Gauge Unification and Quark Masses in a Pati-Salam Model from Branes

We investigate the phase space of parameters in the Pati-Salam model derived in the context of D-branes scenarios, requiring low energy string scale. We find that a non-supersymmetric version complies with a string scale as low as 10 TeV, while in the supersymmetric version the string scale raises up to ~2 x 10^7 TeV. The limited energy region for RGE running demands a large tan(beta) in order to have experimentally acceptable masses for the top and bottom quarks.Comment: 11 pages, LaTeX, 7 figures include

One-loop adjoint masses for branes at non-supersymmetric angles

This proceeding is based on arXiv:1105.0591 [hep-th] where we consider breaking of supersymmetry in intersecting D-brane configurations by slight deviation of the angles from their supersymmetric values. We compute the masses generated by radiative corrections for the adjoint scalars on the brane world-volumes. In the open string channel, the string two-point function receives contributions only from the infrared limits of N~4 and N~2 supersymmetric configurations, via messengers and their Kaluza-Klein excitations, and leads at leading order to tachyonic directions.Comment: 15 pages, 5 figures. Contribution to the proceedings of the Corfu Summer Institute 2011 School and Workshops on Elementary Particle Physics and Gravity, September 4-18 2011 Corfu, Greec

Exact Renormalization Group and Running Newtonian Coupling in Higher Derivative Gravity

We discuss exact renormalization group (RG) in $R^2$-gravity using effective average action formalism. The truncated evolution equation for such a theory on De Sitter background leads to the system of nonperturbative RG equations for cosmological and gravitational coupling constants. Approximate solution of these RG equations shows the appearence of antiscreening and screening behaviour of Newtonian coupling what depends on higher derivative coupling constants.Comment: Latex file, 9 page

Supersymmetry breaking on orbifolds from Wilson lines

We consider five dimensional theories compactified on the orbifold S^1/Z_2 and prove that spontaneous local supersymmetry breaking by Wilson lines and by the Scherk-Schwarz mechanism are equivalent. Wilson breaking is triggered by the SU(2)_R symmetry which is gauged in off-shell N=2 supergravity by auxiliary fields. The super-Higgs mechanism disposes of the would-be Goldstinos which are absorbed by the gravitinos to become massive. The breaking survives in the flat limit, where we decouple all gravitational interactions, and the theory becomes softly broken global supersymmetry.Comment: 9 pages, some comments in the discussion of the super-Higgs effect and some references adde

Quantum Diffeomorphisms and Conformal Symmetry

We analyze the constraints of general coordinate invariance for quantum theories possessing conformal symmetry in four dimensions. The character of these constraints simplifies enormously on the Einstein universe $R \times S^3$. The $SO(4,2)$ global conformal symmetry algebra of this space determines uniquely a finite shift in the Hamiltonian constraint from its classical value. In other words, the global Wheeler-De Witt equation is {\it modified} at the quantum level in a well-defined way in this case. We argue that the higher moments of $T^{00}$ should not be imposed on the physical states {\it a priori} either, but only the weaker condition $\langle \dot T^{00} \rangle = 0$. We present an explicit example of the quantization and diffeomorphism constraints on $R \times S^3$ for a free conformal scalar field.Comment: PlainTeX File, 37 page

Bulk Fermion Stars with New Dimensions

Many efforts have been devoted to the studies of the phenomenology in particle physics with extra dimensions. We propose degenerate fermion stars with extra dimensions and study what features characterized by the size of extra dimensions should appear in its structure. We find that Kaluza-Klein excited modes arise for the larger scale of extra dimensions and examine the conditions on which different layers should be caused in the inside of the stars. We expound how the extra dimensions affect on physical quantities.Comment: 20 pages, 14 figure

Kaluza-Klein States versus Winding States: Can Both Be Above the String Scale?

When closed strings propagate in extra compactified dimensions, a rich spectrum of Kaluza-Klein states and winding states emerges. Since the masses of Kaluza-Klein states and winding states play a reciprocal role, it is often believed that either the lightest Kaluza-Klein states or the lightest winding states must be at or below the string scale. In this paper, we demonstrate that this conclusion is no longer true for compactifications with non-trivial shape moduli. Specifically, we demonstrate that toroidal compactifications exist for which all Kaluza-Klein states as well as all winding states are heavier than the string scale. This observation could have important phenomenological implications for theories with reduced string scales, suggesting that it is possible to cross the string scale without detecting any states associated with spacetime compactification.Comment: 8 pages, LaTeX, no figure

Supersymmetry and Electroweak breaking from extra dimensions at the TeV-scale

We analyze some features of the role that extra dimensions, of radius $R$ in the TeV$^{-1}$ range, can play in the soft breaking of supersymmetry and the spontaneous breaking of electroweak symmetry. We use a minimal model where the gauge and Higgs sector of the MSSM are living in the bulk of five dimensions and the chiral multiplets in a four-dimensional boundary. Supersymmetry is broken in the bulk by the Scherk-Schwarz mechanism and transmitted to the boundary by radiative corrections. The particle spectrum is completely predicted as a function of a unique $R$-charge. The massless sector corresponds to the pure Standard Model and electroweak symmetry is radiatively broken with a light Higgs weighing \simlt 110 GeV. The $\mu$-problem is solved and Higgsinos, gauginos and heavy Higgses acquire masses $\sim 1/R$. Chiral sfermions acquire radiative squared-masses $\sim \alpha_i/R^2$. The effective potential is explicitly computed in the bulk of extra dimensions and some cosmological consequences can be immediately drawn from it. Gauge coupling running and unification is studied in the presence of Scherk-Schwarz supersymmetry breaking. The unification is similar to that in the supersymmetric theory.Comment: 27 pages, Latex, 7 figures. Minor change

Number of Generations in Free Fermionic String Models,

In string theory there seems to be an intimate connection between spacetime and world-sheet physics. Following this line of thought we investigate the family problem in a particular class of string solutions, namely the free fermionic string models. We find that the number of generations $N_g$ is related to the index of the supersymmetry generator of the underlying $N=2$ internal superconformal field theory which is always present in any $N=1$ spacetime supersymmetric string vacuum. We also derive a formula for the index and thus for the number of generations which is sensitive to the boundary condition assignments of the internal fermions and to certain coefficients which determine the weight with which each spin-structure of the model contributes to the one-loop partition function. Finally we apply our formula to several realistic string models in order to derive $N_g$ and we verify our results by constructing explicitly the massless spectrum of these string models.Comment: 17 pages, Plain Tex, no figures