On the Effect of theta_{13} on the Determination of Solar Oscillation Parameters at KamLAND

If the solution to the solar neutrino puzzle falls in the LMA region, KamLAND should be able to measure with good precision the corresponding oscillation parameters after a few years of data taking. Assuming a positive signal, we study their expected sensitivity to the solar parameters (theta_{12},Delta m^2_{21}) when considered in the framework of three-neutrino mixing after taking into account our ignorance on the mixing angle theta_{13}. We find a simple ``scaling'' dependence of the reconstructed theta_{12} range with the value of theta_{13} while the Delta m^2_{12} range is practically unaffected. Our results show that the net effect is approximately equivalent to an uncertainty on the overall neutrino flux normalization of up to sim 10 %.Comment: 6 pages, 2 figures. Typos corrected and reference added. Final version to appear in PL

Large Mixing Induced by the Strong Coupling with a Single Bulk Neutrinos

Neutrino is a good probe of extra dimensions. Large mixing and the apparent lack of very complicated oscillation patterns may be an indication of large couplings between the brane and a single bulk neutrino. A simple and realistic five-dimensional model of this kind is discussed. It requires a sterile in addition to three active neutrinos on the brane, all coupled strongly to one common bulk neutrino, but not directly among themselves. Mindful that sterile neutrinos are disfavored in the atmospheric and solar data, we demand induced mixing to occur among the active neutrinos, but not between the active and the sterile. The size $R$ of the extra dimension is arbitrary in this model, otherwise it contains six parameters which can be used to fit the three neutrino masses and the three mixing angles. However, in the model those six parameters must be suitably ordered, so a successful fit is not guaranteed. It turns out that not only the data can be fitted, but as a result of the ordering, a natural connection between the smallness of the reactor angle $\theta_{13}$ and the smallness of the mass-gap ratio $\Delta M^2_{solar}/\Delta M^2_{atmospheric}$ can be derived.Comment: Misprints above eq. (22) corrected. To appear in PR

Probing neutrino non-standard interactions with atmospheric neutrino data

We have reconsidered the atmospheric neutrino anomaly in light of the laetst data from Super-Kamiokande contained events and from Super-Kamiokande and MACRO up-going muons. We have reanalysed the proposed solution to the atmospheric neutrino anomaly in terms of non-standard neutrino-matter interactions (NSI) as well as the standard nu_mu -> nu_tau oscillations (OSC). Our statistical analysis shows that a pure NSI mechanism is now ruled out at 99%, while the standard nu_mu -> nu_tau OSC mechanism provides a quite remarkably good description of the anomaly. We therefore study an extended mechanism of neutrino propagation which combines both oscillation and non-standard neutrino-matter interactions, in order to derive limits on flavour-changing (FC) and non-universal (NU) neutrino interactions. We obtain that the off-diagonal flavour-changing neutrino parameter epsilon and the diagonal non-universality neutrino parameter epsilon' are confined to -0.03 < epsilon < 0.02 and |epsilon'| < 0.05 at 99.73% CL. These limits are model independent and they are obtained from pure neutrino-physics processes. The stability of the neutrino oscillation solution to the atmospheric neutrino anomaly against the presence of non-standard neutrino interactions establishes the robustness of the near-maximal atmospheric mixing and massive-neutrino hypothesis. The best agreement with the data is obtained for Delta_m^2 = 2.3*10^{-3} eV^2, sin^2(2*theta) = 1, epsilon = 6.7*10^{-3} and epsilon' = 1.1*10^{-3}, although the chi^2 function is quite flat in the epsilon and epsilon' directions for epsilon, epsilon' -> 0.Comment: 26 pages, LaTeX file using REVTeX4, 1 table and 12 figures included. Added a revised analysis which takes into account the new 1489-day Super-Kamiokande and final MACRO data. The bound on NSI parameters is considerably improve

Models of Neutrino Masses: Anarchy versus Hierarchy

We present a quantitative study of the ability of models with different levels of hierarchy to reproduce the solar neutrino solutions, in particular the LA solution. As a flexible testing ground we consider models based on SU(5)xU(1)_F. In this context, we have made statistical simulations of models with different patterns from anarchy to various types of hierachy: normal hierarchical models with and without automatic suppression of the 23 (sub)determinant and inverse hierarchy models. We find that, not only for the LOW or VO solutions, but even in the LA case, the hierarchical models have a significantly better success rate than those based on anarchy. The normal hierachy and the inverse hierarchy models have comparable performances in models with see-saw dominance, while the inverse hierarchy models are particularly good in the no see-saw versions. As a possible distinction between these categories of models, the inverse hierarchy models favour a maximal solar mixing angle and their rate of success drops dramatically as the mixing angle decreases, while normal hierarchy models are far more stable in this respect.Comment: v1: 28 pages, 12 figures; v2: 34 pages, 14 figures, updated previous analysis with the inclusion of recent SNO result

Supersymmetric Seesaw without Singlet Neutrinos: Neutrino Masses and Lepton-Flavour Violation

We consider the supersymmetric seesaw mechanism induced by the exchange of heavy SU(2)_W triplet states, rather than `right-handed' neutrino singlets, to generate neutrino masses. We show that in this scenario the neutrino flavour structure tested at low-energy in the atmospheric and solar neutrino experiments is directly inherited from the neutrino Yukawa couplings to the triplets. This allows us to predict the ratio of the tau --> mu gamma (or tau --> e gamma) and mu --> e gamma decay rates in terms of the low-energy neutrino parameters. Moreover, once the model is embedded in a grand unified model, quark-flavour violation can be linked to lepton-flavour violation.Comment: 26 LaTeX pages, 10 postscript figures, uses epsfig and axodraw. Comments and references adde

Confusing non-standard neutrino interactions with oscillations at a neutrino factory

Most neutrino mass theories contain non-standard interactions (NSI) of neutrinos which can be either non-universal (NU) or flavor-changing (FC). We study the impact of such interactions on the determination of neutrino mixing parameters at a neutrino factory using the so-called ``golden channels'' \pnu{e}\to\pnu{\mu} for the measurement of \theta_{13}. We show that a certain combination of FC interactions in neutrino source and earth matter can give exactly the same signal as oscillations arising due to \theta_{13}. This implies that information about \theta_{13} can only be obtained if bounds on NSI are available. Taking into account the existing bounds on FC interactions, this leads to a drastic loss in sensitivity in \theta_{13}, at least two orders of magnitude. A near detector at a neutrino factory offers the possibility to obtain stringent bounds on some NSI parameters. Such near site detector constitutes an essential ingredient of a neutrino factory and a necessary step towards the determination of \theta_{13} and subsequent study of leptonic CP violation.Comment: 23 pages, 5 figures, improved version, accepted for publication in Phs. Rev. D, references adde

Effects of new physics in neutrino oscillations in matter

A new flavor changing electron neutrino interaction with matter would always dominate the nu_e oscillation probability at sufficiently high neutrino energies. Being suppressed by theta_{13}, the energy scale at which the new effect starts to be relevant may be within the reach of realistic experiments, where the peculiar dependence of the signal with energy could give rise to a clear signature in the nu_e --> nu_tau channel. The latter could be observed by means of a coarse large magnetized detector by exploiting tau --> mu decays. We discuss the possibility of identifying or constraining such effects with a high energy neutrino factory. We also comment on the model independent limits on them.Comment: 11 pages, 5 figure

CP Violation in a Supersymmetric SO(10) x U(2)_{F} Model

A model based on SUSY SO(10) combined with U(2) family symmetry constructed recently by the authors is generalized to include phases in the mass matrices leading to CP violation. In contrast with the commonly used effective operator approach, $\bar{126}$-dimensional Higgs fields are utilized to construct the Yukawa sector. R-parity symmetry is thus preserved at low energies. The symmetric mass textures arising from the left-right symmetry breaking chain of SO(10) give rise to very good predictions for quark and lepton masses and mixings. The prediction for $\sin 2\beta$ agrees with the average of current bounds from BaBar and Belle. In the neutrino sector, our predictions are in good agreement with results from atmospheric neutrino experiments. Our model favors both the LOW and QVO solutions to the solar neutrino anomaly; the matrix element for neutrinoless double beta decay is highly suppressed. The leptonic analog of the Jarlskog invariant, $J_{CP}^{l}$, is predicted to be of $O(10^{-2})$.Comment: RevTeX4; 7 pages; typos corrected; clarification remarks added; more references added. To appear in Physical Review

Horizontal symmetry in Higgs sector of GUT with U(1)_A symmetry

In a series of papers, we pointed out that an anomalous $U(1)_A$ gauge symmetry naturally solves various problems in grand unified theories (GUTs) and that a horizontal gauge symmetry, $SU(2)_H$ or $SU(3)_H$, not only realizes the unification of three generation quarks and leptons in fewer multiplets but also solves the supersymmetric flavor problem. In this paper, we examine the possibility that the Higgs sectors of the GUT symmetry and of the horizontal symmetry are unified, that is, there are some Higgs fields whose vacuum expectation values (VEVs) break both the GUT gauge symmetry and the horizontal symmetry at the same time. Although the scale of the VEVs become too large to suppress the flavor changing neutral current processes sufficiently, the unification is possible. In addition, for the $SU(3)_H$ models, the $SU(3)_H$ gauge anomaly is cancelled in the unified models without introducing additional fields in contrast with the previous models in which the Higgs sectors are not unified.Comment: 35 page