577 research outputs found
Localized Gravity and Higher Curvature Terms
We consider localization of gravity in smooth domain wall solutions of
gravity coupled to a scalar field with a generic potential in the presence of
the Gauss-Bonnet term. We discuss conditions on the scalar potential such that
domain wall solutions are non-singular. We point out that the presence of the
Gauss-Bonnet term does not allow flat solutions with localized gravity that
violate the weak energy condition. We also point out that in the presence of
the Gauss-Bonnet term infinite tension flat domain walls violate positivity. In
fact, for flat solutions unitarity requires that on the solution the scalar
potential be bounded below.Comment: 11 pages, revtex; a minor misprint corrected (to appear in Phys.
Lett. B
Time-like Extra Dimensions without Tachyons or Ghosts
Some time ago Dvali, Gabadadze and Senjanovic (hep-ph/9910207) discussed
brane world scenarios with time-like extra dimensions. In this paper we
construct a solitonic 3-brane solution in the 5-dimensional
Einstein-Hilbert-Gauss-Bonnet theory with the space-time signature (-,+,+,+,-).
The direction transverse to the brane is the second time-like direction. The
solitonic brane is -function like, and has the property that gravity is
completely localized on the brane. That is, there are no propagating degrees of
freedom in the bulk, while on the brane we have purely 4-dimensional Einstein
gravity. In particular, there are no propagating tachyonic or negative norm
states even though the extra dimension is time-like.Comment: 9 pages, revte
A Three-Family SU(6) Type I Compactification
We construct a four dimensional chiral N=1 space-time supersymmetric Type I
vacuum corresponding to a compactification on a toroidal Z_2 X Z_2 X Z_3
orbifold. Using recent results in four dimensional orientifolds, we argue that
this model has a well defined world-sheet description. An interesting feature
of this model is that the gauge group contains an SU(6) subgroup with three
chiral generations. Moreover, this model contains D5-branes and therefore
corresponds to a non-perturbative heterotic vacuum. This is the first example
of a consistent chiral N=1 supersymmetric string vacuum which is
non-perturbative from the heterotic viewpoint, has a perturbative description
in a dual theory, and possesses some phenomenologically interesting
characteristics. We also compute the tree-level superpotential in this theoryComment: 10 pages, revtex, minor misprints corrected, a few sentences modified
(to appear in Phys. Lett. B
Higgs as a Slepton
In theories with TeV scale quantum gravity there is a logical possibility
where the electroweak Higgs can be a fourth generation slepton. Despite maximal
supersymmetry breaking such a scenario turns out to be tightly constrained, and
might be testable already at present collider experiments. Also, a light Higgs
is no longer a prediction of supersymmetry.Comment: 7 pages, no figures, reference adde
Conformal Brane World and Cosmological Constant
We consider a recently proposed setup where a codimension one brane is
embedded in the background of a smooth domain wall interpolating between AdS
and Minkowski minima. Since the volume of the transverse dimension is infinite,
bulk supersymmetry is intact even if brane supersymmetry is completely broken.
On the other hand, in this setup unbroken bulk supersymmetry is incompatible
with non-zero brane cosmological constant, so the former appears to protect the
latter. In this paper we point out that, to have a consistent coupling between
matter localized on the brane and bulk gravity, in this setup generically it
appears to be necessary that the brane world-volume theory be conformal. Thus,
unbroken bulk supersymmetry appears to actually protect not only the
cosmological constant but also conformal invariance on the brane.Comment: 14 pages, revtex; minor misprints correcte
Geometry of Orientifolds with NS-NS B-flux
We discuss geometry underlying orientifolds with non-trivial NS-NS B-flux. If
D-branes wrap a torus with B-flux the rank of the gauge group is reduced due to
non-commuting Wilson lines whose presence is implied by the B-flux. In the case
of D-branes transverse to a torus with B-flux the rank reduction is due to a
smaller number of D-branes required by tadpole cancellation conditions in the
presence of B-flux as some of the orientifold planes now have the opposite
orientifold projection. We point out that T-duality in the presence of B-flux
is more subtle than in the case with trivial B-flux, and it is precisely
consistent with the qualitative difference between the aforementioned two
setups. In the case where both types of branes are present, the states in the
mixed (e.g., 59) open string sectors come with a non-trivial multiplicity,
which we relate to a discrete gauge symmetry due to non-zero B-flux, and
construct vertex operators for the the mixed sector states. Using these results
we revisit K3 orientifolds with B-flux (where K3 is a T^4/Z_M orbifold) and
point out various subtleties arising in some of these models. For instance, in
the Z_2 case the conformal field theory orbifold does not appear to be the
consistent background for the corresponding orientifolds with B-flux. This is
related to the fact that non-zero B-flux requires the presence of both O5^- as
well as O5^+ planes at various Z_2 orbifold fixed points, which appears to be
inconsistent with the presence of the twisted B-flux in the conformal field
theory orbifold. We also consider four dimensional N=2 and N=1 supersymmetric
orientifolds. We construct consistent four dimensional models with B-flux which
do not suffer from difficulties encountered in the K3 cases.Comment: 79 pages, revte
A Remark on Brane Stabilization in Brane World
In this note we discuss dynamical mechanisms for brane stabilization in the
brane world context. In particular, we consider supersymmetry preserving brane
stabilization, and also brane stabilization accompanied by supersymmetry
breaking. These mechanisms are realized in some four dimensional N=1
supersymmetric orientifold models. For illustrative purposes we consider two
explicit orientifold models previously constructed in hep-th/9806008. In both
of these models branes are stabilized at a finite distance from the orientifold
planes. In the first model brane stabilization occurs via supersymmetry
preserving non-perturbative gauge dynamics. In the second model supersymmetry
is dynamically broken, and brane stabilization is due to an interplay between
non-perturbatively generated superpotential and tree-level Kahler potential.Comment: 11 pages, revtex, a clarifying remark adde
Toward Solving the Cosmological Constant Problem?
We discuss the cosmological constant problem in the context of higher
codimension brane world scenarios with infinite-volume extra dimensions. In
particular, by adding higher curvature terms in the bulk action we are able to
find smooth solutions with the property that the 4-dimensional part of the
brane world-volume is flat for a range of positive values of the brane tension.Comment: 45 pages, revtex, 8 eps figure
Non-commutative Unification in Brane World
We point out that in (open) string compactifications with non-zero NS-NS
B-field we can have large Kaluza-Klein thresholds even in the small volume
limit. In this limit the corresponding gauge theory description is in terms of
a compactification on a non-commutative space (e.g., a torus or an orbifold
thereof). Based on this observation we discuss a brane world scenario of
non-commutative unification via Kaluza-Klein thresholds. In this scenario, the
unification scale can be lowered down to the TeV-range, yet the corresponding
compactification radii are smaller than the string length. We discuss a
potential application of this scenario in the context of obtaining mixing
between different chiral generations which is not exponentially suppressed - as
we point out, such mixing is expected to be exponentially suppressed in certain
setups with large volume compactifications. We also point out that T-duality is
broken by certain non-perturbative twisted open string sectors which are
supposed to give rise to chiral generations, so that in the case of a small
volume compactification with a rational B-field we cannot T-dualize to a large
volume description. In this sense, the corresponding field theoretic picture of
unification via Kaluza-Klein thresholds in this setup is best described in the
non-commutative language.Comment: 15 pages, revtex; misprints corrected, clarifying remarks added (to
appear in Phys. Lett. B
Diluting solutions of the cosmological constant problem
We discuss the cosmological constant problem in the context of higher
codimension brane world scenarios with infinite-volume extra dimensions.Comment: 11 pages, Revtex, reference adde
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