Stephan J. Huber

Fermion masses, mixings and proton decay in a Randall-Sundrum model

We consider a Randall–Sundrum model in which the Standard Model fermions and gauge bosons correspond to bulk fields. We show how the observed charged fermion masses and CKM mixings can be explained, without introducing hierarchical Yukawa couplings. We then study the impact on the mass scales associated with non-renormalizable operators responsible for proton decay, neutrino masses, and flavor changing neutral currents. Although mass scales as high as 1011–1012 GeV are in principle possible, dimensionless couplings of order 10−8 are still needed to adequately suppress proton decay. Large neutrino mixings seem to require new physics beyond the Standard Model.

Flavor violation and warped geometry

Extra dimensions have interesting consequences for flavor physics. We consider a setup where the standard model fermions and gauge fields reside in the bulk of a warped extra dimension. Fermion masses and mixings are explained by flavor dependent fermion locations, without relying on hierarchical Yukawa couplings. We discuss various flavor violating processes induced by (Kaluza–Klein) gauge boson exchange and non-renormalizable operators. Experimental constraints are satisfied with a Kaluza–Klein scale of about 10 TeV. Some processes, such as muon–electron conversion, are within reach of next generation experiments.

Baryogenesis in the two-Higgs doublet model

We consider the generation of the baryon asymmetry in the two-Higgs doublet model. Investigating the thermal potential in the presence of CP violation, as relevant for baryogenesis, we find a strong first-order phase transition if the extra Higgs states are heavier than about 300 GeV. The mass of the lightest Higgs can be as large as about 200 GeV. We compute the bubble wall properties, including the profile of the relative complex phase between the two Higgs vevs. The baryon asymmetry is generated by top transport, which we treat in the WKB approximation. We find a baryon asymmetry consistent with observations. The neutron electric dipole moment is predicted to be larger than about 10(-27) ecm and can reach the current experimental bound. Low values of tan beta are favored.

Electroweak baryogenesis: Concrete in a SUSY model with a gauge singlet

SUSY models with a gauge singlet easily allow for a strong first order electroweak phase transition (EWPT) if the vevs of the singlet and Higgs fields are of comparable size. We discuss the profile of the stationary expanding bubble wall and CP-violation in the effective potential, in particular transitional CP-violation inside the bubble wall during the EWPT. The dispersion relations for charginos contain CP-violating terms in the WKB approximation. These enter as source terms in the Boltzmann equations for the (particle-antiparticle) chemical potentials and fuel the creation of a baryon asymmetry through the weak sphaleron in the hot phase. This is worked out for concrete parameters.

Higgs mechanism and bulk gauge boson masses in the Randall-Sundrum model

Assuming the breaking of gauge symmetries by the Higgs mechanism, we consider the associated bulk gauge boson masses in the Randall-Sundrum background. With the Higgs field confined on the TeV-brane, the W and Z boson masses are naturally an order of magnitude smaller than their Kaluza-Klein excitation masses. The electroweak precision data require the lowest excited state to lie above about 30 TeV, with fermions on the TeV-brane. This bound is reduced to about 10 TeV if the fermions reside sufficiently close to the Planck-brane. Thus, some tuning of parameters is needed. We also discuss the bulk Higgs case, where the bounds are an order of magnitude smaller.

Electroweak Phase Transition and Baryogenesis in the nMSSM

We analyze the nMSSM with CP violation in the singlet sector. We study the static and dynamical properties of the electroweak phase transition. We conclude that electroweak baryogenesis in this model is generic in the sense that if the present limits on the mass spectrum are applied, no severe additional tuning is required to obtain a strong first-order phase transition and to generate a sufficient baryon asymmetry. For this we determine the shape of the nucleating bubbles, including the profiles of CP-violating phases. The baryon asymmetry is calculated using the advanced transport theory to first and second order in gradient expansion presented recently. Still, first and second generation sfermions must be heavy to avoid large electric dipole moments.

The Baryon asymmetry in the Standard Model with a low cut-off

We study the generation of the baryon asymmetry in a variant of the standard model, where the Higgs field is stabilized by a dimension-six interaction. Analyzing the one-loop potential, we find a strong first order electroweak phase transition for Higgs masses up to at least 170 GeV. Dimension-six operators induce also new sources of CP violation. We compute the baryon asymmetry in the WKB approximation. Novel source terms in the transport equations enhance the generated baryon asymmetry. For a wide range of parameters the model predicts a baryon asymmetry close to the observed value.

Neutrino oscillations and rare processes in models with a small extra dimension

Abstract We discuss Dirac neutrino masses and mixings in a scenario where both the standard model fermions and right-handed neutrinos are bulk fields in a non-factorizable geometry in five dimensions. We show how the atmospheric and solar neutrino anomalies can be satisfactorily resolved, and in particular how bimaximal mixing is realized. We also consider rare processes such as neutron–antineutron oscillations and μ → e + γ , which may occur at an observable rate.

Kaluza-Klein excitations of W and Z at the LHC?

Abstract Deviations from standard electroweak physics arise in the framework of a Randall–Sundrum model, with matter and gauge fields in the bulk and the Higgs field localized on the TeV brane. We focus in particular on modifications associated with the weak mixing angle. Comparison with the electroweak precision data yields a rather stringent lower bound of about 10 TeV on the masses of the lowest Kaluza–Klein excitation of the W- and Z-bosons. With some optimistic assumptions the bound could be lowered to about 7 TeV.

Production of Δ- and N∗-resonances in the one-boson exchange model☆

Abstract Motivated by the renewed interest on the creation of nuclear resonances in the studies of relativistic heavy-ion collisions we calculate the total and differential cross sections of the reactions N + N → Δ + N, N + N → Δ + Δ and N + N → N ∗ + N and the subsequent decay of the resonances into pions and nucleons employing the one-boson exchange model. The parameters of the model are adjusted to the elastic nucleonnucleon scattering. As far as data are available, the calculations agree well with experiment and hence the direct pion production seems to play an essential role very close to the threshold only. Finally, we present a simple parametrization of the theoretical results of the total as well as of the differential cross section.