479 research outputs found
Homotopy Structure of 5d Vacua
It is shown that flat zero-energy solutions (vacua) of the 5d Kaluza-Klein
theory admit a non-trivial homotopy structure generated by certain Kaluza-Klein
excitations. These vacua consist of an infinite set of homotopically different
spacetimes denoted by , among which
and are especially identified as and
, the ground states of the 5d Kaluza-Klein theory and the 5d general
relativity, respectively (where represents the -dimensional Minkowski
space).Comment: 8 page
Parameterized Post-Newtonian coefficients for Brans-Dicke gravity with d+1 dimensions
We present calculations of Post-Newtonian parameters for Brans-Dicke
tensor-scalar gravity in an arbitrary number of compact extra dimensions in
both the Jordan and Einstein conformal frames. We find that the parameter
gamma, which measures the amount of spacetime curvature per unit mass, becomes
a function of omega, the coefficient of the scalar kinetic term in the
Brans-Dicke Lagrangian. Experiment has placed strong constraints on gamma which
require that omega become negative in the Jordan frame for any number of extra
dimensions, highlighting that this formulation is not physical. We also confirm
the well-known result that a compact extra dimension can be equivalently viewed
as a massless scalar `dilaton.' In the Einstein frame, we find that the
behavior of gamma as constrained by experiment replicates that which is
predicted by string theory.Comment: 9 pages, accepted in Classical and Quantum Gravit
Consistent Group and Coset Reductions of the Bosonic String
Dimensional reductions of pure Einstein gravity on cosets other than tori are
inconsistent. The inclusion of specific additional scalar and p-form matter can
change the situation. For example, a D-dimensional Einstein-Maxwell-dilaton
system, with a specific dilaton coupling, is known to admit a consistent
reduction on S^2= SU(2)/U(1), of a sort first envisaged by Pauli. We provide a
new understanding, by showing how an S^3=SU(2) group-manifold reduction of
(D+1)-dimensional Einstein gravity, of a type first indicated by DeWitt, can be
broken into in two steps; a Kaluza-type reduction on U(1) followed by a
Pauli-type coset reduction on S^2. More generally, we show that any
D-dimensional theory that itself arises as a Kaluza U(1) reduction from (D+1)
dimensions admits a consistent Pauli reduction on any coset of the form G/U(1).
Extensions to the case G/H are given. Pauli coset reductions of the bosonic
string on G= (G\times G)/G are believed to be consistent, and a consistency
proof exists for S^3=SO(4)/SO(3). We examine these reductions, and arguments
for consistency, in detail. The structures of the theories obtained instead by
DeWitt-type group-manifold reductions of the bosonic string are also studied,
allowing us to make contact with previous such work in which only singlet
scalars are retained. Consistent truncations with two singlet scalars are
possible. Intriguingly, despite the fact that these are not supersymmetric
models, if the group manifold has dimension 3 or 25 they admit a superpotential
formulation, and hence first-order equations yielding domain-wall solutions.Comment: Latex, 5 figures, 45 pages, minor correction
Gauge theories as a geometrical issue of a Kaluza-Klein framework
We present a geometrical unification theory in a Kaluza-Klein approach that
achieve the geometrization of a generic gauge theory bosonic component.
We show how it is possible to derive the gauge charge conservation from the
invariance of the model under extra-dimensional translations and to geometrize
gauge connections for spinors, thus we can introduce the matter just by free
spinorial fields. Then, we present the applications to i)a pentadimensional
manifold , so reproducing the original Kaluza-Klein theory,
unless some extensions related to the rule of the scalar field contained in the
metric and the introduction of matter by spinors with a phase dependence from
the fifth coordinate, ii)a seven-dimensional manifold , in which we geometrize the electro-weak model by
introducing two spinors for any leptonic family and quark generation and a
scalar field with two components with opposite hypercharge, responsible of
spontaneous symmetry breaking.Comment: 37 pages, no figure
Solutions of multigravity theories and discretized brane worlds
We determine solutions to 5D Einstein gravity with a discrete fifth
dimension. The properties of the solutions depend on the discretization scheme
we use and some of them have no continuum counterpart. In particular, we find
that the neglect of the lapse field (along the discretized direction) gives
rise to Randall-Sundrum type metric with a negative tension brane. However, no
brane source is required. We show that this result is robust under changes in
the discretization scheme. The inclusion of the lapse field gives rise to
solutions whose continuum limit is gauge fixed by the discretization scheme. We
find however one particular scheme which leads to an undetermined lapse
reflecting the reparametrization invariance of the continuum theory. We also
find other solutions, with no continuum counterpart with changes in the metric
signature or avoidance of singularity. We show that the models allow a
continuous mass spectrum for the gravitons with an effective 4D interaction at
small scales. We also discuss some cosmological solutions.Comment: 19 page
Research and management challenges following soil and landscape decontamination at the onset of the reopening of the Difficult-to-Return Zone, Fukushima (Japan)
Twelve years after the nuclear accident that occurred at the
Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in March 2011, radiocesium
contamination (with a large dominance of 137Cs, with a 30-year
half-life) remains a major concern in various municipalities of north-eastern
Japan. The Japanese authorities completed an unprecedented soil
decontamination programme in residential and cultivated areas affected by
the main radioactive plume (8953âkm2). They implemented a
complex remediation programme scheme to remediate soils that are fundamental
to life on Earth, relying on different decision rules depending on the waste
type, its contamination level and its region of origin, after delineating
different zones exposed to contrasted radiation rates. The central objective
was not to expose local inhabitants to radioactive doses exceeding 1âmSvâyrâ1 in addition to the natural levels. At the onset of the full
reopening of the Difficult-to-Return Zone (DTRZ) in spring 2023, the current review
provides an update of a previous synthesis published in 2019
(Evrard et al., 2019). Although this ambitious soil remediation
and reconstruction programme has almost been completed in the 12 municipalities
of Fukushima Prefecture in which an evacuation order was imposed in at least
one neighbourhood in 2011, from the 147â443 inhabitants who lived there
before the accident, only 29.9â% of them had returned by 2020. Waste
generated by decontamination and tsunami cleaning/demolition work is planned
to have been fully transported to (interim) storage facilities by the end of
2023. The cost of the operations conducted between 2011 and 2020 for the
so-called ânuclear recoveryâ operations (including decontamination) was
estimated by the Board of Audit of Japan in 2023 as JPYâ6122.3Â billion
(âŒâEURâ44 billion). Decontamination of cropland was shown to
have impacted soil fertility, and potassium fertilisation is recommended to
limit the transfer of residual radiocesium to new crops. In forests that
cover 71â% of the surface area of Fukushima Prefecture and that were
not targeted by remediation, radiocesium is now found in the upper mineral
layer of the soil in a quasi-equilibrium state. Nevertheless, 137Cs
concentrations in forest products (including wood for heating and
construction, wild plants, wildlife game, mushrooms) often keep exceeding
the threshold values authorised in Japan, which prohibits their exploitation
in the area affected by the main plume. Radionuclides from forests were shown
to be exported in dissolved and particle-bound forms to downstream river
systems and floodplains, although multiple monitoring records showed the
continuous decrease in radiocesium concentrations in both river water and
sediment across the main plume between 2011 and 2021. Fish contamination is now
generally found to be below the threshold limits although reputational damage
remains a major concern for local fishing communities. The remobilisation of
radiocesium from sediment accumulated in reservoirs of the region is also of
potential concern as it may lead to secondary contamination of fish or
irrigation waters supplied to decontaminated fields. Overall, this synthesis
demonstrates the need to continue monitoring post-accidental radiocesium
transfer in these environments and to keep sharing data in order to refine
our predictive understanding of radiocesium mobility and consolidate the
tools available to model contaminant transfer in ecosystems. In forests in
particular, novel countermeasures and wood uses remain to be developed and
tested. Furthermore, the hydrologic connectivity between soils under
different ecosystems greatly influences long-term radiocesium
transport. The consequences of extreme phenomena (e.g. typhoons, forest
fires) that may become more frequent in the future as a result of global
change in these contaminated environments should be further anticipated.</p
Kaluza-Klein and Gauss-Bonnet cosmic strings
We make a systematic investigation of stationary cylindrically symmetric
solutions to the five-dimensional Einstein and Einstein-Gauss-Bonnet equations.
Apart from the five-dimensional neutral cosmic string metric, we find two new
exact solutions which qualify as cosmic strings, one corresponding to an
electrically charged cosmic string, the other to an extended superconducting
cosmic string surrounding a charged core. In both cases, test particles are
deflected away from the singular line source. We extend both kinds of solutions
to exact multi-cosmic string solutions.Comment: 26 pages, LaTex, no figure
O adsorption and incipient oxidation of the Mg(0001) surface
First principles density functional calculations are used to study the early
oxidation stages of the Mg(0001) surface for oxygen coverages 1/16 <= Theta <=
3 monolayers. It is found that at very low coverages O is incorporated below
the topmost Mg layer in tetrahedral sites. At higher oxygen-load the binding in
on-surface sites is increased but at one monolayer coverage the on-surface
binding is still about 60 meV weaker than for subsurface sites. The subsurface
octahedral sites are found to be unfavorable compared to subsurface tetrahedral
sites and to on-surface sites. At higher coverages oxygen adsorbs both under
the surface and up. Our calculations predict island formation and clustering of
incorporated and adsorbed oxygen in agreement with previous calculations. The
calculated configurations are compared with the angle-scanned x-ray
photoelectron diffraction experiment to determine the geometrical structure of
the oxidized Mg(0001) surface.Comment: 10 pages, 5 figure
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