Limits on the size of extra dimensions

We give a brief summary of present bounds on the size of possible extra-dimensions from collider experiments.Comment: 13 pages, late

Large Dimensions and String Physics in Future Colliders

We review the status of low-scale string theories and large extra-dimensions. After an overview on different string realizations, we discuss some of the main important problems and we summarize present bounds on the size of possible extra-dimensions from collider experiments.Comment: 58 pages, latex, 8 eps figure

Kaluza-Klein states at the LHC in models with localized fermions

We give a brief review of some aspects of physics with TeV size extra-dimensions. We focus on a minimal model with matter localized at the boundaries for the study of the production of Kaluza-Klein excitations of gauge bosons. We briefly discuss different ways to depart from this simple analysis.Comment: 6 pages, 2 figure

Dirac Gauginos in General Gauge Mediation

We extend the formulation by Meade, Seiberg and Shih of general gauge mediation of supersymmetry breaking to include Dirac masses for the gauginos. These appear through mixing of the visible sector gauginos with additional states in adjoint representations. We illustrate the method by reproducing the existing results in the literature for the gaugino and sfermion masses when preserving R-symmetry. We then explain how the generation of same sign masses for the two propagating degrees of freedom in the adjoint scalars can be achieved. We end by commenting on the use of the formalism for describing U(1) mixing.Comment: 22 pages, no figures. V2: minor corrections

A New Gauge Mediation Theory

We propose a class of models with gauge mediation of supersymmetry breaking, inspired by simple brane constructions, where R-symmetry is very weakly broken. The gauge sector has an extended N=2 supersymmetry and the two electroweak Higgses form an N=2 hypermultiplet, while quarks and leptons remain in N=1 chiral multiplets. Supersymmetry is broken via the D-term expectation value of a secluded U(1) and it is transmitted to the Standard Model via gauge interactions of messengers in N=2 hypermultiplets: gauginos thus receive Dirac masses. The model has several distinct experimental signatures with respect to ordinary models of gauge or gravity mediation realizations of the Minimal Supersymmetric Standard Model (MSSM). First, it predicts extra states as a third chargino that can be observed at collider experiments. Second, the absence of a D-flat direction in the Higgs sector implies a lightest Higgs behaving exactly as the Standard Model one and thus a reduction of the `little' fine-tuning in the low tan(beta) region. This breaking of supersymmetry can be easily implemented in string theory modelsComment: 23 pages, 3 figures, uses axodraw.sty. v2: a mistake in the radiative generation of the scalar masses is corrected. The main conclusions are unchange

Split extended supersymmetry from intersecting branes

We study string realizations of split extended supersymmetry, recently proposed in hep-ph/0507192. Supersymmetry is broken by small ($\epsilon$) deformations of intersection angles of $D$-branes giving tree-level masses of order $m_0^2\sim \epsilon M_s^2$, where $M_s$ is the string scale, to localized scalars. We show through an explicit one-loop string amplitude computation that gauginos acquire hierarchically smaller Dirac masses $m_{1/2}^D \sim m_0^2/M_s$. We also evaluate the one-loop Higgsino mass, $\mu$, and show that, in the absence of tree-level contributions, it behaves as $\mu\sim m_0^4/M_s^3$. Finally we discuss an alternative suppression of scales using large extra dimensions. The latter is illustrated, for the case where the gauge bosons appear in N=4 representations, by an explicit string model with Standard Model gauge group, three generations of quarks and leptons and gauge coupling unification.Comment: 32 pages, 3 figure

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

D-brane Models with Non-Linear Supersymmetry

We study a class of type I string models with supersymmetry broken on the world-volume of some D-branes and vanishing tree-level potential. Despite the non-supersymmetric spectrum, supersymmetry is non-linearly realized on these D-branes, while it is spontaneously broken in the bulk by Scherk-Schwarz boundary conditions. These models can easily accommodate 3-branes with interesting gauge groups and chiral fermions. We also study the effective field theory and in particular we compute the four-fermion couplings of the localized Goldstino with the matter fermions on the brane.We study a class of type I string models with supersymmetry broken on the world-volume of some D-branes and vanishing tree-level potential. Despite the non-supersymmetric spectrum, supersymmetry is non-linearly realized on these D-branes, while it is spontaneously broken in the bulk by Scherk-Schwarz boundary conditions. These models can easily accommodate 3-branes with interesting gauge groups and chiral fermions. We also study the effective field theory and in particular we compute the four-fermion couplings of the localized Goldstino with the matter fermions on the brane.We study a class of type I string models with supersymmetry broken on the world-volume of some D-branes and vanishing tree-level potential. Despite the non-supersymmetric spectrum, supersymmetry is non-linearly realized on these D-branes, while it is spontaneously broken in the bulk by Scherk–Schwarz boundary conditions. These models can easily accommodate 3-branes with interesting gauge groups and chiral fermions. We also study the effective field theory and in particular we compute the four-fermion couplings of the localized goldstino with the matter fermions on the brane

Contact interactions in D-brane models

We compute the tree-level four-point scattering amplitudes in string models where matter fields live on D-brane intersections. Extracting the contribution of massless modes, we are left with dimension-six four-fermion operators which in general receive contributions from three different sources: exchange of massive Kaluza--Klein excitations, winding modes and string oscillator states. We compute their coefficients and extract new bounds on the string scale in the brane-world scenario. This is contrasted with the situation where matter fields arise from open strings with both ends confined on the same collection of D-branes, in which case the exchange of massive string modes leads to dimension-eight operators that have been studied in the past. When matter fields live on brane intersections, the presence of dimension-six operators increases the lower bound on the string scale to 2--3 TeV, independently of the number of large extra dimensions.Comment: 27 pages, 1 eps figure, uses JHEP.cls. One reference added. Use of low energy precision electroweak data increased the bounds on the string scale to 2--3 Te

Two-loop RGEs with Dirac gaugino masses

The set of renormalisation group equations to two loop order for general supersymmetric theories broken by soft and supersoft operators is completed. As an example, the explicit expressions for the RGEs in a Dirac gaugino extension of the (N)MSSM are presented.Comment: 10 pages + 24 pages of RGEs in appendix; no figure