Stability of the lepton-flavor mixing matrix against quantum corrections

Recent neutrino experiments suggest strong evidence of tiny neutrino masses and the lepton-flavor mixing. Neutrino-oscillation solutions for the atmospheric neutrino anomaly and the solar neutrino deficit can determine the texture of the neutrino mass matrix according to the neutrino mass hierarchies as Type A: , Type B: , and Type C: , where is the i-th generation neutrino mass. In this paper we study the stability of the lepton-flavor mixing matrix against quantum corrections for all three types of mass hierarchy in the minimal supersymmetric Standard Model with an effective dimension-five operator which gives the Majorana masses of neutrinos. The relative sign assignments of neutrino masses in each type play crucial role for the stability against quantum corrections. We find that the lepton-flavor mixing matrix of Type A is stable against quantum corrections, and that of Type B with the same (opposite) signs of and are unstable (stable). For Type C, the lepton-flavor-mixing matrix approaches the definite unitary matrix according to the relative sign assignments of the neutrino mass eigenvalues as the effects of quantum corrections become large enough to neglect the squared mass differences of neutrinos. Received: 24 June 1999 / Revised version: 23 December 1999 / Published online: 17 March 2000     

Solar magnetohydrodynamic modes and parametric resonance in neutrino spin–flavor conversion

Consequences of parametric resonances on neutrino resonant spin–flavor precession (RSFP) arising from global magnetohydrodynamic waves in the Sun are investigated. We show that for typical magnetic field profiles which generate an RSFP solution to the solar neutrino anomaly, the effects of the parametric resonance can be found for neutrinos of which the energy is of order 0.1 to 1 MeV. This opens the possibility of investigating these effects using real time experiments, like Borexino or Hellaz. Received: 4 March 1999 / Published online: 8 December 1999     

Nearly degenerate mass and bimaximal mixing of neutrinos in the SO(3) gauge model of leptons

A realistic scheme for masses and mixing of leptons is investigated in the model with gauged SO(3) lepton flavor symmetry. Within this scheme, a nearly bimaximal neutrino mixing pattern with a maximal CP-violating phase is found to be the only possible solution for reconciling both solar and atmospheric neutrino flux anomalies. Three Majorana neutrino masses are nearly degenerate and large enough to play a significant cosmological role. Lepton flavor-violating effects via SO(3) gauge interactions can be as large as the present experimental limits. Masses of the SO(3) gauge bosons are bounded to be above 24 TeV when the SO(3) gauge boson mixing angle and coupling constant are taken to be the same as those ( and g) in the electroweak theory. Received: 10 February 1999 / Published online: 8 September 1999     

Vacuum solutions of neutrino anomalies through a softly broken U(1) symmetry

We discuss an extended model which naturally leads to mass scales and mixing angles relevant for understanding both the solar and atmospheric neutrino anomalies in terms of the vacuum oscillations of the three known neutrinos. The model uses a softly broken –– symmetry and contains a heavy scale GeV. The –– symmetric neutrino masses solve the atmospheric neutrino anomaly while breaking of –– generates the highly suppressed radiative mass scale needed for the vacuum solution of the solar neutrino problem. All the neutrino masses in the model are inversely related to , thus providing seesaw-type of masses without invoking any heavy right-handed neutrinos. The possible embedding of the model into an SU(5) grand unified theory is discussed. Received: 5 August 1999 / Revised version: 18 November 1999 / Published online: 6 April 2000     

Charge and magnetic moment of the neutrino in the background field method and in the linear R_{\xi}^L gauge

We present a computation of the charge and the magnetic moment of the neutrino in the recently developed electro-weak Background Field Method and in the linear gauge. First, we deduce a formal Ward-Takahashi identity which implies the immediate cancellation of the neutrino electric charge. This Ward-Takahashi identity is as simple as that for QED. The computation of the (proper and improper) one loop vertex diagrams contributing to the neutrino electric charge is also presented in an arbitrary gauge, checking in this way the Ward-Takahashi identity previously obtained. Finally, the calculation of the magnetic moment of the neutrino, in the minimal extension of the Standard Model with massive Dirac neutrinos, is presented, showing its gauge parameter and gauge structure independence explicitly. Received: 6 July 1999 / Revised version: 27 October 1999 / Published online: 27 January 2000     

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. Received: 6 November 1998 / Published online: 18 June 1999     

A 4-neutrino model with a Higgs triplet

We take as a starting point the Gelmini–Roncadelli model enlarged by a term with explicit lepton number violation in the Higgs potential and add a neutrino singlet field that is coupled via a scalar doublet to the usual leptons. This scenario allows us to take into account all three present indications in favor of neutrino oscillations provided by the solar, atmospheric, and LSND neutrino oscillation experiments. Furthermore, it suggests a model which reproduces naturally one of the two 4-neutrino mass spectra favored by the data. In this model, the solar neutrino problem is solved by large mixing MSW transitions, and the atmospheric neutrino problem by transitions of into a sterile neutrino. Received: 11 May 1999 / Published online: 3 February 2000     

Inertial effects on Berry's phase of neutrino oscillations

The Berry phase of mixed states, here neutrino oscillations, is calculated in an accelerating and rotating reference frame. It turns out to depend on the vacuum mixing angle, the mass-squared difference and on the coupling between the momentum of the neutrino and the spinorial connection. Berry's phase for solar neutrinos and its geometrical aspects are also discussed. Received: 15 December 1999 / Revised version: 9 February 2000 / Published online: 18 May 2000     

Test of non-standard neutrino properties with the BOREXINO source experiments

We calculate the event rates induced by high-intensity radioactive sources of (Cr) and of (Sr), to be located near the BOREXINO detector. Calculations are performed both in the standard case and assuming non-standard properties of neutrinos, including flavor oscillations, neutrino electromagnetic interactions, and deviations from the standard vector and axial couplings in the -e interaction. It is shown that, in some cases, the current limits on non-standard neutrino properties can be significantly improved. Received: 11 January 1999 / Published online: 22 March 1999     

Tritium decay and the hypothesis of tachyonic neutrinos

Numerous recent measurements indicate an excess of counts near the endpoint of the electron energy spectrum in tritium decay. We show that this effect is expected if the neutrino is a tachyon. Results of calculations, based on a unitary (causal) theory of tachyons, are presented. The hypothesis of tachyonic neutrinos also offers a natural explanation of the vector-axial (V-A) structure of the weak leptonic current in neutrino interactions. Received: 12 October 1998 / Published online: 11 March 1999     

Lepton flavor changing in neutrinoless \tau decays

Neutrino oscillations, as recently reported by the Super-Kamiokande collaboration, imply that lepton numbers could be violated, and are some typical examples. We point out that in these neutrinoless modes, the GIM cancelation is much milder with only a logarithmic behavior where are the neutrino masses. This is in sharp contrast with the vanishingly small amplitude strongly suppressed by the quadratic power . In comparison with the hopelessly small branching ratio B, the B could be larger than . The latter mode, if measurable, could give one more constraint to the lepton mixing angle and the neutrino mass ratio , and therefore is complementary to neutrino oscillation experiments. Received: 29 October 1998 / Published online: 11 March 1999     

Search for heavy Majorana neutrinos at future lepton colliders

A nonzero neutrino mass may be a sign of new physics beyond the standard model (SM). To explain the small neutrino mass, we can extend the SM using right-handed Majorana neutrinos in a low-scale seesaw mechanism, and the CP violation effect can be induced due to the CP phase in the interference of heavy Majorana neutrinos. The existence of heavy Majorana neutrinos may lead to lepton number violation processes, which can be used to search for the signals of heavy Majorana neutrinos. In this paper, we focus on the CP violation effect related to two generations of heavy Majorana neutrinos at \begin{document}$ 15 $\end{document} GeV \begin{document}$ <m_{N_1}< 70 $\end{document} GeV in the pair production of W bosons and rare decays. It is valuable to investigate Majorana neutrino production signals and the related CP violation effects in rare W boson decays at future lepton colliders.     

Detection of Supernova Neutrinos on the Earth for Large θ_(13)

Supernova （SN） neutrinos detected on the Earth are subject to the shock wave effects, the Mikheyev- Smirnov-Wolfenstein （MSW） effects, the neutrino collective effects and the Earth matter effects. Considering the recent experimental result about the large mixing angle 013 （-8.8°） provided by the Daya Bay Collaboration and applying the available knowledge for the neutrino conversion probability in the high resonance region of SN, PH , which is in the form of hypergeometric function in the case of large 813, we deduce the expression of PH taking into account the shock wave effects. It is found that PH is not zero in a certain range of time due to the shock wave effects. After considering all the four physical effects and scanning relevant parameters, we calculate the event numbers of SN neutrinos for the ＂Garehing＂ distribution of neutrino energy spectrum. From the numerical results, it is found that the behaviors of neutrino event numbers detected on the Earth depend on the neutrino mass hierarchy and neutrino spectrum parameters including the dimensionless pinching parameter βa （where a refers to neutrino flavor）, the average energy 〈Ea〉, and the SN neutrino luminosities La. Finally, we give the ranges of SN neutrino event numbers that will be detected at the Daya Bay experiment.     

A new global analysis of deep inelastic scattering data

A new QCD analysis of deep inelastic scattering (DIS) data is presented. All available neutrino and antineutrino cross sections are reanalyzed and included in the fit, along with charged-lepton DIS and Drell–Yan data. A massive factorization scheme is used to describe the charm component of the structure functions. Next-to-leading-order parton distribution functions are provided. In particular, the strange-sea density is determined with a higher accuracy with respect to other global fits. Received: 16 March 1999 / Published online: 8 December 1999     

Linear seesaw model with T7 symmetry for neutrino mass and mixing

We propose a low-scale Standard Model extension with \begin{document}$T_7\times Z_4 \times Z_3\times Z_2$\end{document} symmetry that can successfully explain observed neutrino oscillation results within the \begin{document}$3 \sigma$\end{document} range. Small neutrino masses are obtained via the linear seesaw mechanism. Normal and inverted neutrino mass orderings are considered with three lepton mixing angles in their experimentally allowed \begin{document}$3\sigma$\end{document} ranges. The model provides a suitable correlation between the solar and reactor neutrino mixing angles, which is consistent with the \begin{document}${\rm{TM}}_2$\end{document} pattern. The prediction for the Dirac phase is \begin{document}$\delta_{\rm CP}\in (295.80, 330.0)^\circ$\end{document} for both normal and inverted orderings, including its experimentally maximum value, while those for the two Majorana phases are \begin{document}$\eta_1\in (349.60, 356.60)^\circ,\, \eta_2=0$\end{document} for normal ordering and \begin{document}$\eta_1\in (3.44, 10.37)^\circ, \, \eta_2=0$\end{document} for inverted ordering. In addition, the predictions for the effective neutrino masses are consistent with the present experimental bounds.     

Higgs bosons in the left–right model

The model with the gauge group, containing one bidoublet and two triplets in the Higgs sector, is considered. The link between the constants determining the physical Higgs boson interactions and the neutrino oscillation parameters is found. It is shown that the observation of the ultrahigh-energy neutrinos with the help of the processes , gives us information on the singly charged Higgs bosons. The processes of the doubly charged Higgs bosons production, , are investigated. From the point of view of detecting the neutral Higgs bosons the process of the electron–muon recharge is studied. Received: 29 January 1999 / Revised version: 20 September 1999 / Published online: 3 February 2000     

Stellar energy loss rates associated with photoneutrino processes in the minimal extension of the standard model

Using the minimal extension of the standard model and considering the charge radius and the anapole moments of a neutrino, we derive analytical expressions for the stellar energy loss rates associated with the production of a neutrino pair \begin{document}$ e^- + \gamma \rightarrow e^- + \nu_e +\overline{\nu_e} $\end{document} in hot plasma under three limiting regimes (nondegenerate, intermediate, and degenerate electrons) of the temperature, electron chemical potential, and plasma energy. The obtained results reveal the presence of an extra contribution of approximately \begin{document}$10$\end{document}% based on the considered calculations.     

A non-renormalizable B-L model with Q4 × Z4 × Z2 flavor symmetry for cobimaximal neutrino mixing

We construct a non-renormalizable gauge \begin{document}$ B-L $\end{document} model based on \begin{document}$ Q_4\times Z_4\times Z_2 $\end{document} symmetry that successfully explains the cobimaximal lepton mixing scheme. Small active neutrino masses and both neutrino mass hierarchies are produced via the type-I seesaw mechanism at the tree-level. The model is predictive; hence, it reproduces the cobimaximal lepton mixing scheme, and the reactor neutrino mixing angle \begin{document}$ \theta_{13} $\end{document} and the solar neutrino mixing angle \begin{document}$ \theta_{12} $\end{document} can obtain best-fit values from recent experimental data. Our model also predicts the effective neutrino mass parameters of \begin{document}$ m_{\beta }\in (8.80, 9.05)\, \mathrm{meV} $\end{document} and \begin{document}$ \langle m_{ee}\rangle \in (3.65, 3.95)\, \mathrm{meV} $\end{document} for normal ordering (NO) and \begin{document}$ m_{\beta }\in (49.16, 49.21)\, \mathrm{meV} $\end{document} and \begin{document}$ \langle m_{ee}\rangle \in (48.59, 48.67)\, \mathrm{meV} $\end{document} for inverted ordering (IO), which are highly consistent with recent experimental constraints.     

Leading-order QCD analysis of neutrino-induced dimuon events

The results of a leading-order QCD analysis of neutrino-induced charm production are presented. They are based on a sample of 4111 - and 871 -induced opposite-sign dimuon events with , and , observed in the CHARM II detector exposed to the CERN wide band neutrino and antineutrino beams. The analysis yields the value of the charm quark mass and the Cabibbo–Kobayashi–Maskawa matrix element . The strange quark content of the nucleon is found to be suppressed with respect to non-strange sea quarks by a factor . Received: 1 February 1999 / Published online: 14 October 1999     

Update of results from the SuperKamiokande detector (June 1999)

The data collected in the SuperKamiokande detector as of June 1999 are presented. This review covers the complete spectrum of neutrino interactions from solar neutrinos, through the entire spectrum of atmospheric neutrinos, and ending with the neutrino beam produced at KEK for a long-baseline experiment. Different interpretations of these data as demonstrations of neutrino oscillations are discussed. The results of a search for nucleon decay are also summarized.