On Neutrino Masses and Family Replication

The old issue of why there are more than one family of quarks and leptons is reinvestigated with an eye towards the use of anomaly as a tool for constraining the number of families. It is found that, by assuming the existence of right-handed neutrinos (which would imply that neutrinos will have a mass) and a new chiral SU(2) gauge theory, strong constraints on the number of families can be obtained. In addition, a model, based on that extra SU(2), is constructed where it is natural to have one "very heavy" fourth neutrino and three almost degenerate light neutrinos whose masses are all of the Dirac type.Comment: RevTex, 12 pages with 1 figure, minor changes to the text and added acknowledgment

A new mechanism for a naturally small Dirac neutrino mass

A mechanism is proposed in which a right-handed neutrino zero mode and a right-handed charged lepton zero mode can be localized at the same place along an extra compact dimension while having markedly different spreads in their wave functions: a relatively narrow one for the neutrino and a rather broad one for the charged lepton. In their overlaps with the wave function for the left-handed zero modes, this mechanism could produce a natural large hierarchy in the effective Yukawa couplings in four dimensions, and hence a large disparity in masses.Comment: 6 pages (2 with figures), twocolumn forma

Neutrino Masses and Oscillations in Models with Large Extra Dimensions

We discuss the profile of neutrino masses and mixings in models with large extra dimensions when right handed neutrinos are present in the branes along with the usual standard model particles. In these models, string scale must be bigger than $10^{8}$ GeV to have desired properties for the neutrinos at low energies. The lightest neutrino mass is zero and there is oscillations to sterile neutrinos that are different from other models with the bulk neutrino.Comment: Minor changes. 9 pages, latex file, uses epsf style, two figures included. To appear at Phys. Lett.

Best Possible Results in a Class of Inequalities, II

AbstractWe give a sufficient condition on a lower triangular infinite matrix A with nonnegative entries, and a positive sequence b = (bn), for an inequality of the form ||A(b|x|)||p ≤ K||x||p, x ∈ ℓp, to be best possible, in the sense that there is no positive sequence d = (dn) such that (dnb−1n) is a monotone unbounded sequence, and an inequality of the form above holds with b replaced by d. This condition permits easy proofs of "best possible" theorems that generalize a previous result concerning Hardy′s inequality

The coupling constants for an electroweak model with a SU(4)PS⊗SU(4)EWSU(4)_{PS} \otimes SU(4)_{EW} unification symmetry

We introduce the sequence of spontaneous symmetry breaking of a coupling between Pati-Salam and electroweak symmetries $SU(4)_{PS} \otimes SU(4)_{EW}$ in order to establish a mathematically consistent relation among the coupling constants at grand unification energy scale. With the values of baryon minus lepton quantum numbers of known quarks and leptons, by including right-handed neutrinos, we can find the mixing angle relations at different energy levels up to the electromagnetic $U(1)_{EM}$ scale.Comment: 8 page

Neutrino Mass and New Physics

We review the present state of and future outlook for our understanding of neutrino masses and mixings. We discuss what we think are the most important perspectives on the plausible and natural scenarios for neutrinos and what may have the most promise to throw light on the flavor problem of quarks and leptons. We focus on the seesaw mechanism which fits into the big picture of particle physics such as supersymmetry and grand unification providing a unified approach to flavor problem of quarks and leptons. We argue that in combination with family symmetries, this may be at the heart of a unified understanding of flavor puzzle. We also discuss other new physics ideas such as neutrinos in models with extra dimensions and possible theoretical implications of sterile neutrinos. We outline some tests for the various schemes.Comment: 90 pages and 9 figures; With permission from the Annual Review of Nuclear and Particle Science. Final version of this material is scheduled to appear in the Annual Review of Nuclear and Particle Science Vol. 56, to be published in November 2006 by Annual Reviews (http://www.annualreviews.org); some references and parts of text update

Higgs Sector of the Minimal Left-Right Symmetric Model

We perform an exhaustive analysis of the most general Higgs sector of the minimal left-right symmetric model (MLRM). We find that the CP properties of the vacuum state are connected to the Higgs spectrum: if CP is broken spontaneously, the MLRM does not approach the Standard Model in the limit of a decoupling left-right symmetry breaking scale. Depending on the size of the CP phases scenarios with extra non-decoupling flavor-violating doublet Higgses or very light SU(2) triplet Higgses emerge, both of which are ruled out by phenomenology. For zero CP phases the non-standard Higgses decouple only if a very unnatural fine-tuning condition is fulfilled. We also discuss generalizations to a non-minimal Higgs sector.Comment: brief discussion of non-minimal Higgs sectors added, journal versio

Radiative magnification of neutrino mixings and a natural explanation of the neutrino anomalies

We show that the neutrino mixing pattern with the large mixing required for the atmospheric neutrino problem and the small mixing angle MSW solution for the solar neutrino problem can be naturally generated through radiative magnification, even though all the mixing angles at the seesaw scale may be small. This can account for the neutrino anomalies as well as the CHOOZ constraints in the context of quark-lepton unified theories, where the quark and lepton mixing angles are expected to be similar in magnitude at the high scale. We also indicate the 4$\nu$ mixing scenarios for which this mechanism of radiative magnification can provide a natural explanation.Comment: 14 pages RevTex, 2 eps figure

The Absorptive Extra Dimensions

It is well known that gravity and neutrino oscillation can be used to probe large extra dimensions in a braneworld scenario. We argue that neutrino oscillation remains a useful probe even when the extra dimensions are small, because the brane-bulk coupling is likely to be large. Neutrino oscillation in the presence of a strong brane-bulk coupling is vastly different from the usual case of a weak coupling. In particular, some active neutrinos could be absorbed by the bulk when they oscillate from one kind to another, a signature which can be taken as the presence of an extra dimension. In a very large class of models which we shall discuss, the amount of absorption for all neutrino oscillations is controlled by a single parameter, a property which distinguishes extra dimensions from other mechanisms for losing neutrino fluxes.Comment: Introduction enlarged; conclusions added. To appear in Phys. Rev.

Up-Down Unification, Neutrino Masses and Rare Lepton Decays

In a recent paper, we showed that tree level up-down unification of fermion Yukawa couplings is a natural consequence of a large class of supersymmetric models. They can lead to viable quark masses and mixings for moderately large values of $\tan\beta$ with interesting and testable predictions for CP violation in the hadronic sector. In this letter, we extend our discussion to the leptonic sector focusing on one particular class of these models, the supersymmetric left-right model with the seesaw mechanism for neutrino masses. We show that fitting the solar and the atmospheric neutrino data considerably restricts the Majorana-Yukawa couplings of the leptons in this model and leads to predictions for the decay $\tau\to \mu +\gamma$, which is found to be accessible to the next generation of rare decay searches. We also show that the resulting parameter space of the model is consistent with the requirements of generating adequate baryon asymmetry through lepton-number violating decays of the right-handed neutrino.Comment: 16 pages, latex, 6 figures, typos correcte