Electroweak symmetry breaking in TeV-scale string models

We propose a scenario of the electroweak symmetry breaking by one-loop radiative corrections in a class of string models with D3-branes at non-supersymmetric orbifold singularities with the string scale in TeV region. As a test example, we consider a simple model based on a D3-brane at locally C^3/Z_6 orbifold singularity, and the electroweak Higgs doublet fields are identified with the massless bosonic modes of the open string on that D3-brane. They have Yukawa couplings with three generations of left-handed quarks and right-handed up-type quarks which are identified with the massless fermionic modes of the open string on the D3-brane. We calculate the one-loop correction to the Higgs mass due to the non-supersymmetric string spectrum and interactions, and qualitatively suggest that the negative mass squared can be generated. The problems which must be solved to proceed quantitative calculations are pointed out.Comment: 9 pages, Talk given at 2nd Cairo International Conference on High Energy Physics (CICHEP II), Cairo, Egypt, 14-17 Jan 200

Brane SUSY Breaking and the Gravitino Mass

Supergravity models with spontaneously broken supersymmetry have been widely investigated over the years, together with some notable non-linear limits. Although in these models the gravitino becomes naturally massive absorbing the degrees of freedom of a Nambu-Goldstone fermion, there are cases in which the naive counting of degrees of freedom does not apply, in particular because of the absence of explicit gravitino mass terms in unitary gauge. The corresponding models require non-trivial de Sitter-like backgrounds, and it becomes of interest to clarify the fate of their Nambu-Goldstone modes. We elaborate on the fact that these non-trivial backgrounds can accommodate, consistently, gravitino fields carrying a number of degrees of freedom that is intermediate between those of massless and massive fields in a flat spacetime. For instance, in a simple supergravity model of this type with de Sitter background, the overall degrees of freedom of gravitino are as many as for a massive spin-3/2 field in flat spacetime, while the gravitino remains massless in the sense that it undergoes null-cone propagation in the stereographic picture. On the other hand, in the ten-dimensional USp(32) Type I Sugimoto model with "brane SUSY breaking", which requires a more complicated background, the degrees of freedom of gravitino are half as many of those of a massive one, and yet it somehow behaves again as a massless one.Comment: 18 page

Mass Generation Mechanism in Supersymmetric Composite Model with Three Generations

We propose a supersymmetric composite model with three generations in which supersymmetry and electroweak symmetry are broken dynamically, and masses of quarks and leptons are generated without introducing any mass scales by hand. All the mass scales in the model are expected to be generated dynamically. The mechanism to have mass hierarchy is explicitly described, although the roughly estimated mass spectrum of quarks and leptons does not exactly coincide with the realistic one.Comment: 15 pages, 2 figure

Radiative Electroweak Symmetry Breaking in TeV-Scale String Models

We examine the possibility of necessary and inevitable radiative electroweak symmetry breaking by one-loop radiative corrections in a class of string models which are the string realization of "brane world" scenario. As an example, we consider a simple quasi-realistic model based on a D3-brane at non-supersymmetric C^3/Z_6 orbifold singularity, in which the electroweak Higgs doublet fields are identified with the massless bosonic modes of the open string on that D3-brane. We calculate the one-loop correction to the Higgs potential, and find that its vacuum expectation value can be realized in this specific model.Comment: 6 pages. Contribution to the proceedings of SCGT06, Intnational Workshop on "Origin of Mass and Strong Coupling Gauge Theories" 21-24 November 2006, Nagoya, Japa