Theodore N. Tomaras

A D-brane alternative to unification

We propose a minimal embedding of the Standard Model spectrum in a D-brane configuration of type I string theory. The SU(3) color and SU(2) weak interactions arise from two different collections of branes. The model is neither grand unified nor supersymmetric but it naturally leads to the right prediction of the weak angle for a string scale of the order of a few TeV. It requires two Higgs doublets and guarantees proton stability.

Dynamics of magnetic vortices

Abstract The canonical conservation laws of linear and angular momentum in the ferromagnetic continuum have been known to be plagued by certain ambiguities which are resolved in this paper by constructing conservation laws as suitable moments of a topological density. The resulting canonical structure is then shown to be analogous to that encountered in the familiar Hall effect and explains the unusual features of the dynamics of magnetic vortices without resorting to a detailed solution of the underlying nonlinear equations. Thus, in the absence of external magnetic fields, a magnetic vortex is shown to be spontaneously pinned around a fixed guiding center. The guiding center would drift in a direction perpendicular to an applied magnetic field gradient, provided that dissipation can be neglected, with a Hall velocity that is calculated explicitly in terms of the initial configuration of the vortex. In the presence of dissipation, the vortex undergoes skew deflection at an angle δ ≠ 90° with respect to the applied field gradient. The angle δ is related to the winding number of the vortex according to the well-known golden rule of bubble dynamics.

D-branes and the Standard Model

We perform a systematic study of the Standard Model embedding in a D-brane configuration of type I string theory at the TeV scale. We end up with an attractive model and we study several phenomenological questions, such as gauge coupling unification, proton stability, fermion masses and neutrino oscillations. At the string scale, the gauge group is U(3)color×U(2)weak×U(1)1×U(1)bulk. The corresponding gauge bosons are localized on three collections of branes; two of them describe the strong and weak interactions, while the last Abelian factor lives on a brane which is extended in two large extra dimensions with a size of a few microns. The hypercharge is a linear combination of the first three U(1)s. All remaining U(1)s get masses at the TeV scale due to anomalies, leaving the baryon and lepton numbers as (perturbatively) unbroken global symmetries at low energies. The conservation of baryon number assures proton stability, while lepton number symmetry guarantees light neutrino masses that involve a right-handed neutrino in the bulk. The model predicts the value of the weak angle which is compatible with the experiment when the string scale is in the TeV region. It also contains two Higgs doublets that provide tree-level masses to all fermions of the heaviest generation, with calculable Yukawa couplings; one obtains a naturally heavy top and the correct ratio mb/mτ. We also study neutrino masses and mixings in relation to recent solar and atmospheric neutrino data.

Induced Gravity on RS Branes

It is shown that a localized four-dimensional Einstein term, induced by quantum corrections, modifies significantly the law of gravity in a Randall-Sundrum brane world. In particular, the short-distance behavior of gravity changes from five- to four-dimensional, while, depending on the values of parameters, there can be an intermediate range where gravity behaves as in five dimensions. The spectrum of graviton fluctuations around the brane, their relative importance for the gravitational force, and the relevance of their emission in the bulk for the brane cosmology are analysed. Finally, constraints on parameters are derived from energy loss in astrophysical and particle physics processes.

D-brane standard model

The minimal embedding of the Standard Model in type I string theory is described. The SU(3) color and SU(2) weak interactions arise from two different collections of branes. The correct prediction of the weak angle is obtained for a string scale of 6-8 TeV. Two Higgs doublets are necessary and proton stability is guaranteed. It predicts two massive vector bosons with masses at the TeV scale, as well as a new superweak interaction.

Induced brane gravity: realizations and limitations

The induced 4d gravity on a brane world is analyzed. The case of a thick brane is considered and the unexpected appearance of a new threshold scale, much larger than the thickness scale is found. In cases of phenomenological interest, this new length scale turns out to be in the submillimeter range. The effect of R2 corrections, both in the bulk and on the brane, is also studied. It is shown that they introduce new threshold scales and may induce drastic modifications to the leading behavior. A concrete string/D-brane realization of the induced gravity scenario is also presented.

Brane-bulk energy exchange: a model with the present universe as a global attractor

The role of brane-bulk energy exchange and of an induced gravity term on a single braneworld of negative tension and vanishing effective cosmological constant is studied. It is shown that for the physically interesting cases of dust and radiation a unique global attractor which can realize our present universe (accelerating and 0 < Ωm0 < 1) exists for a wide range of the parameters of the model. For Ωm0 = 0.3, independently of the other parameters, the model predicts that the equation of state for the dark energy today is wDE,0 = −1.4, while Ωm0 = 0.03 leads to wDE,0 = −1.03. In addition, during its evolution, wDE crosses the wDE = −1 line to smaller values.

On n-point Amplitudes in N=4 SYM

The computation of n-point planar amplitudes in N=4 SYM at strong coupling is known to be reduced to the search for solutions of the integrable 2d SO(4,2) ?-model with growing asymptotics on the world-sheet and to the study of their Whitham deformations induced by an -regularization, which breaks both integrability and SO(4,2) symmetry. A multi-parameter (moduli) family of such solutions is constructed for n = 4. They all correspond to the same s and t and some are related by SO(4,2) transformations. Nevertheless, they lead to different regularized areas, whose minimum is the Alday-Maldacena solution. A brief review of results on n-point amplitudes is also provided, with special emphasis on the underlying equivalence of the above regularized minimal area in AdS and a double contour integral along the same boundary, two purely geometric quantities.

One-loop effective action around De Sitter space

Abstract The non-local one-loop contribution to the gravitational effective action around De Sitter space is computed using the background field method with pure trace external gravitational fields and it is shown to vanish. The calculation is performed in a generic covariant gauge and the result is verified to be gauge invariant.

Back reaction in light cone QED

We consider the back reaction of quantum electrodynamics upon an electric field E(x{sub +})=-A{sub -}{prime}(x{sub +}) which is parallel to x{sup 3} and depends only on the light cone coordinate x{sub +}=(x{sup 0}+x{sup 3})/2. Novel features are that the mode functions have simple expressions for arbitrary A{sub -}(x{sub +}) and that one cannot ignore the usual light cone ambiguity at zero + momentum. Each mode of definite canonical momentum k{sub +} experiences pair creation at the instant when its kinetic momentum p{sub +}=k{sub +}-eA{sub -}(x{sub +}) vanishes, at which point operators from the surface at x{sub -}=-{infinity} play a crucial role. Our formalism permits a more explicit and complete derivation of the rate of particle production than is usually given. We show that the system can be understood as the infinite boost limit of the analogous problem of an electric field which is homogeneous on surfaces of constant x{sup 0}.