246 research outputs found
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
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 ()
deformations of intersection angles of -branes giving tree-level masses of
order , where is the string scale, to localized
scalars. We show through an explicit one-loop string amplitude computation that
gauginos acquire hierarchically smaller Dirac masses . We also evaluate the one-loop Higgsino mass, , and show that,
in the absence of tree-level contributions, it behaves as . 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
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
Radiative Symmetry Breaking in Brane Models
We propose a way to generate the electroweak symmetry breaking radiatively in
non-supersymmetric type I models with string scale in the TeV region. By
identifying the Higgs field with a tree-level massless open string state, we
find that a negative squared mass term can be generated at one loop. It is
finite, computable and typically a loop factor smaller than the string scale,
that acts as an ultraviolet cutoff in the effective field theory. When the
Higgs open string has both ends confined on our world brane, its mass is
predicted to be around 120 GeV, i.e. that of the lightest Higgs in the minimal
supersymmetric model for large and . Moreover, the string
scale turns out to be one to two orders of magnitude higher than the weak
scale. We also discuss possible effects of higher order string threshold
corrections that might increase the string scale and the Higgs mass.Comment: 16 pages, Latex2e, 2 eps figures. Some comments on the large
(longitudinal) radius limit of the mass term at the origin are developed, and
new references adde
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