792 research outputs found
Arc stability and melting characteristics of welding wire for use with 2219 aluminum alloy plate
Arc stability and melting characteristics of welding wire for arc welding of 2219 aluminum alloy plat
Non-tachyonic Scherk-Schwarz compactifications, cosmology and moduli stabilization
It is well-known that Scherk-Schwarz compactifications in string theory have
a tachyon in the closed string spectrum appearing for a critical value of a
compact radius. The tachyon can be removed by an appropriate orientifold
projection in type II strings, giving rise to tachyon-free compactifications.
We present explicit examples of this type in various dimensions, including six
and four-dimensional chiral examples, with softly broken supersymmetry in the
closed sector and non-BPS configurations in the open sector. These vacua are
interesting frameworks for studying various cosmological issues. We discuss
four-dimensional cosmological solutions and moduli stabilization triggered by
nonperturbative effects like gaugino condensation on D-branes and fluxes.Comment: 36 pages, LaTeX; added reference
Triviality and the (Supersymmetric) See-Saw
For the D=5 Majorana neutrino mass operator to have a see-saw ultraviolet
completion that is viable up to the Planck scale, the see-saw scale is bounded
above due to triviality limits on the see-saw couplings. For supersymmetric
see-saw models, with realistic neutrino mass textures, we compare constraints
on the see-saw scale from triviality bounds, with those arising from
experimental limits on induced charged-lepton flavour violation, for both the
CMSSM and for models with split supersymmetry.Comment: 27 pages, 7 figures, references adde
Anomaly Cancellation and Conformality in Quiver Gauge Theories
Abelian quiver gauge theories provide nonsupersymmetric candidates for the
conformality approach to physics beyond the standard model. Written as , gauge theories, however, they have mixed and
triangle anomalies. It is shown how to construct explicitly
a compensatory term which restores gauge invariance of
under . It can
lead to a negative contribution to the U(1) -function and hence to
one-loop conformality at high energy for all dimensionless couplings.Comment: 15 pages. minor clarification
On cosmologically induced hierarchies in string theory
We propose, within a perturbative string theory example, a cosmological way
to generate a large hierarchy between the observed Planck mass and the
fundamental string scale. Time evolution results in three large space
dimensions, one additional dimension transverse to our world and five small
internal dimensions with a very slow time evolution. The evolution of the
string coupling and internal space generate a large Planck mass. However due to
an exact compensation between the time evolution of the internal space and that
of the string coupling, the gauge and Yukawa couplings on our Universe are time
independent.Comment: 12 pages, LaTeX, interpretation of the solution clarified, typos
corrected, references adde
Effective description of brane terms in extra dimensions
We study how theories defined in (extra-dimensional) spaces with localized
defects can be described perturbatively by effective field theories in which
the width of the defects vanishes. These effective theories must incorporate a
``classical'' renormalization, and we propose a renormalization prescription a
la dimensional regularization for codimension 1, which can be easily used in
phenomenological applications. As a check of the validity of this setting, we
compare some general predictions of the renormalized effective theory with
those obtained in a particular ultraviolet completion based on deconstruction.Comment: 28 page
Flavour in supersymmetry: horizontal symmetries or wave function renormalisation
We compare theoretical and experimental predictions of two main classes of
models addressing fermion mass hierarchies and flavour changing neutral
currents (FCNC) effects in supersymmetry: Froggatt-Nielsen (FN) U(1) gauged
flavour models and Nelson-Strassler/extra dimensional models with hierarchical
wave functions for the families. We show that whereas the two lead to identical
predictions in the fermion mass matrices, the second class generates a stronger
suppression of FCNC effects. We prove that, whereas at first sight the FN setup
is more constrained due to anomaly cancelation conditions, imposing unification
of gauge couplings in the second setup generates conditions which precisely
match the mixed anomaly constraints in the FN setup. Finally, we provide an
economical extra dimensional realisation of the hierarchical wave functions
scenario in which the leptonic FCNC can be efficiently suppressed due to the
strong coupling (CFT) origin of the electron mass.Comment: 23 page
Non anomalous U(1)_H gauge model of flavor
A non anomalous horizontal gauge symmetry can be responsible for the
fermion mass hierarchies of the minimal supersymmetric standard model. Imposing
the consistency conditions for the absence of gauge anomalies yields the
following results: i) unification of leptons and down-type quarks Yukawa
couplings is allowed at most for two generations. ii) The term is
necessarily somewhat below the supersymmetry breaking scale. iii) The
determinant of the quark mass matrix vanishes, and there is no strong
problem. iv) The superpotential has accidental and symmetries. The
prediction allows for an unambiguous test of the model at low
energy.Comment: 5 pages, RevTex. Title changed, minor modifications. Final version to
appear in Phys. Rev.
Unification, KK-thresholds and the top Yukawa coupling in F-theory GUTs
In a class of F-theory SU(5) GUTs the low energy chiral mass spectrum is
obtained from rank one fermion mass textures with a hierarchical structure
organised by U(1) symmetries embedded in the exceptional E_8 group. In these
theories chiral fields reside on matter `curves' and the tree level masses are
computed from integrals of overlapping wavefuctions of the particles at the
triple intersection points. This calculation requires knowledge of the exact
form of the wavefuctions. In this work we propose a way to obtain a reliable
estimate of the various quantities which determine the strength of the Yukawa
couplings. We use previous analysis of KK threshold effects to determine the
(ratios of) heavy mass scales of the theory which are involved in the
normalization of the wave functions. We consider similar effects from the
chiral spectrum of these models and discuss possible constraints on the
emerging matter content. In this approach, we find that the Yukawa couplings
can be determined solely from the U(1) charges of the states in the
`intersection' and the torsion which is a topological invariant quantity. We
apply the results to a viable SU(5) model with minimal spectrum which satisfies
all the constraints imposed by our analysis. We use renormalization group
analysis to estimate the top and bottom masses and find that they are in
agreement with the experimental values.Comment: 28 pages, 2 figure
Supersymmetric codimension-two branes and U(1)_R mediation in 6D gauged supergravity
We construct a consistent supersymmetric action for brane chiral and vector
multiplets in a six-dimensional chiral gauged supergravity. A nonzero brane
tension can be accommodated by allowing for a brane-localized Fayet-Iliopoulos
term proportional to the brane tension. When the brane chiral multiplet is
charged under the bulk U(1)_R, we obtain a nontrivial coupling to the extra
component of the U(1)_R gauge field strength as well as a singular scalar
self-interaction term. Dimensionally reducing to 4D on a football
supersymmetric solution, we discuss the implication of such interactions for
obtaining the U(1)_R D-term in the 4D effective supergravity. By assuming the
bulk gaugino condensates and nonzero brane F- and/or D-term for the uplifting
potential, we have all the moduli stabilized with a vanishing cosmological
constant. The brane scalar with nonzero R charge then gets a soft mass of order
the gravitino mass. The overall sign of the soft mass squared depends on the
sign of the R charge as well as whether the brane F- or D-term dominates.Comment: 28 pages, no figures, version to appear in JHE
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