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S. M. Barr

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

At least 37 records · Page 2Linked to original sources
arXiv2007

Radiative Fermion Mass Hierarchy in a Non-supersymmetric Unified Theory

In non-supersymmetric grand unified models a ``radiative fermion mass hierarchy" can be achieved in which the spectrum of quark and lepton masses is determined entirely by physics at the unification scale, with many relations following from the unified gauge symmetry, and with the masses of the lightest family arising from loops. A simple, realistic, and predictive model of this kind is presented. A "doubly lopsided" structure, known to lead to bilarge neutrino mixing, plays a crucial role in the radiative hierarchy.

hep-ph↗
arXiv2007

Anthropic tuning of the weak scale and of m_u/m_d in two-Higgs-doublet models

It is shown that in a model in which up-type and down-type fermions acquire mass from different Higgs doublets, the anthropic tuning of the Higgs mass parameters can explain the fact that the observed masses of the $d$ and $u$ quarks are nearly the same with $d$ slightly heavier. If Yukawa couplings are assumed not to "scan" (vary among domains), this would also help explain why the t quark is much heavier than the b quark. It is also pointed out that the existence of dark matter invalidates some earlier anthropic arguments against the viability of domains where the Standard Model Higgs has positive $μ^2$, but makes other even stronger arguments possible.

hep-ph↗
arXiv2006

Extra $Z$ bosons and low-energy tests of unification

If there is an extra U(1) gauge symmetry broken at low energies, then it may be possible from the charges of the known quarks and leptons under this U(1) to make inferences about how much gauge unification occurs at high scales and about the unification group. (For instance, there are certain observed properties of an extra $U(1)'$ that would be inconsistent with unification in four dimensions at high scales.) A general analysis is presented. Two criteria used in this analysis are (1) the degree to which the generator of the extra U(1) mixes with hypercharge, and (2) the ratio of the extra U(1) charge of the "10" and the "$\bar{5}$" of quarks and leptons.

hep-ph↗
arXiv2005

Flat-Directions in Grand Unification With $U(1)_R$ Symmetry

It is shown that in SO(10) and SU(5) models having a $U(1)_R$ symmetry, the requirement of breaking the unified group to the Standard Model leads to flat directions in the scalar potential. These can lead to a ``cosmological modulus problem". This is relevant to grand unified models of inflation, where $U(1)_R$ symmetries are often used to insure the flatness of the inflaton potential. A way that the modulus problem might be avoided is discussed.

hep-ph↗
arXiv2005

A Prediction from the Type III See-saw Mechanism

A simple ansatz that is well-motivated by group-theoretical considerations is proposed in the context of the type III neutrino see-saw mechanism. It results in predictions for m_s/m_b and m_b/m_tau that relate these quantities to the masses and mixings of neutrinos.

hep-ph↗
arXiv2005

The Origin of a Peculiar Extra U(1)

The origin of a family-independent "extra U(1)", discovered by Barr, Bednarz, and Benesh and independently by Ma, and whose phenomenology has recently been studied by Ma and Roy, is discussed. Even though it satisfies anomaly constraints in a highly economical way, with just a single extra triplet of leptons per family, this extra U(1) cannot come from four-dimensional grand unification. However, it is shown here that it can come from a Pati-Salam scheme with an extra U(1), which explains the otherwise surprising cancellation of anomalies.

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arXiv2005

How a Non-hierarchical Neutrino Mass Matrix Can Arise

One puzzle of neutrino masses and mixings is that they do not exhibit the kind of strong "hierarchy" that is found for the quarks and charged leptons. Neutrino mass ratios and mixing angles are not small. A possible reason for this is proposed here. It is based on the fact that typical realistic grand unified models contain particles with unification-scale masses which, when integrated out, can yield a neutrino mass matrix that is not of the standard seesaw form.

hep-ph↗
arXiv2004

Resonant leptogenesis in a predictive SO(10) grand unified model

An SO(10) grand unified model considered previously by the authors featuring lopsided down quark and charged lepton mass matrices is successfully predictive and requires that the lightest two right-handed Majorana neutrinons be nearly degenerate in order to obtain the LMA solar neutrino solution. Here we use this model to test its predictions for baryogenesis through resonant-enhanced leptogenesis. With the conventional type I seesaw mechanism, the best predictions for baryogenesis appear to fall a factor of three short of the observed value. However, with a proposed type III seesaw mechanism leading to three pairs of massive pseudo-Dirac neutrinos, resonant leptogenesis is decoupled from the neutrino mass and mixing issues with successful baryogenesis easily obtained.

hep-ph↗
arXiv2004

Time-varying Quark Masses and Cosmological Axion Energy

The possibility is examined that the masses of the light quarks $u$ and $d$ have varied over the course of the universe's evolution. Such a variation would have an effect on axion cosmology, and can be the basis of a solution of the so-called axion energy problem. If this is the case, a scalar force somewhat weaker than gravity could exist that has a range of order 1 km.

hep-ph↗
arXiv2004

A General Analysis of Corrections to the Standard See-saw Formula in Grand Unified Models

In realistic grand unified models there are typically extra vectorlike matter multiplets at the GUT scale that are needed to explain the family hierarchy. These contain neutrinos that, when integrated out, can modify the usual neutrino see-saw formula. A general analysis is given. It is noted that such modifications can explain why the neutrinos do not exhibit a strong family hierarchy like the other types of fermions.

hep-ph↗
arXiv2004

Leptogenesis in the type III seesaw mechanism

It is shown that the type III seesaw mechanism proposed recently can have certain advantages over the conventional (or type I) seesaw mechanism for leptogenesis. In particular a resonant enhancement of leptogenesis via heavy quasi-Dirac right-handed neutrino pairs can occur without a special flavor form or "texture" of the mass matrices being assumed. Some of the requirements for neutrino mixing and leptogenesis are effectively decoupled.

hep-ph↗
arXiv2003

A different see-saw formula for neutrino masses

In a wide class of unified models there is an additional (and possibly dominant) term in the neutrino mass formula that under the simplest assumption takes the form M_ν = (M_N + M_N^T)u/M_G, where M_N is the neutrino Dirac mass matrix, and u = O(M_W). This makes possible highly predictive models. A generalization of this form yields realistic neutrino masses and mixings more readily than the usual see-saw formula in some models.

hep-ph↗
arXiv2003

Explaining Why the u and d Quark Masses are Similar

An approach is suggested for modeling quark and lepton masses and mixing in the context of grand unified theories that explains the curious fact that m_u ~ m_d even though m_t >> m_b. The structure of the quark mass matrices is such as to allow a non-Peccei-Quinn solution of the Strong CP Problem.

hep-ph↗
arXiv2003

Atmospheric neutrino mixing and b-tau Unification

Extrapolating the tau and b masses in the MSSM tends to give for their ratio at the GUT scale a number around 1.2, for most viable values \tan β, rather than the minimal SU(5) prediction of 1. We suggest that this may be due to large off-diagonal elements in the charged lepton mass matrix M_L that can also explain the large atmospheric neutrino mixing angle. Several simple models with definite predictions for m_{tau}/m_b(M_{GUT}) are presented.

hep-ph↗
arXiv2002

Lifting a Realistic SO(10) Grand Unified Model to Five Dimensions

It has been shown recently that the problem of rapid proton decay induced by dimension five operators arising from the exchange of colored Higgsinos can be simply avoided in grand unified models where a fifth spatial dimension is compactified on an orbifold. Here we demonstrate that this idea can be used to solve the Higgsino-mediated proton decay problem in any realistic SO(10) model by lifting that model to five dimensions. A particular SO(10) model that has been proposed to explain the pattern of quark and lepton masses and mixings is used as an example. The idea is to break the SO(10) down to the Pati-Salam symmetry by the orbifold boundary conditions. The entire four-dimensional SO(10) model is placed on the physical SO(10) brane except for the gauge fields, the 45 and a single 10 of Higgs fields, which are placed in the five-dimensional bulk. The structure of the Higgs superpotential can be somewhat simplified in doing so, while the Yukawa superpotential and mass matrices derived from it remain essentially unaltered.

hep-ph↗
arXiv2002

Unifying flipped SU(5) in five dimensions

It is shown that embedding a four-dimensional flipped SU(5) model in a five-dimensional SO(10) model, preserves the best features of both flipped SU(5) and SO(10). The missing partner mechanism, which naturally achieves both doublet-triplet splitting and suppression of d=5 proton decay operators, is realized as in flipped SU(5), while the gauge couplings are unified as in SO(10). The masses of down quarks and charged leptons, which are independent in flipped SU(5), are related by the SO(10). Distinctive patterns of quark and lepton masses can result. The gaugino mass M_1 is independent of M_3 and M_2, which are predicted to be equal.

hep-ph↗
arXiv2002

Four Puzzles of Neutrino Mixing

There are four puzzling questions about by the magnitudes of neutrino mixings and mass splittings. A brief sketch is given of the various kinds of models of neutrino masses and how they answer these questions. Special attention is given to so-called "lopsided" models.

hep-ph↗