270 research outputs found
Rotating D-branes and O-planes
We review orientifold constructions in the presence of magnetic backgrounds
both in the open and closed sectors. Generically, the resulting orientifold
models have a nice geometric description in terms of rotated D-branes and/or
O-planes. In the case of multiple magnetic backgrounds, some amount of
supersymmetry is recovered if the magnetic fields are suitably chosen and part
of the original D-branes and/or O-planes are transmuted into new ones.Comment: Contribution to the Proceedings of the first SP2002 Conference and of
the 35 Internationsl Symposium Ahrenshoo
Stringy Instanton Effects in Models with Rigid Magnetised D-branes
We compute instantonic effects in globally consistent T^6/Z2xZ2 orientifold
models with discrete torsion and magnetised D-branes. We consider fractional
branes and instantons wrapping the same rigid cycles. We clarify and analyse in
detail the low-energy effective action on D-branes in these models. We provide
explicit examples where instantons induce linear terms in the charged fields,
or non-perturbative mass terms are generated. We also find examples where the
gauge theory on fractional branes has conformal symmetry at one-loop, broken by
instantonic mass terms at a hierarchically small energy scale.Comment: 60 pages. Refs added. Typos corrected in some eqs. Modified comments
in subsection 4.
Magnetized Type I Orbifolds in Four Dimensions
I review the basic features of four dimensional Z_2 x Z_2 (shift)
orientifolds with internal magnetic fields, describing two examples with N=1
supersymmetry. As in the corresponding six-dimensional examples, D9-branes
magnetized along four internal directions can mimic D5-branes, even in presence
of multiplets of image branes localized on different fixed tori. Chiral
low-energy spectra can be obtained if the model also contains D5-branes
parallel to the magnetized directions.Comment: 4 pages, LATEX; misprints correcte
Type I vacua with brane supersymmetry breaking
We show how chiral type I models whose tadpole conditions have no
supersymmetric solution can be consistently defined introducing antibranes with
non-supersymmetric world volumes. At tree level, the resulting stable non-BPS
configurations correspond to tachyon-free spectra, where supersymmetry is
broken at the string scale on some (anti)branes but is exact in the bulk, and
can be further deformed by the addition of brane-antibrane pairs of the same
type. As a result, a scalar potential is generated, that can stabilize some
radii of the compact space. This setting has the novel virtue of linking
supersymmetry breaking to the consistency requirements of an underlying
fundamental theory.Comment: 45 pages. Late
Non-perturbative transitions among intersecting-brane vacua
We investigate the transmutation of D-branes into Abelian magnetic
backgrounds on the world-volume of higher-dimensional branes, within the
framework of global models with compact internal dimensions. The phenomenon,
T-dual to brane recombination in the intersecting-brane picture, shares some
similarities to inverse small-instanton transitions in non-compact spaces,
though in this case the Abelian magnetic background is a consequence of the
compactness of the internal manifold, and is not ascribed to a zero-size
non-Abelian instanton growing to maximal size. We provide details of the
transition in various supersymmetric orientifolds and non-supersymmetric
tachyon-free vacua with Brane Supersymmetry Breaking, both from brane
recombination and from a field theory Higgs mechanism viewpoints.Comment: 52 pages, 2 figures. Typos correcte
Type-I strings on magnetised orbifolds and brane transmutation
In the presence of internal magnetic fields, a D9 brane can acquire a D5 (or
anti-D5) R-R charge, and can therefore contribute to the corresponding tadpole.
In the resulting vacua, supersymmetry is generically broken and tachyonic
instabilities are present. However, suitable choices for the magnetic fields,
corresponding to self-dual configurations in the internal space, can yield new
chiral supersymmetric vacua with gauge groups of reduced rank, where the
magnetic energy saturates, partly or fully, the negative tension of the O5+
planes. These models contain Green-Schwarz couplings to untwisted R-R forms not
present in conventional orientifolds.Comment: typos corrected, references added, to appear in Phys. Lett.
Non-Supersymmetric Type I Strings with Zero Vacuum Energy
We study open descendants of non-supersymmetric type IIB asymmetric (freely
acting) orbifolds with zero cosmological constant. A generic feature of these
models is that supersymmetry remains unbroken on the brane at all mass levels,
while it is broken in the bulk in a way that preserves Fermi-Bose degeneracy in
both the massless and massive (closed string) spectrum. This property remains
valid in the heterotic dual of the type II model but only for the massless
excitations. A possible application of these constructions concerns scenarios
of low-energy supersymmetry breaking with large dimensions.Comment: 22 pages, TeX, harvmac. Minor corrections. Final version to appear on
Nucl.Phys.
Contact interactions in low scale string models with intersecting -branes
We evaluate the tree level four fermion string amplitudes in the TeV string
mass scale models with intersecting -branes. The coefficient functions of
contact interactions subsuming the contributions of string Regge resonance and
winding mode excitations are obtained by subtracting out the contributions from
the string massless and massive momentum modes. Numerical applications are
developed for the Standard Model like solution of Cremades, Ibanez, and
Marchesano for a toroidal orientifold with four intersecting -brane stacks.
The chirality conserving contact interactions of the quarks and leptons are
considered in applications to high energy collider and flavor changing neutral
current phenomenology. The two main free parameters consist of the string and
compactification mass scales, and . Useful constraints on these
parameters are derived from predictions for the Bhabha scattering differential
cross section and for the observables associated to the mass shifts of the
neutral meson systems and the lepton number
violating three-body leptonic decays of the charged leptons and .Comment: 34 pages, 7 figure
Aspects of Type 0 String Theory
A construction of compact tachyon-free orientifolds of the non-supersymmetric
Type 0B string theory is presented. Moreover, we study effective
non-supersymmetric gauge theories arising on self-dual D3-branes in Type 0B
orbifolds and orientifolds.Comment: 9 pages, LATEX; submitted to Proceedings of Strings '9
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