741 research outputs found
Modulus stabilization of generalized Randall Sundrum model with bulk scalar field
We study the stabilization of inter-brane spacing modulus of generalized
warped brane models with a nonzero brane cosmological constant. Employing
Goldberger-Wise stabilization prescription of brane world models with a bulk
scalar field, we show that the stabilized value of the modulus generally
depends on the value of the brane cosmological constant. Our result further
reveals that the stabilized modulus value corresponding to a vanishingly small
cosmological constant can only resolve the gauge hierarchy problem
simultaneously. This in turn vindicates the original Randall-Sundrum model
where the 3-brane cosmological constant was chosen to be zero.Comment: 12 Pages, 1 figure, Revtex, Version to appear in Euro. Phys. Let
Scalar Kaluza-Klein modes in a multiply warped braneworld
The Kaluza-Klein (KK) modes of a massive scalar field on a 3-brane embedded
in six dimensional multiply warped spacetime are determined. Due to the
presence of warping along both the extra dimensions the KK mass spectrum splits
into two closely spaced branches which is a distinct feature of this model
compared to the five dimensional Randall-Sundrum model. This new cluster of the
KK mode spectrum is expected to have interesting phenomenological implications
for the upcoming collider experiments. Such a scenario may also be extended for
even larger number of orbifolded extra dimensions.Comment: 10 pages, Revte
de Sitter branes with a bulk scalar
We propose new braneworld models arising from a scalar field in the bulk. In
these examples, the induced on--brane line element is de Sitter (or anti de
Sitter) and the bulk (five dimensional) Einstein equations can be exactly
solved to obtain warped spacetimes. The solutions thus derived are single and
two-brane models -- one with {\em thin} branes while the other one of the {\em
thick} variety. The field profiles and the potentials are obtained and analysed
for each case. We note that for the {\em thick} brane scenario the field
profile resembles a kink, whereas for one or more {\em thin} branes, it is
finite and bounded in the domain of the extra dimension. We have also addressed
the localisation of gravity and other matter fields on the brane for these
braneworld models.Comment: 18 pages, 5 figures. Substantial changes and new results. To appear
in GR
Cosmology and Fermion Confinement in a Scalar-Field-Generated Domain Wall Brane in Five Dimensions
We consider a brane generated by a scalar field domain wall configuration in
4+1 dimensions, interpolating, in most cases, between two vacua of the field.
We study the cosmology of such a system in the cases where the effective
four-dimensional brane metric is de Sitter or anti de Sitter, including a
discussion of the bulk coordinate singularities present in the de-Sitter case.
We demonstrate that a scalar field kink configuration can support a brane with
dS cosmology, despite the presence of coordinate singularities in the
metric. We examine the trapping of fermion fields on the domain wall for
nontrivial brane cosmology.Comment: 29 pages, 12 figures; minor changes, accepted by JHE
Braneworlds in six dimensions: new models with bulk scalars
Six dimensional bulk spacetimes with 3-- and 4--branes are constructed using
certain non--conventional bulk scalars as sources. In particular, we
investigate the consequences of having the phantom (negative kinetic energy)
and the Brans--Dicke scalar in the bulk while obtaining such solutions. We find
geometries with 4--branes with a compact on--brane dimension (hybrid
compactification) which may be assumed to be small in order to realize a
3--brane world. On the other hand, we also construct, with similar sources,
bulk spacetimes where a 3--brane is located at a conical singularity.
Furthermore, we investigate the issue of localization of matter fields (scalar,
fermion, graviton, vector) on these 3-- and 4--branes and conclude with
comments on our six dimensional models.Comment: 24 pages, 1 figure, Replaced to match version published in Class.
Quant. Gra
Geometry of deformations of branes in warped backgrounds
The `braneworld' (described by the usual worldvolume action) is a D
dimensional timelike surface embedded in a N dimensional () warped,
nonfactorisable spacetime. We first address the conditions on the warp factor
required to have an extremal flat brane in a five dimensional background.
Subsequently, we deal with normal deformations of such extremal branes. The
ensuing Jacobi equations are analysed to obtain the stability condition. It
turns out that to have a stable brane, the warp factor should have a minimum at
the location of the brane in the given background spacetime. To illustrate our
results we explicitly check the extremality and stability criteria for a few
known co-dimension one braneworld models. Generalisations of the above
formalism for the cases of (i) curved branes (ii) asymmetrical warping and
(iii) higher co-dimension braneworlds are then presented alongwith some typical
examples for each. Finally, we summarize our results and provide perspectives
for future work along these lines.Comment: 21 pages. Version matching final version. Accepted for publication in
Class. Quant. Gra
Navigating Copper-Atom-Pair Structural Effect inside a Porous Organic Polymer Cavity for Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural
In recent times, selective hydrogenation of biomass-derived 5-hydroxymethylfurfural (5-HMF) to produce the novel difuranic polyol scaffold 2,5-dihydroxymethylfuran (DHMF) has attracted the interest of the many researchers due to its peculiar symmetrical structure and its widespread application as a monomer for the preparation of cross-linked polyesters and polyurethane. Copper-based catalysts have been explored for selective catalytic hydrogenation; however, hurdles are still associated with the strongly reducing H2 atmosphere and oxidizing C–O bond that make the Cu0 and Cux+ surface active species unstable, limiting the rational design of highly efficient integrated catalyst systems. To address this, herein, we built catalytic systems for 5-HMF hydrogenation with stable and balanced Cu0 and Cux+ active surface species inside the nanocage of a catechol-based porous organic polymer (POP) endowed with large surface areas, impressive stabilities, and spatial restriction inhibiting nanoparticle aggregation. Batch reactor screening identified that a superior catalytic performance (DHMF selectivity of 98%) has been achieved with our newly designed Cu@C-POP at 150 °C temperature and 20 bar H2 pressure, which was also higher than that of other reported copper catalysts. Comprehensive characterization understanding with H2-TPR and X-ray photoelectron spectroscopy (XPS) study revealed that substantially boosted activity is induced by the presence of the bulk CuOx phase and atomically dispersed Cu species incorporating isolated Cu ions, which are further confirmed through the positive binding energy shift of Cu 2p3/2 XPS spectra (∼0.4 eV). The Cu environment in our catalytic systems comprises a predominantly square planar geometry (probably Jahn–Teller distorted OH), which we gleaned from the extended X-ray absorption for fine structure (EXAFS) analysis featuring two adjacent copper atoms with the valence state in between of 0 and +2, as validated by XANES absorption edge positions. EXAFS studies further revealed a lowering of the Cu coordination number for the most active Cu@C-POP-B catalyst, suggesting the presence of metal vacancies. Density functional theory calculations showed that the presence of Cu metal vacancies stabilized the reaction intermediates formed during 5-HMF hydrogenation and decreased the hydrogenation barriers, resulting in an enhanced catalytic activity of the Cu@C-POP-B catalyst
Fermion localization on branes with generalized dynamics
In this letter we consider a specific model of braneworld with nonstandard
dynamics diffused in the literature, specifically we focus our attention on the
matter energy density, the energy of system, the Ricci scalar and the thin
brane limit. As the model is classically stable and capable of localize
gravity, as a natural extension we address the issue of fermion localization of
fermions on a thick brane constructed out from one scalar field with
nonstandard kinetic terms coupled with gravity. The contribution of the
nonstandard kinetic terms in the problem of fermion localization is analyzed.
It is found that the simplest Yukawa coupling support
the localization of fermions on the thick brane. It is shown that the zero mode
for left-handed can be localized on the thick brane depending on the values for
the coupling constant .Comment: 6 pages, 3 figure
Particle creation in the presence of a warped extra dimension
Particle creation in spacetimes with a warped extra dimension is studied. In
particular, we investigate the dynamics of a conformally coupled, massless
scalar field in a five dimensional warped geometry where the induced metric on
the 3--branes is that of a spatially flat cosmological model. We look at
situations where the scale of the extra dimension is assumed (i) to be time
independent or (ii) to have specific functional forms for time dependence. The
warp factor is chosen to be that of the Randall--Sundrum model. With particular
choices for the functional form of the scale factor (and also the function
characterising the time evolution of the extra dimension) we obtain the , the particle number and energy densities after solving (wherever
possible, analytically but, otherwise, numerically) the conformal scalar field
equations. The behaviour of these quantities for the massless and massive
Kaluza--Klein modes are examined. Our results show the effect of a warped extra
dimension on particle creation and illustrate how the nature of particle
production on the brane depends on the nature of warping, type of cosmological
evolution as well as the temporal evolution of the extra dimension.Comment: 21 pages, 10 figures, minor corrections, new references added,
version to appear in JCA
Reducing Constraints in a Higher Dimensional Extension of the Randall and Sundrum Model
In order to investigate the phenomenological implications of warped spaces in
more than five dimensions, we consider a dimensional extension to
the Randall and Sundrum model in which the space is warped with respect to a
single direction by the presence of an anisotropic bulk cosmological constant.
The Einstein equations are solved, giving rise to a range of possible spaces in
which the additional spaces are warped. Here we consider models in
which the gauge fields are free to propagate into such spaces. After carrying
out the Kaluza Klein (KK) decomposition of such fields it is found that the KK
mass spectrum changes significantly depending on how the additional
dimensions are warped. We proceed to compute the lower bound on the KK mass
scale from electroweak observables for models with a bulk
gauge symmetry and models with a bulk gauge
symmetry. It is found that in both cases the most favourable bounds are
approximately TeV, corresponding to a mass of the first gauge
boson excitation of about 4-6 TeV. Hence additional warped dimensions offer a
new way of reducing the constraints on the KK scale.Comment: 27 pages, 15 figures, v3: Additional comments in sections 1, 2 and 4.
New appendix added. Five additional figures. References adde
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