB) observations. Considering the fact that both the neutrinoless double beta decay experiment and the cosmological data show a preference for degenerate neutrinos, we give the lower limits for the breaking scale ηB–L with the neutrino masses 0.06 eV⩽m¯=(m12+m22+m32)1/2⩽1.0 eV, where the full possible cases of degenerate neutrinos are included. We obtain ηB–L≳3.3×1015 GeV, 5.3×1015 GeV and 9.5×1015 GeV for m¯=0.2 eV, 0.4 eV and 1.0 eV respectively in the SD process, and find the B–L cosmic string has a very small contribution to the BAU in the QD process.

First evidence for neutrinoless double beta decay, with enriched 76Ge in Gran Sasso 1990-2003

The HEIDELBERG-MOSCOW experiment operating 11 kg of enriched 76Ge in the GRAN SASSO Underground Laboratory, is one of the long-running underground experiments. It is the most sensitive double beta decay experiment since more than ten years. The measurement, and the analysis of the data taken from 2 August 1990-20 May 2003, which has been performed in the second half of 2003, is presented here. The duty cycle of the experiment was ∼80%, the collected statistics is 71.7kgy. The background achieved in the energy region of the Q value for double beta decay is O.11events/kgykeV. The two-neutrino accompanied half-life is determined on the basis of more than 100 000 events. The confidence level for the neutrinoless signal has been improved to a 4.2 σ level. Fundamental consequences of this first evidence for neutrinoless double beta decay is that total lepton number is not conserved, and further, that the neutrino is a Majorana particle, and further that only a degenerate neutrino mass model is allowed. The effective neutrino mass deduced is (0.2-0.6)eV.

Constraining neutrino masses by CMB experiments alone

It is shown that a subelectronvolt upper limit can be derived on the neutrino mass from the CMB data alone in the Lambda CDM model with the power-law adiabatic perturbations, without the aid of any other cosmological data. Assuming the flatness of the universe, the constraint we can derive from the current WMAP observations is \sum m_nu < 2.0 eV at the 95% confidence level for the sum over three species of neutrinos (m_nu < 0.66 eV for the degenerate neutrinos). This constraint modifies little even if we abandon the flatness assumption for the spatial curvature. We argue that it would be difficult to improve the limit much beyond \sum m_nu \lesssim 1.5 eV using only the CMB data, even if their statistics are substantially improved. However, a significant improvement of the limit is possible if an external input is introduced that constrains the Hubble constant from below. The parameter correlation and the mechanism of CMB perturbations that give rise to the limit on the neutrino mass are also elucidated.

CP violating vacuum transition and Big Bang Nucleosynthesis

An analysis of primordial nucleosynthesis is made in the perspective of transition in the early universe from quark gluon to a hadronic phase in a CP violating vacuum. The universe opaque to color, quarks and anti quarks binds into globally colorless hadrons. u, d and s quarks are considered in a sea of degenerate neutrinos for the case of μve = μvμ = μvτ. The nn/np ratio is calculated for a transition temperature ˜ 100 − 200MeV for various values of neutrino degeneracy ξve = μve/T, μve being the chemical potential of electron type neutrino. The limiting value of ξve is found to be 2.38, if the upper bound of fractional helium abundance Yp is 0.26.

Neutrino Mean Free Path in Strange Matter at High Temperatures

We investigate the condition that the neutrino energiesEν can be considered to be small enough to be neglected in theenergy momentum conservation equation in the reactionsd+νe→u+e− and u+e−→d+νe. It canbe simply expressed as Eν<4αcμe/π, whereαc is the strong coupling constant and μe is theelectron chemical potential. Then for Eν<4αcμe/π,we generalize the mean free path for nondegenerate neutrinos givenby Iwamoto and obtain a formula that is valid for both nondegenerate anddegenerate neutrinos. We also model the mean free path for bothnondegenerate and degenerate neutrinos with energiesEν>4αcμe/π.

From hierarchical to partially degenerate neutrinos via type II upgrade of type I see-saw models

We propose a type II upgrade of type I see-saw models leading to new classes of models where partially degenerate neutrinos are as natural as hierarchical ones. The additional type II contribution to the neutrino mass matrix, which determines the neutrino mass scale, is forced to be proportional to the unit matrix by a SO(3) flavour symmetry. The type I see-saw part of the neutrino mass matrix, which controls the mass squared differences and mixing angles, may be governed by sequential right-handed neutrino dominance and a natural alignment for the SO(3)-breaking vacuum. We focus on classes of models with bi-large mixing originating from the neutrino mass matrix although we also briefly discuss other classes of models where large mixing stems from the charged lepton mass matrix. We study renormalization group corrections to the neutrino mass squared differences and mixings and find that the low energy values do not depend sensitively on the high energy values for partially degenerate neutrinos with a mass scale up to about 0.15 eV. Our scenario predicts the effective mass for neutrinoless double beta decay to be approximately equal to the neutrino mass scale and therefore neutrinoless double beta decay will be observable if the neutrino mass spectrum is partially degenerate. We also find that all observable CP phases become small as the neutrino mass scale increases.

A4symmetry and neutrinos with very different masses

The ${A}_{4}$ model of leptons was proposed originally for nearly degenerate neutrino masses. With minimal modification, it is shown to accommodate as well the cases of normal hierarchy and inverted hierarchy. In all cases, this new model predicts ${\mathrm{sin}}^{2}2{\ensuremath{\theta}}_{\mathrm{atm}}=1$ and ${\mathrm{tan}}^{2}{\ensuremath{\theta}}_{\mathrm{sol}}=0.5$ for ${U}_{e3}=0.$ It also predicts their deviations from 1 and 0.5 respectively for ${U}_{e3}\ensuremath{\ne}0.$

Decoupling supernova and neutrino oscillation physics with LAr TPC detectors

Core collapse supernovae are a huge source of all flavour neutrinos. The flavourcomposition, energy spectrum and time structure of the neutrino burst from a galacticsupernova can provide information about the explosion mechanism and the mechanisms ofproto neutron star cooling. Such data can also give information about the intrinsicproperties of the neutrino such as flavour oscillations. One important question isto understand to what extent the supernova and the neutrino physics can bedecoupled in the observation of a single supernova. On one hand, the understandingof the supernova explosion mechanism is still plagued by uncertainties whichhave an impact on the precision with which one can predict time-, energy- andflavour-dependent neutrino fluxes. On the other hand, the neutrino mixing propertiesare not fully known, since the type of mass hierarchy and the value of theθ13 angle are unknown, and in fact large uncertainty still exists on the prediction ofthe actual effect of neutrino oscillations in the event of a supernova explosion.In this paper we discuss the possibility of probing the neutrino mixing angleθ13 and the type of mass hierarchy from the detection of supernova neutrinos with a liquidargon TPC detector. Moreover, describing the supernova neutrino emission by a set of fiveparameters (average energy of the different neutrino flavours, their relative luminosity andthe total supernova binding energy), we quantitatively study how it is possible to constrainthese parameters. A characteristic feature of the liquid argon TPC is the accessibility tofour independent detection channels ((1) elastic scattering off electrons, (2) chargedneutrino and (3) antineutrino and (4) neutral currents on argon nuclei) which have differentsensitivities to electron neutrino, anti-electron neutrino and other neutrino flavours (muonand tau (anti)neutrinos). This allows us to over-constrain the five supernova andthe flavour mixing parameters and to some extent disentangle neutrino fromsupernova physics. Numerically, we find that a very massive liquid argon detector(O(100 kton)) is needed to perform accurate measurements of these parameters, speciallyin supernova scenarios where the average energies of electron and non-electronneutrinos are similar (almost degenerate neutrinos) or if no information about theθ13 mixing angle and type of mass hierarchy is available.

Sequential dominance

We review the mechanism of sequential right-handed neutrino dominance proposed in the framework of the type I see-saw mechanism to account for bi-large neutrino mixing and a neutrino mass hierarchy in a natural way. We discuss how sequential dominance (SD) may also be applied to the right-handed charged leptons, which alternatively allows for bi-large lepton mixing from the charged lepton sector. We review how such SD models may be upgraded to include type II see-saw contributions, resulting in a partially degenerate neutrino mass spectrum with the bi-large lepton mixing arising from SD. We also summarize the model-building applications and the phenomenological implications of SD.

Type II leptogenesis and the neutrino mass scale

We discuss the effect of the neutrino mass scale on baryogenesis via the out-of-equilibrium decay of the lightest right-handed (s)neutrinos in type II see–saw models. We calculate the type II contributions to the decay asymmetries for minimal scenarios based on the Standard Model and on the Minimal Supersymmetric Standard Model, where the additional direct mass term for the neutrinos arises from a Higgs triplet vacuum expectation value. The result in the supersymmetric case is new and we correct the previous result in the scenario based on the Standard Model. We confirm and generalize our results by calculating the decay asymmetries in an effective approach, which is independent of the realization of the type II contribution. We derive a general upper bound on the decay asymmetry in type II see–saw models and find that it increases with the neutrino mass scale, in sharp contrast to the type I case which leads to an upper bound of about 0.1 eV on the neutrino mass scale. We find a lower bound on the mass of the lightest right-handed neutrino, significantly below the corresponding type I bound for partially degenerate neutrinos. This lower bound decreases with increasing neutrino mass scale, making leptogenesis more consistent with the gravitino constraints in supersymmetric models.

Supersymmetric threshold corrections to Δm2⊙

For nearly degenerate neutrinos, quantum corrections can modify the tree-level masses via low energy supersymmetric threshold corrections comparable to the solar oscillation mass scale. We numerically calculate corrections to neutrino masses in minimal supergravity (mSugra) and Gauge Mediated Supersymmetry Breaking (GMSB) scenarios and identify parameter spaces in the high energy regime for which the solar neutrino mass splitting becomes too large compared to the LMA solution. We show that such considerations can give bounds on GMSB and mSugra models which can be useful. On the contrary, if we start from degenerate mass eigenvalues at the tree level, these threshold corrections being generation dependent, can also produce the required mass splitting at solar scale for regions of parameter space.

Thermal leptogenesis in a model with mass varying neutrinos

In this paper we consider the possibility of neutrino mass varying during the evolution of the Universe and study its implications on leptogenesis. Specifically, we take the minimal seesaw model of neutrino masses and introduce a coupling between the right-handed neutrinos and the dark energy scalar field, the Quintessence. In our model, the right-handed neutrino masses change as the Quintessence scalar evolves. We then examine in detail the parameter space of this model allowed by the observed baryon number asymmetry. Our results show that it is possible to lower the reheating temperature in this scenario in comparison with the case that the neutrino masses are unchanged, which helps solve the gravitino problem. Furthermore, a degenerate neutrino mass patten with $m_i$ larger than the upper limit given in the minimal leptogenesis scenario is permitted.

Almost degenerate neutrinos and bi-large lepton mixing

A scenario is proposed for bi-large lepton mixing in the framework of nearly threefold degenerate Majorana neutrinos. In our proposal, we impose Z3 symmetry in the neutrino sector at a high energy scale to account for the threefold degenerate neutrinos and the maximal mixing between νμ and ντ. In order to obtain the atmospheric neutrino mass splitting while keeping the maximal mixing between νμ and ντ, we introduce a small perturbation to the neutrino mass matrix without breaking Z3 symmetry. On the other hand, the solar neutrino mixing arises due to the non-diagonal charged lepton mass matrix, and the desirable large mixing and mass splitting for the solar neutrino oscillation can be obtained by radiative corrections.

WMAP, neutrino degeneracy, and non-Gaussianity constraints on isocurvature perturbations in the curvaton model of inflation

In the curvaton model of inflation, where a second scalar field, the "curvaton", is responsible for the observed inhomogeneity, a non-zero neutrino degeneracy may lead to a characteristic pattern of isocurvature perturbations in the neutrino, cold dark matter and baryon components. We find the current data can only place upper limits on the level of isocurvature perturbations. These can be translated into upper limits on the neutrino degeneracy parameter. In the case that lepton number is created before curvaton decay, we find that the limit on the neutrino degeneracy parameter is comparable with that obtained from Big-bang nucleosynthesis. For the case that lepton number is created by curvaton decay we find that the absolute value of the non-Gaussianity parameter, |f_nl|, must be less than 10 (95% confidence interval).

A preference for a non-zero neutrino mass from cosmological data

We present results from the analysis of cosmic microwave background (CMB), large-scale structure (galaxy redshift survey) and X-ray galaxy cluster (baryon fraction and X-ray luminosity function) data, assuming a geometrically flat cosmological model and allowing for tensor components and a non-negligible neutrino mass. From a combined analysis of all data, assuming three degenerate neutrino species, we measure a contribution of neutrinos to the energy density of the Universe, Omega(nu)h(2) = 0.0059(-0.0027)(+0.0033) (68 per cent confidence limits), with zero falling on the 99 per cent confidence limit. This corresponds to similar to4 per cent of the total mass density of the Universe and implies a species-summed neutrino mass Sigma(i)m(i) = 0.56(-0.26)(+0.30) eV, or m(nu) similar to 0.2 eV per neutrino. We examine possible sources of systematic uncertainty in the results. Combining the CMB, large-scale structure and cluster baryon fraction data, we measure an amplitude of mass fluctuations on 8 h(-1) Mpc scales of sigma(8) = 0.74(-0.07)(+0.12), which is consistent with measurements based on the X-ray luminosity function and other studies of the number density and evolution of galaxy clusters. This value is lower than that obtained when fixing a negligible neutrino mass (sigma(8) = 0.86(-0.07)(+0.08)). The combination of CMB, large-scale structure and cluster baryon fraction data also leads to remarkably tight constraints on the Hubble constant, H-0 = 68.4(-1.4)(+2.0) km s(-1) Mpc(-1), mean matter density, Omega(m) = 0.31 +/- 0.02, and physical baryon density, Omega(b) h(2) = 0.024 +/- 0.001, of the Universe.

New “square root” model of lepton family cyclic symmetry

Following the newly formulated notion of form invariance of the neutrino mass matrix, a complete model of leptons is constructed. It is based on a specific unitary 3 X 3 matrix U in family space, such that U^2 is the simple discrete symmetry nu_e to -nu_e, nu_mu to nu_tau. Thus U also generates the cyclic group Z_4. The charged-lepton mass matrix is nearly diagonal while the neutrino mass matrix is of the form suitable for explaining maximal (large) mixing in atmospheric (solar) neutrino oscillations in the context of three nearly degenerate neutrino masses. Observable lepton flavor violation is predicted. Quarks may be treated in the same way as the charged leptons.