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S. M. Barr

Publications and source records attributed to S. M. Barr.

At least 55 records · Page 3Linked to original sources
arXiv2002

Family Unification in Five and Six Dimensions

In family unification models, all three families of quarks and leptons are grouped together into an irreducible representation of a simple gauge group, thus unifying the Standard Model gauge symmetries and a gauged family symmetry. Large orthogonal groups, and the exceptional groups $E_7$ and $E_8$ have been much studied for family unification. The main theoretical difficulty of family unification is the existence of mirror families at the weak scale. It is shown here that family unification without mirror families can be realized in simple five-dimensional and six-dimensional orbifold models similar to those recently proposed for SU(5) and SO(10) grand unification. It is noted that a family unification group that survived to near the weak scale and whose coupling extrapolated to high scales unified with those of the Standard model would be evidence accessible in principle at low energy of the existence of small (Planckian or GUT-scale) extra dimensions.

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arXiv2002

Flavon exchange effects in models with abelian flavor symmetry

In models with abelian flavor symmetry the small mixing angles and mass ratios of quarks and leptons are typically given by powers of small parameters characterizing the spontaneous breaking of flavor symmetry by "flavon" fields. If the scale of the breaking of flavor symmetry is near the weak scale, flavon exchange can lead to interesting flavor-violating and CP violating effects. These are studied. It is found that d_e, mu -> e + gamma, and mu-e conversion on nuclei can be near present limits. For significant range of parameters mu-e conversion can be the most sensitive way to look for such effects.

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arXiv2002

Eliminating the d=5 proton decay operators from SUSY GUTs

A general analysis is made of the question whether the d=5 proton decay operators coming from exchange of colored Higgsinos can be completely eliminated in a natural way in supersymmetric grand unified models. It is shown that they can indeed be in SO(10) while at the same time naturally solving the doublet-triplet splitting problem, having only two light Higgs doublets, and using no more than a single adjoint Higgs field. Accomplishing all of this requires that the vacuum expectation value of the adjoint Higgs field be proportional to the generator I_{3R} rather than to B-L, as is usually assumed. It is shown that such models can give realistic quark and lepton masses. We also point out a new mechanism for solving the μproblem in the context of SO(10) SUSY GUTs.

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arXiv2001

Bimaximal Neutrino Mixings from Lopsided Mass Matrices

Current solar and atmospheric neutrino oscillation data seem to favor a bimaximal pattern for neutrino mixings where the matrix elements U_{e2} and U_{μ3} are of order one, while U_{e3} is much smaller. We show that such a pattern can be obtained quite easily in theories with ``lopsided'' mass matrices for the charged leptons and the down type quarks. A relation connecting the solar and atmospheric neutrino mixing angles is derived, \tan^2θ_{atm} \simeq 1+ \tan^2θ_{sol}, which predicts \sin^2 2θ_{atm} \simeq 0.97 corresponding to the best fit LMA solution for solar neutrinos. Predictive schemes in SO(10) realizing these ideas are presented. A new class of SO(10) models with lopsided mass matrices is found which makes use of an adjoint VEV along the I_{3R} direction, rather than the traditional B-L direction.

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arXiv2001

Obtaining the large angle MSW solution to the solar neutrino problem in models

The large mixing angle (LMA) MSW solution to the solar neutrino problem seems favored by the data at the moment over the small mixing angle (SMA) MSW solution and the vacuum (VAC) solution. In this paper the various main types of models of neutrino masses and mixings are studied from the point of view of how naturally they can give the LMA solution. Special attention is given to a very simple type of "lopsided" SU(5) model.

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arXiv2001

The Cosmological Constant, False Vacua, and Axions

It is suggested that the true ground state of the world has exactly vanishing vacuum energy and that the cosmological constant that seems to have been observed is due to our region of the universe being stuck in a false vacuum, whose energy is split from the true vacuum by non-renormalizable operators that are suppressed by powers of the Planck scale. It is shown that conventional invisible axion models typically have the features needed to realize this possibility. In invisible axion models the same field and the same potential can explain both the cosmological constant (or dark energy) and the dark matter. It is also shown that the idea can be realized in non-axion models, an example of which is given having $Λ= M_W^7/M_{Pl}^3$, which accords well with the observed value.

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arXiv2001

Flavor without Flavor Symmetry

Non-trivial patterns of quark and lepton masses and mixings can arise without there being any underlying flavor symmetry that distinguishes among the three families. Two realistic examples are given.

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arXiv2001

Realization of the Large Mixing Angle Solar Neutrino Solution in an SO(10) Supersymmetric Grand Unified Model

An SO(10) supersymmetric grand unified model proposed earlier leading to the solar solution involving ``just-so'' vacuum oscillations is reexamined to study its ability to obtain the other possible solar solutions. It is found that all four viable solar neutrino oscillation solutions can be achieved in the model simply by modification of the right-handed Majorana neutrino mass matrix, M_R. Whereas the small mixing and vacuum solutions are easily obtained with several texture zeros in M_R, the currently-favored large mixing angle solution requires a nearly geometric hierarchical form for M_R that leads by the seesaw formula to a light neutrino mass matrix which has two or three texture zeros. The form of the matrix which provides the ``fine-tuning'' necessary to achieve the large mixing angle solution can be understood in terms of Froggatt-Nielsen diagrams for the Dirac and right-handed Majorana neutrino mass matrices. The solution fulfils several leptogenesis requirements which in turn can be responsible for the baryon asymmetry in the universe.

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arXiv2001

A General Classification of Three-Neutrino Models and U_e3

A classification of models with three light neutrinos is given. This classification includes virtually all of the three-neutrino models proposed in the last few years, of which there are approximately one hundred. The essential ideas, attractive features, and characteristic problems of the different classes of model are discussed. The classification is based principally on how the large ν_μ - ν_τ mixing is obtained. A general discussion of the mixing parameter U_{e3} is then given, showing what values are to be expected for it in each type of model.

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arXiv2000

The scaling of lepton dipole moments with lepton mass

The dipole moments of the leptons and quarks are matrices in flavor space, which can potentially reveal as much about the flavor structure of the theory as do the mass matrices. The off-diagonal elements of the dipole matrices lead to flavor-changing decays such as μ-> e γ, while the imaginary parts of the diagonal elements give rise to electric dipole moments. We analyze the scaling of the leptonic dipole moments with the lepton masses in theories beyond the standard model. While in many models the dipole moments scale roughly as lepton mass, it is shown that simple models exist in which the dipoles scale as the cube of the mass or in other ways. An explicit example with cubic scaling is presented, which is motivated on independent grounds from large angle neutrino oscillation data. Our results have great significance for the observability of the electric dipole moments of e, μand τ, and the rare decays μ-> e γ, and τ-> μγand will be tested in several forthcoming experiments.

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arXiv2000

Explicit SO(10) Supersymmetric Grand Unified Model

A complete set of Higgs and matter superfields is introduced with well-defined SO(10) properties and U(1) \times Z_2 \times Z_2 family charges from which the Higgs and Yukawa superpotentials are constructed. The Higgs fields solve the doublet-triplet splitting problem, while the structures of the four Dirac fermion mass matrices obtained involve just six effective Yukawa operators. The right-handed Majorana matrix, M_R, arises from one Higgs field coupling to several pairs of superheavy conjugate neutrino singlets. In terms of 10 input parameters to the mass matrices, the model accurately yields the 20 masses and mixings of the lightest quarks and leptons, as well as the masses of the 3 heavy right-handed neutrinos. The bimaximal atmospheric and solar neutrino vacuum solutions are favored in this simplest version with a moderate hierarchy in M_R. The large mixing angle MSW solution is obtainable, on the other hand, with a considerably larger hierarchy in M_R which is also necessary to obtain baryogenesis through the leptogenesis mechanism.

hep-ph↗
arXiv2000

Explicit SO(10) Supersymmetric Grand Unified Model for the Higgs and Yukawa Sectors

A complete set of fermion and Higgs superfields is introduced with well-defined SO(10) properties and U(1) x Z_2 x Z_2 family charges from which the Higgs and Yukawa superpotentials are constructed. The structures derived for the four Dirac fermion and right-handed Majorana neutrino mass matrices coincide with those previously obtained from an effective operator approach. Ten mass matrix input parameters accurately yield the twenty masses and mixings of the quarks and leptons with the bimaximal atmospheric and solar neutrino vacuum solutions favored in this simplest version.

hep-ph↗
arXiv2000

Construction of a Minimal Higgs SO(10) SUSY GUT Model

A full account is given of the procedure used by the authors to construct an SO(10) supersymmetric grand unified model of the fermion mass matrices. Various features of the model which gives remarkably accurate results for the quark and lepton masses and mixings were presented earlier in separate publications. The construction of the matrices is first discussed in the framework of effective operators, from which one naturally obtains the maximal ν_μ- ν_τmixing, while the small angle or maximal mixing solutions for the solar neutrinos depend upon the nature of the Majorana matrix. A set of Higgs and fermion superfields is then introduced from which the Higgs and Yukawa superpotentials uniquely give the structure of the mass matrices previously obtained. The right-handed Majorana matrix arises from one Higgs field coupling to several pairs of superheavy conjugate neutrino singlets. For the simple version considered, 10 input parameters accurately yield the 20 masses and mixings of the quarks and leptons, and the 3 masses of the right-handed neutrinos.

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arXiv2000

A mass relation for neutrinos

A generalization of the well-known Georgi-Jarlskog relation m_μ/m_τ = 3 (m_s/m_b) to neutrinos is found in the context of SO(10). This new relation is m_{ν_μ}/m_{ν_τ} = 16 (m_c/m_t), which is consistent with present data, assuming the small-angle MSW solution to the solar neutrino problem.

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arXiv2000

SO(10) and Large nu_mu - nu_tau Mixing

A general approach to understanding the large mixing seen in atmospheric neutrinos is explained, as well as a highly predictive SO(10) model which implements this approach. It is also seen how bimaximal mixing naturally arises in this scheme. (Talk presented at NNN99, SUNY Stony Brook, Sept. 22-26, 1999)

hep-ph↗
arXiv1999

Examining the cancellation mechanism of neutron EDM in a model with dilaton-dominated susy breaking

We examine the cancellation mechanism between the different contributions to the electric dipole moment of the neutron in a model with dilaton-dominated SUSY breaking. We find these accidental cancellations occur at few points in parameter space. For a wide region of this space we must constrain the phase of $μ$ to be of order $10^{-1}$ and have the phases of $A$ and $μ$ strongly correlated in order to have small neutron EDM. Moreover, we consider the indirect CP violation parameter $ε$ in this region where the electric dipole moment is less than the experimental limit and find that we can generate $ε$ of order $10^{-6}$.

hep-ph↗
arXiv1999

Bimaximal Mixing in an SO(10) Minimal Higgs Model

An SO(10) SUSY GUT model was previously presented based on a minimal set of Higgs fields. The quark and lepton mass matrices derived fitted the data extremely well and led to large mixing of muon- and tau-neutrinos in agreement with the atmospheric neutrino data and to the small-angle MSW solution for the solar neutrinos. Here we show how a slight modification leading to a non-zero up quark mass can result in bimaximal mixing for the atmospheric and solar neutrinos. The "just-so" vacuum solution is slightly favored over the large-angle MSW solution on the basis of the hierarchy required for the right-handed Majorana matrix and the more nearly-maximal mixing angles obtained.

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