1,768 research outputs found
Bianchi Type I Cosmology in N=2, D=5 Supergravity
The dynamics and evolution of Bianchi type I space-times is considered in the
framework of the four-dimensional truncation of a reduced theory obtained from
the N=2,D=5 supergravity. The general solution of the gravitational field
equations can be represented in an exact parametric form. All solutions have a
singular behavior at the initial/final moment, except when the space-time
geometry reduces to the isotropic flat case. Generically the obtained
cosmological models describe an anisotropic, expanding or collapsing, singular
Universe with a non-inflationary evolution for all times.Comment: revised version to appear in PR
10th September 2005: a bolide airblast in the Gulf of Naples (Southern Italy).
On 10th September 2005 at 17:11 local time (15:11 GMT) a loud boom was heard on the Ischia island. A clear seismic signal was also recorded by the seismic monitoring network of the Neapolitan volcanic areas (Ischia, Campi Flegrei and Mt. Vesuvius) and on a regional station (Mt. Massico). On the base of the seismic recordings and on acoustic phenomena reports, we relate this event to the airblast of a bolide at about 15 Km SW of Ischia at an elevation of about 11.5 Km. The location has been obtained through probabilistic non-linear traveltime inversion in a realistic atmospheric model including wind eff ect. We will show, using statistical estimators, how the traveltime pattern is due to both atmospheric winds and bolide trajectory. Using the same reasoning we discard a human origin (supersonic jet or sea-air missile). In addition, we propose also a new algorithm for a fast acoustic traveltime computation for a moving source
M/M/ queues in semi-Markovian random environment
In this paper we investigate an M/M/ queue whose parameters depend on
an external random environment that we assume to be a semi-Markovian process
with finite state space. For this model we show a recursive formula that allows
to compute all the factorial moments for the number of customers in the system
in steady state. The used technique is based on the calculation of the raw
moments of the measure of a bidimensional random set. Finally the case when the
random environment has only two states is deeper analyzed. We obtain an
explicit formula to compute the above mentioned factorial moments when at least
one of the two states has sojourn time exponentially distributed.Comment: 17 pages, 2 figure
Enhanced Nuclear Engineering Simulators
Engineering simulation is a sophisticated multi-purpose technology allowing the users of simulators to run a variety of engineering activities due to the possibility of modifying the simulated plant architecture and components, to adjust parameters, to test alternative solutions. Engineering Simulators (ES) have been built and used worldwide for a variety of purposes:
- Development and refinement of the plant design or plant modifications
- Safety analyses focused on the overall system behaviour
- Verification and Validation (V&V) of systems and components
- Development of Operational and Emergency Procedures
- Pre-Training of operators and supervisors
- High level education and Communication activities
- Human Factor Engineering Analysis
- Adaptive Control System training Engineering Simulators also play a role in developing and maintaining key nuclear skills, as knowledge repositories and tools for training at various levels of expertise
Application of best estimate plus uncertainty in review of research reactor safety analysis
To construct and operate a nuclear research reactor, the licensee is required to obtain the authorization from the regulatory body. One of the tasks of the regulatory authority is to verify that the safety analysis fulfils safety requirements. Historically, the compliance with safety requirements was assessed using a deterministic approach and conservative assumptions. This provides sufficient safety margins with respect to the licensing limits on boundary and operational conditions. Conservative assumptions were introduced into safety analysis to account for the uncertainty associated with lack of knowledge. With the introduction of best estimate computational tools, safety analyses are usually carried out using the best estimate approach. Results of such analyses can be accepted by the regulatory authority only if appropriate uncertainty evaluation is carried out. Best estimate computer codes are capable of providing more realistic information on the status of the plant, allowing the prediction of real safety margins. The best estimate plus uncertainty approach has proven to be reliable and viable of supplying realistic results if all conditions are carefully followed. This paper, therefore, presents this concept and its possible application to research reactor safety analysis. The aim of the paper is to investigate the unprotected loss-of-flow transients "core blockage" of a miniature neutron source research reactor by applying best estimate plus uncertainty methodology. The results of our calculations show that the temperatures in the core are within the safety limits and do not pose any significant threat to the reactor, as far as the melting of the cladding is concerned. The work also discusses the methodology of the best estimate plus uncertainty approach when applied to the safety analysis of research reactors for licensing purposes
Supersymmetric Brane World Scenarios from Off-Shell Supergravity
Using N=2 off-shell supergravity in five dimensions, we supersymmetrize the
brane world scenario of Randall and Sundrum. We extend their construction to
include supersymmetric matter at the fixpoints.Comment: 15 pages, no figures, late
Supergravity interacting with bosonic p-branes and local supersymmetry
We study the coupling of supergravity with a purely bosonic brane source
(bosonic p-brane). The interaction, described by the sum of their respective
actions, is self-consistent if the bosonic p-brane is the pure bosonic limit of
a super-p-brane. In that case the dynamical system preserves 1/2 of the local
supersymmetry characteristic of the `free' supergravity.Comment: 11 pages, RevTe
M-Theory as a Holographic Field Theory
We suggest that M-theory could be non-perturbatively equivalent to a local
quantum field theory. More precisely, we present a ``renormalizable'' gauge
theory in eleven dimensions, and show that it exhibits various properties
expected of quantum M-theory, most notably the holographic principle of
't~Hooft and Susskind. The theory also satisfies Mach's principle: A
macroscopically large space-time (and the inertia of low-energy excitations) is
generated by a large number of ``partons'' in the microscopic theory. We argue
that at low energies in large eleven dimensions, the theory should be
effectively described by eleven-dimensional supergravity. This effective
description breaks down at much lower energies than naively expected, precisely
when the system saturates the Bekenstein bound on energy density. We show that
the number of partons scales like the area of the surface surrounding the
system, and discuss how this holographic reduction of degrees of freedom
affects the cosmological constant problem. We propose the holographic field
theory as a candidate for a covariant, non-perturbative formulation of quantum
M-theory.Comment: 27 pp. v2: typos corrected; a small paragraph on naturalness of small
cosmological constant in four dimensions added at end of sect 5.1; final
version to appear in Phys. Rev.
Alleviating the non-ultralocality of coset sigma models through a generalized Faddeev-Reshetikhin procedure
The Faddeev-Reshetikhin procedure corresponds to a removal of the
non-ultralocality of the classical SU(2) principal chiral model. It is realized
by defining another field theory, which has the same Lax pair and equations of
motion but a different Poisson structure and Hamiltonian. Following earlier
work of M. Semenov-Tian-Shansky and A. Sevostyanov, we show how it is possible
to alleviate in a similar way the non-ultralocality of symmetric space sigma
models. The equivalence of the equations of motion holds only at the level of
the Pohlmeyer reduction of these models, which corresponds to symmetric space
sine-Gordon models. This work therefore shows indirectly that symmetric space
sine-Gordon models, defined by a gauged Wess-Zumino-Witten action with an
integrable potential, have a mild non-ultralocality. The first step needed to
construct an integrable discretization of these models is performed by
determining the discrete analogue of the Poisson algebra of their Lax matrices.Comment: 31 pages; v2: minor change
Metastable de Sitter vacua in N=2 to N=1 truncated supergravity
We study the possibility of achieving metastable de Sitter vacua in general
N=2 to N=1 truncated supergravities without vector multiplets, and compare with
the situations arising in N=2 theories with only hypermultiplets and N=1
theories with only chiral multiplets. In N=2 theories based on a quaternionic
manifold and a graviphoton gauging, de Sitter vacua are necessarily unstable,
as a result of the peculiar properties of the geometry. In N=1 theories based
on a Kahler manifold and a superpotential, de Sitter vacua can instead be
metastable provided the geometry satisfies some constraint and the
superpotential can be freely adjusted. In N=2 to N=1 truncations, the crucial
requirement is then that the tachyon of the mother theory be projected out from
the daughter theory, so that the original unstable vacuum is projected to a
metastable vacuum. We study the circumstances under which this may happen and
derive general constraints for metastability on the geometry and the gauging.
We then study in full detail the simplest case of quaternionic manifolds of
dimension four with at least one isometry, for which there exists a general
parametrization, and study two types of truncations defining Kahler
submanifolds of dimension two. As an application, we finally discuss the case
of the universal hypermultiplet of N=2 superstrings and its truncations to the
dilaton chiral multiplet of N=1 superstrings. We argue that de Sitter vacua in
such theories are necessarily unstable in weakly coupled situations, while they
can in principle be metastable in strongly coupled regimes.Comment: 40 pages, no figure
- âŠ