636 research outputs found
A comment on discrete Kalb-Ramond field on orientifold and rank reduction
We show that the rank reduction of the gauge group on orientifolds in
presence of non vanishing discrete Kalb-Ramond field can be explained by the
presence of an induced field strength in a non trivial bundle on the branes.
This field strength is also necessary for the tadpole cancellation and the
number of branes is left unchanged by the presence of the discrete Kalb-Ramond
background.Comment: v2: added references, improved introduction and corrected misprints;
15 page
Non-tachyonic open descendants of the 0B string theory
We use the crosscap constraint to construct open descendants of the 0B string
compactified on and on free of tachyons both in the closed
and in the open unoriented sectors. In four dimensions the construction results
in a Chan-Paton gauge group with three
generations of chiral fermions in the representations
.Comment: 13 pages, Latex. Minor corrections. Final version to appear on
Phys.Lett.
Vanishing Perturbative Vacuum Energy in Non-Supersymmetric Orientifolds
We present a novel source for supersymmetry breaking in orientifold models,
and show that it gives a vanishing contribution to the vacuum energy at genus
zero and three-half. We also argue that all the corresponding perturbative
contributions to the vacuum energy from higher-genus Riemann surfaces vanish
identically.Comment: 13 pages, 1 figure. Final discussion and Refs adde
Discrete Deformations in Type I Vacua
We study supersymmetric orientifolds where the world-sheet parity
transformation is combined with a conjugation of some compact complex
coordinates. We investigate their T-duality relation to standard orientifolds
and discuss the origin of continuous and discrete moduli. In contrast to
standard orientifolds, the antisymmetric tensor describes a continuous
deformation, while the off-diagonal part of the metric is frozen to quantized
values and is responsible for the rank reduction of the gauge group. We also
give a geometrical interpretation of some recently constructed six-dimensional
permutational orientifolds.Comment: 12 pages, TeX, harvmac, 4 figures, eq.(3.15) change
Comments on Open-String Orbifolds with a Non-Vanishing B_{ab}
We study the effect of a non-vanishing flux for the NS-NS antisymmetric
tensor in open-string orbifolds. As in toroidal models, the total dimension of
the Chan-Paton gauge group is reduced proportionally to the rank of ,
both on D9 and on D5-branes, while the Moebius amplitude involves some signs
that, in the case, allow one to connect continuously groups
to groups on each set of D-branes. In this case,
non-universal couplings between twisted scalars and gauge vectors arise, as
demanded by the generalised Green-Schwarz mechanism. We also comment on the
role of the NS-NS antisymmetric tensor in a recently proposed type scenario,
where supersymmetry is broken on the D-branes, while it is preserved in the
bulk.Comment: 28 pages, harvmac. Misprints corrected. References added. Version to
appear on Nucl. Phys.
Aspects of Type I Compactifications and Type I-Heterotic Duality
We review the construction of open descendants of the type IIB superstring on
the Z-orbifold. It results in a chiral four-dimensional model with gauge group
and three generations of matter in the representations. As a test of type I - heterotic duality, that reduces
to a weak/weak duality in D=4, a heterotic model on the same orbifold is also
presented. The massless spectrum reproduces exactly the one found in the type I
case apart from additional twisted matter charged with respect to the SO(8)
gauge group. The puzzle is solved by noting that at generic points in the
moduli space these states get masses.Comment: Talk delivered at the V Korean-Italian Meeting on Relativistic
Astrophysics, 8 pages, Late
Open Strings and Supersymmetry Breaking
We review several mechanisms for supersymmetry breaking in orientifold
models. In particular, we focus on non-supersymmetric open-string realisations
that correspond to consistent flat-space solutions of the classical equations
of motion. In these models, the one-loop vacuum energy can typically fixed by
the size of the compact extra dimensions, and can thus be tuned to extremely
small values if enough extra dimensions are large.Comment: Contribution to the proceedings of the XVI SIGRAV conference. V2 refs
added, V3 one ref adde
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
Non-Supersymmetric Open String Vacua
We review the construction of non-supersymmetric open string vacua in various
dimensions. They can be obtained either projecting the (compactified)
non-supersymmetric 0B theory, or applying the Scherk-Schwarz mechanism to open
strings. Generically, these vacua generate a non-vanishing cosmological
constant. However, one can construct particular kinds of Scherk-Schwarz
compactifications with vanishing cosmological constant, at least for low
orders, based on asymmetric orbifolds. A generic feature of these models is
that supersymmetry remains unbroken on the branes at all mass level, while it
is broken in the bulk in a way that preserves Fermi-Bose degeneracy at each
mass level in the perturbative string spectrum.Comment: 8 pages, Latex + JHEP.cls. Contribution to the Proceedings of the
Trieste Meeting of the TMR Network on Physics beyond the S
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