184 research outputs found
Dynamical Instability of Brane Solutions with a Self-Tuning Cosmological Constant
A five-dimensional solution to Einstein's equations coupled to a scalar field
has been proposed as a partial solution to the cosmological constant problem:
the effect of arbitrary vacuum energy (tension) of a 3-brane is cancelled;
however, the scalar field becomes singular at some finite proper distance in
the extra dimension. We show that in the original model with a vanishing bulk
potential for the scalar, the solution is a saddle point which is unstable to
expansion or contraction of the brane world. We construct exact time-dependent
solutions which generalize the static solution, and demonstrate that they do
not conserve energy on the brane; thus they do not have an effective 4-D field
theoretic description. When a bulk scalar field potential is added, the
boundary conditions on the brane cannot be trivially satisfied, raising hope
that the self-tuning mechanism may still give some insight into the
cosmological constant problem in this case.Comment: 11 pages, 2 figure
Technical note: Introduction of a superconducting gravimeter as novel hydrological sensor for the Alpine research catchment Zugspitze
GFZ (German Research Centre for Geosciences) set up the Zugspitze Geodynamic Observatory Germany with a worldwide unique installation of a superconducting gravimeter at the summit of Mount Zugspitze on top of the Partnach spring catchment. This high alpine catchment is well instrumented, acts as natural lysimeter and has significant importance for water supply to its forelands, with a large mean annual precipitation of 2080ĝ€¯mm and a long seasonal snow cover period of 9 months, while showing a high sensitivity to climate change. However, regarding the majority of alpine regions worldwide, there is only limited knowledge on temporal water storage variations due to sparsely distributed hydrological and meteorological sensors and the large variability and complexity of signals in alpine terrain. This underlines the importance of well-equipped areas such as Mount Zugspitze serving as natural test laboratories for improved monitoring, understanding and prediction of alpine hydrological processes. The observatory superconducting gravimeter, OSG 052, supplements the existing sensor network as a novel hydrological sensor system for the direct observation of the integral gravity effect of total water storage variations in the alpine research catchment at Zugspitze. Besides the experimental set-up and the available data sets, the gravimetric methods and gravity residuals are presented based on the first 27 months of observations from 29 December 2018 to 31 March 2021. The snowpack is identified as being a primary contributor to seasonal water storage variations and, thus, to the gravity residuals with a signal range of up to 750ĝ€¯nms-2 corresponding to 1957ĝ€¯mm snow water equivalent measured with a snow scale at an altitude of 2420ĝ€¯m at the end of May 2019. Hydro-gravimetric sensitivity analysis reveal a snow-gravimetric footprint of up to 4ĝ€¯km distance around the gravimeter, with a dominant gravity contribution from the snowpack in the Partnach spring catchment. This shows that the hydro-gravimetric approach delivers representative integral insights into the water balance of this high alpine site. © Copyright
Dynamical Supersymmetry Breaking in Standard-like Models with Intersecting D6-branes
We address dynamical supersymmetry breaking within a N=1 supersymmetric
Standard-like Model based on a Z_2 x Z_2 Type IIA orientifold with intersecting
D6-branes. The model possesses an additional, confining gauge sector with the
USp(2)_A x USp(2)_B x USp(4) gauge group, where the gaugino condensation
mechanism allows for the breaking of supersymmetry and stabilizes moduli. We
derive the leading contribution to the non-perturbative effective
superpotential and determine numerically the minima of the supergravity
potential. These minima break supersymmetry and fix two undetermined moduli,
which in turn completely specify the gauge couplings at the string scale. For
this specific construction the minima have a negative cosmological constant. We
expect that for other supersymmetric Standard-like models with intersecting
D6-branes, which also possess confining gauge sectors, the supersymmetry
breaking mechanism would have qualitatively similar features.Comment: 19 pages, 1 figur
Intersecting brane world models from D8-branes on (T^2 x T^4/Z_3)/Omega R_1 type IIA orientifolds
We present orientifold models of type IIA string theory with D8-branes
compactified on a two torus times a four dimensional orbifold. The orientifold
group is chosen such that one coordinate of the two torus is reversed when
applying worldsheet parity. RR tadpole cancellation requires D8-branes which
wrap 1-cycles on the two torus and transform non-trivially under the orbifold
group. These models are T-dual to orientifolds with D4-branes only which admit
large volume compactifications. The intersections of the D8-branes are chosen
in such a way that supersymmetry is broken in the open string sector and chiral
fermions arise. Stability of the models is discussed in the context of NSNS
tadpoles. Two examples with the SM gauge group and two left-right symmetric
models are given.Comment: 30 pages, 7 figure
Braneworld models with a non-minimally coupled phantom bulk field: a simple way to obtain the -1-crossing at late times
We investigate general braneworld models, with a non-minimally coupled
phantom bulk field and arbitrary brane and bulk matter contents. We show that
the effective dark energy of the brane-universe acquires a dynamical nature, as
a result of the non-minimal coupling which provides a mechanism for an indirect
"bulk-brane interaction" through gravity. For late-time cosmological evolution
and without resorting to special ansatzes or to specific areas of the parameter
space, we show that the -1-crossing of its equation-of-state parameter is
general and can be easily achieved. As an example we provide a simple, but
sufficiently general, approximate analytical solution, that presents the
crossing behavior.Comment: 11 pages, 2 figure
Generalised permutation branes
We propose a new class of non-factorising D-branes in the product group GxG
where the fluxes and metrics on the two factors do not necessarily coincide.
They generalise the maximally symmetric permutation branes which are known to
exist when the fluxes agree, but break the symmetry down to the diagonal
current algebra in the generic case. Evidence for the existence of these branes
comes from a Lagrangian description for the open string world-sheet and from
effective Dirac-Born-Infeld theory. We state the geometry, gauge fields and, in
the case of SU(2)xSU(2), tensions and partial results on the open string
spectrum. In the latter case the generalised permutation branes provide a
natural and complete explanation for the charges predicted by K-theory
including their torsion.Comment: 33 pages, 6 figures, v2: Extended discussion of K-theory
interpretation of our branes for products of higher rank groups in the
conclusions; v3: Correction of formula (35) and adjustment of the discussion
below equation (45) (no change of result). Footnote 9 points out a previously
unnoticed subtlety and provides a reference to a more detailed discussio
DBI analysis of generalised permutation branes
We investigate D-branes on the product GxG of two group manifolds described
as Wess-Zumino-Novikov-Witten models. When the levels of the two groups
coincide, it is well known that there exist permutation D-branes which are
twisted by the automorphism exchanging the two factors. When the levels are
different, the D-brane charge group demands that there should be
generalisations of these permutation D-branes, and a geometric construction for
them was proposed in hep-th/0509153. We give further evidence for this proposal
by showing that the generalised permutation D-branes satisfy the
Dirac-Born-Infeld equations of motion for arbitrary compact, simply connected
and simple Lie groups G.Comment: 19 pages, computation in section 3.5.1 corrected, conclusions
unchange
Positive tension 3-branes in an bulk
In this work, we review and extend the so-called consistency conditions for
the existence of a braneworld scenario in arbitrary dimensions in the
Brans-Dicke (BD) gravitational theory. After that, we consider the particular
case of a five-dimensional scenario which seems to have phenomenological
interesting implications. We show that, in the BD framework, it is possible to
achieve necessary conditions pointing to the possibility of accommodating
branes with positive tensions in an AdS bulk by the presence of the additional
BD scalar field, avoiding in this way the necessity of including unstable
objects in the compactification scheme. Furthermore, in the context of time
variable brane tension, it is shown that the brane tension may change its sign,
following the bulk cosmological constant sign.Comment: 15 pages, new version to appear in JHE
Orientifolds of K3 and Calabi-Yau Manifolds with Intersecting D-branes
We investigate orientifolds of type II string theory on K3 and Calabi-Yau
3-folds with intersecting D-branes wrapping special Lagrangian cycles. We
determine quite generically the chiral massless spectrum in terms of
topological invariants and discuss both orbifold examples and algebraic
realizations in detail. Intriguingly, the developed techniques provide an
elegant way to figure out the chiral sector of orientifold models without
computing any explicit string partition function. As a new example we derive a
non-supersymmetric Standard-like Model from an orientifold of type IIA on the
quintic Calabi-Yau 3-fold with wrapped D6-branes. In the case of supersymmetric
intersecting brane models on Calabi-Yau manifolds we discuss the D-term and
F-term potentials, the effective gauge couplings and the Green-Schwarz
mechanism. The mirror symmetric formulation of this construction is provided
within type IIB theory. We finally include a short discussion about the lift of
these models from type IIB on K3 to F-theory and from type IIA on Calabi-Yau
3-folds to M-theory on G_2 manifolds.Comment: 82 pages, harvmac, 5 figures. v2: references added. v3: T^6
orientifold corrected, JHEP versio
Searching singlet extensions of the supersymmetric standard model in orbifold compactification of heterotic string
We search for supersymmetric standard model realizations with extra singlets
and extra using the heterotic string compactification on the orbifold with two Wilson lines. We analyze the vacuum restabilization
mechanism for three representative Pati-Salam string models obtained in the
literature and present detailed results for the effective superpotential
compatible with the string selection rules. An automated selection of
semi-realistic vacua along flat directions in the non-Abelian singlet modes
field space is performed by requiring the presence of massless pairs of
electroweak Higgs bosons having trilinear superpotential couplings with
massless singlet modes and the decoupling of color triplet exotic modes needed
to suppress and number violating processes.Comment: revtex4 format, 21 pages, 7 tables, shortened version added
reference
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