Stephen M. Barr

Electroweak Fermion Number Violation and the Production of Stable Particles in the Early Universe

The effect of electroweak fermion number violation in the early universe is to distribute a fermion number excess over all the electroweak doublets. If baryogenesis is due to physics above the weak scale, this can result in the production of heavy stable particles in numbers comparable to the number of baryons. In particular, there may be enough stable technibaryons present today to account for the missing mass of the universe.

Natural Suppression of Higgsino-Mediated Proton Decay in Supersymmetric SO(10) ∗

In supersymmetric grand unified theories, proton decay mediated by the color-triplet Higgsino is generally problematic and requires some fine-tuning of parameters. We present a mechanism which naturally suppresses such dimension-5 operators in the context of SUSY SO(10). The mechanism, which implements natural doublet-triplet splitting using the adjoint Higgs boson, converts these dimension-5 operators effectively into dimension 6. By explicitly computing the Higgs spectrum and the resulting threshold uncertainties we show that the successful prediction of sin[sup 2][theta][sub [ital W]] is maintained [ital as] [ital a] [ital prediction] in this scheme. It is argued that only a weak suppression of the Higgsino-mediated proton decay is achievable within SUSY SU(5) without fine-tuning, in contrast to a strong suppression in SUSY SO(10).

The anthropic principle and the mass scale of the Standard Model

In theories in which different regions of the universe can have different values of the the physical parameters, we would naturally find ourselves in a region which has parameters favorable for life. We explore the range of anthropically allowed values of the mass parameter in the Higgs potential, µ 2 . For µ 2 0, baryon stability requires that |µ| << MP, the Planck Mass. Smaller values of � µ 2 � may or may not be allowed depending on issues of element synthesis and stellar evolution. We conclude that the observed value of µ 2 is reasonably typical of the anthropically allowed range, and that anthropic arguments provide a plausible explanation for the closeness of the QCD scale and the weak scale.

Minimality Condition and Atmospheric Neutrino Oscillations

A structure is proposed for the mass matrices of the quarks and leptons that arises in a natural way from the assumption that the breaking of SO(10) gauge symmetry is achieved by the smallest possible set of vacuum expectation values. This structure explains well many features of the observed spectrum of quarks and leptons. It reproduces the Georgi-Jarlskog mass relations and leads to a charm quark mass in reasonable agreement with data. It also predicts a large mixing angle between {nu}{sub {mu}} and {nu}{sub {tau}} , as suggested by atmospheric neutrino data. The mixing angles of the electron neutrino are predicted to be small. {copyright} {ital 1998} {ital The American Physical Society}

Large neutrino mixing angles in unified theories

Typically in unified theories the neutrino mixing angles, like the Cabibbo-Kobayashi-Maskawa (CKM) angles of the quarks, are related to the small mass ratios between fermions of different generations and are therefore quite small. A new approach for explaining the intergenerational mass hierarchies is proposed here which, while giving small CKM angles, naturally leads to neutrino angles of order unity. Such large mixing angles may be required for a resolution of the atmospheric neutrino anomaly and may also be relevant for the solar neutrino puzzle. The mechanism presented here provides a framework in which novel approaches to the fermion mass question can arise. In particular, within this framework a variant of the texture idea allows highly predictive models to be constructed, an illustrative example of which is given. It is shown how the neutrino mixing angles may be completely determined in such schemes.

Minimal SO(10) Unification

It is shown that the doublet-triplet splitting problem can be solved in SO(10) with only a single adjoint Higgs field (as seems to be required by superstring theory), while at the same time avoiding the existence of light fields that would destroy the unification of gauge couplings. {copyright} {ital 1997} {ital The American Physical Society}

A general classification of three-neutrino models and Ue3

Abstract 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 large ν μ − ν τ mixing is obtained. A general discussion of the mixing parameter U e 3 is then given, showing what values are to be expected for it in each type of model.

Upper bound on baryogenesis scale from neutrino masses

Abstract We examine the constraints on baryogenesis if anomalous weak baryon violation is in thermal equilibrium at high temperatures. If neutrinos have Majorana masses, there is an upper bound on the scale of baryogenesis: T0≲1012 GeV(1 eV/mν)2, where mν is the mass of the lightest neutrino, and no baryon number is generated at temperatures below T0.

Fermion masses in SO(10) with a single adjoint Higgs field

It has recently been shown how to break

Explicit SO(10) supersymmetric grand unified model for the higgs and yukawa sectors

\mathrm{SO}(10)