Neutrinos Have Mass - So What?

In this brief review, I discuss the new physics unveiled by neutrino oscillation experiments over the past several years, and discuss several attempts at understanding the mechanism behind neutrino masses and lepton mixing. It is fair to say that, while significant theoretical progress has been made, we are yet to construct a coherent picture that naturally explains non-zero, yet tiny, neutrino masses and the newly revealed, puzzling patterns of lepton mixing. I discuss what the challenges are, and point to the fact that more experimental input (from both neutrino and non-neutrino experiments) is dearly required - and that new data is expected to reveal, in the next several years, new information. Finally, I draw attention to the fact that neutrinos may have only just begun to reshape fundamental physics, given the fact that we are still to explain the LSND anomaly and because the neutrino oscillation phenomenon is ultimately sensitive to very small new-physics effects.Comment: invited brief review, 15 pages, 1 eps figure, typo corrected, reference adde

Can a CPT Violating Ether Solve ALL Electron (Anti)Neutrino Puzzles?

Assuming that CPT is violated in the neutrino sector seems to be a viable alternative to sterile neutrinos when it comes to reconciling the LSND anomaly with the remainder of the neutrino data. There are different (distinguishable) ways of incorporating CPT violation into the standard model, including postulating m different from \bar{m}. Here, I investigate the possibility of introducing CPT violation via Lorentz-invariance violating effective operators (``Ether'' potentials) which modify neutrino oscillation patterns like ordinary matter effects. I argue that, within a simplified two-flavor like oscillation analysis, one cannot solve the solar neutrino puzzle and LSND anomaly while still respecting constraints imposed by other neutrino experiments, and comment on whether significant improvements should be expected from a three-flavor analysis. If one turns the picture upside down, some of the most severe constrains on such CPT violating terms can already be obtained from the current neutrino data, while much more severe constraints can arise from future neutrino oscillation experiments.Comment: 10 pages, 1 eps figure; version to appear in PRD. Comment added, mistake corrected, results and conclusions unchange

Deviation of Atmospheric Mixing from Maximal and Structure in the Leptonic Flavor Sector

I attempt to quantify how far from maximal one should expect the atmospheric mixing angle to be given a neutrino mass-matrix that leads, at zeroth order, to a nu_3 mass-eigenstate that is 0% nu_e, 50% nu_mu, and 50% nu_tau. This is done by assuming that the solar mass-squared difference is induced by an "anarchical" first order perturbation, an approach than can naturally lead to experimentally allowed values for all oscillation parameters. In particular, both |cos 2theta_atm| (the measure for the deviation of atmospheric mixing from maximal) and |U_e3| are of order sqrt(Delta m^2_sol/Delta m^2_atm) in the case of a normal neutrino mass-hierarchy, or of order Delta m^2_sol/Delta m^2_atm in the case of an inverted one. Hence, if any of the textures analyzed here has anything to do with reality, next-generation neutrino experiments can see a nonzero cos 2theta_atm in the case of a normal mass-hierarchy, while in the case of an inverted mass-hierarchy only neutrino factories should be able to see a deviation of sin^2 2theta_atm from 1.Comment: 12 pages, no figures, references and acknowledgments adde

Neutrino Masses and Lepton Flavour Violation in Thick Brane Scenarios

We address the issue of lepton flavour violation and neutrino masses in the ``fat-brane'' paradigm, where flavour changing processes are suppressed by localising different fermion field wave-functions at different positions (in the extra dimensions) in a thick brane. We study the consequences of suppressing lepton number violating charged lepton decays within this scenario for lepton masses and mixing angles. In particular, we find that charged lepton mass matrices are constrained to be quasi-diagonal. We further consider whether the same paradigm can be used to naturally explain small Dirac neutrino masses by considering the existence of three right-handed neutrinos in the brane, and discuss the requirements to obtain phenomenologically viable neutrino masses and mixing angles. Finally, we examine models where neutrinos obtain a small Majorana mass by breaking lepton number in a far away brane and show that, if the fat-brane paradigm is the solution to the absence of lepton number violating charged lepton decays, such models predict, in the absence of flavour symmetries, that charged lepton flavour violation will be observed in the next round of rare muon/tau decay experiments.Comment: 33 pages, 9 eps figure

Establishing a nu_{mu,tau} Component in the Solar Neutrino Flux

We point out that the recoil electron kinetic energy spectra in the nu-e elastic scattering are different for incident nu_{e} or nu_{mu,tau}, and hence one can in principle establish the existence of the nu_{mu,tau} component in the solar neutrino flux by fitting the shape of the spectrum. This would be a new model-independent test of the solar neutrino oscillation in a single experiment, free from astrophysical and nuclear physics uncertainties. For the ^7Be neutrinos, it is possible to determine the nu_{mu,tau} component at BOREXINO or KamLAND, if the background is sufficiently low. Note that this effect is different from the distortion in the incident neutrino energy spectrum, which has been discussed in the literature.Comment: 12 pages, 3 figures, uses psfig. Figures reorganized, one corrected, conclusions unchange

Leptogenesis from N~\widetilde{N}-dominated early universe

We investigate in detail the leptogenesis by the decay of coherent right-handed sneutrino $\widetilde{N}$ having dominated the energy density of the early universe, which was originally proposed by HM and TY. Once the $\widetilde{N}$ dominant universe is realized, the amount of the generated lepton asymmetry (and hence baryon asymmetry) is determined only by the properties of the right-handed neutrino, regardless of the history before it dominates the universe. Moreover, thanks to the entropy production by the decay of the right-handed sneutrino, thermally produced relics are sufficiently diluted. In particular, the cosmological gravitino problem can be avoided even when the reheating temperature of the inflation is higher than 10^{10}\GeV, in a wide range of the gravitino mass m_{3/2}\simeq 10\MeV--100\TeV. If the gravitino mass is in the range m_{3/2}\simeq 10\MeV--1\GeV as in the some gauge-mediated supersymmetry breaking models, the dark matter in our universe can be dominantly composed of the gravitino. Quantum fluctuation of the $\widetilde{N}$ during inflation causes an isocurvature fluctuation which may be detectable in the future.Comment: 16 page

How precisely can we reduce the three-flavor neutrino oscillation to the two-flavor one only from (\delta m^2_{12})/(\delta m^2_{13}) <~ 1/15 ?

We derive the reduction formula, which expresses the survival rate for the three-flavor neutrino oscillation by the two-flavor one, to the next-to-leading order in case there is one resonance due to the matter effect. We numerically find that the next-to-leading reduction formula is extremely accurate and the improvement is relevant for the precision test of solar neutrino oscillation and the indirect measurment of CP violation in the leptonic sector. We also derive the reduction formula, which is slightly different from that previously obtained, in case there are two resonances. We numerically verify that this reduction formula is quite accurate and is valid for wider parameter region than the previously obtained ones are.Comment: 28pages, 8figures, revtex4. to appear in PR

The minimal 3+2 neutrino model versus oscillation anomalies

We study the constraints imposed by neutrino oscillation experiments on the minimal extension of the Standard Model that can explain neutrino masses, which requires the addition of just two singlet Weyl fermions. The most general renormalizable couplings of this model imply generically four massive neutrino mass eigenstates while one remains massless: it is therefore a minimal 3+2 model. The possibility to account for the confirmed solar, atmospheric and long-baseline oscillations, together with the LSND/MiniBooNE and reactor anomalies is addressed. We find that the minimal model can fit oscillation data including the anomalies better than the standard 3ν model and similarly to the 3+2 phenomenological models, even though the number of free parameters is much smaller than in the latter. Accounting for the anomalies in the minimal model favours a normal hierarchy of the light states and requires a large reactor angle, in agreement with recent measurements. Our analysis of the model employs a new parametrization of seesaw models that extends the Casas-Ibarra one to regimes where higher order corrections in the light-heavy mixings are significant

Can three-flavor oscillations solve the solar neutrino problem?

The most promising solution to the solar neutrino problem are neutrino oscillations, which usually are analyzed within the reduced 2-flavor scheme, because the solutions found therein reasonably well reproduce the recent data of Super-Kamiokande about the recoil-electron energy spectrum, zenith-angle and seasonal variations, and the event rate data of all the neutrino detectors. In this work, however, a survey of the complete parameter space of 3-flavor oscillations is performed. Basically eight new additional solutions could be identified, where the best one with \Delta m(12)^2=2.7x10^(-10) eV^2, \Delta m(13)^2=1.0x10^(-5) eV^2, \Theta(12)=23, and \Theta(13)=1.3 (denoted SVO) is slightly more probable than any 2-flavor solution. However, including the results of the atmospheric neutrino problem excludes all 3-flavour solutions apart from the SLMA-solution (\Delta m(12)^2=7.9x10^(-6) eV^2, \Delta m(13)^2=2.5x10^(-4) eV^2, \Theta(12)=1.4, and \Theta(13)=20). Besides, the ability of SNO and Borexino to discriminate the various 2- and 3-flavor solutions is investigated. Only with very good statistics in these experiments the correct solution to the solar neutrino problem can be identified unambiguously.Comment: 22 pages, 19 figures, REVTeX, submitted to Phys.Rev.D, article with better resolved figures available under http://www.mpa-garching.mpg.de/~schlattl/public.htm