1,311 research outputs found
Two-loop Euler-Heisenberg effective actions from charged open strings
We present the multiloop partition function of open bosonic string theory in
the presence of a constant gauge field strength, and discuss its low-energy
limit. The result is written in terms of twisted determinants and differentials
on higher-genus Riemann surfaces, for which we provide an explicit
representation in the Schottky parametrization. In the field theory limit, we
recover from the string formula the two-loop Euler-Heisenberg effective action
for adjoint scalars minimally coupled to the background gauge field.Comment: 32 pages, 3 eps figures, plain LaTeX. References added, minor changes
to the text. Published version, affiliation correcte
Practical Bayesian Modeling and Inference for Massive Spatial Datasets On Modest Computing Environments
With continued advances in Geographic Information Systems and related
computational technologies, statisticians are often required to analyze very
large spatial datasets. This has generated substantial interest over the last
decade, already too vast to be summarized here, in scalable methodologies for
analyzing large spatial datasets. Scalable spatial process models have been
found especially attractive due to their richness and flexibility and,
particularly so in the Bayesian paradigm, due to their presence in hierarchical
model settings. However, the vast majority of research articles present in this
domain have been geared toward innovative theory or more complex model
development. Very limited attention has been accorded to approaches for easily
implementable scalable hierarchical models for the practicing scientist or
spatial analyst. This article is submitted to the Practice section of the
journal with the aim of developing massively scalable Bayesian approaches that
can rapidly deliver Bayesian inference on spatial process that are practically
indistinguishable from inference obtained using more expensive alternatives. A
key emphasis is on implementation within very standard (modest) computing
environments (e.g., a standard desktop or laptop) using easily available
statistical software packages without requiring message-parsing interfaces or
parallel programming paradigms. Key insights are offered regarding assumptions
and approximations concerning practical efficiency.Comment: 20 pages, 4 figures, 2 table
On the Gauge/Gravity Correspondence and the Open/Closed String Duality
In this article we review the conditions for the validity of the
gauge/gravity correspondence in both supersymmetric and non-supersymmetric
string models. We start by reminding what happens in type IIB theory on the
orbifolds C^2/Z_2 and C^3/(Z_2 x Z_2), where this correspondence beautifully
works. In these cases, by performing a complete stringy calculation of the
interaction among D3 branes, it has been shown that the fact that this
correspondence works is a consequence of the open/closed duality and of the
absence of threshold corrections. Then we review the construction of type 0
theories with their orbifolds and orientifolds having spectra free from both
open and closed string tachyons and for such models we study the validity of
the gauge/gravity correspondence, concluding that this is not a peculiarity of
supersymmetric theories, but it may work also for non-supersymmetric models.
Also in these cases, when it works, it is again a consequence of the
open/closed string duality and of vanishing threshold corrections.Comment: Invited review article for Int. J. Mod. Phys. A, 95 pages, 2 figures,
3 tables, LaTeX. References and acknowledgements adde
Interactions of a String Inspired Graviton Field
We continue to explore the possibility that the graviton in two dimensions is
related to a quadratic differential that appears in the anomalous contribution
of the gravitational effective action for chiral fermions. A higher dimensional
analogue of this field might exist as well. We improve the defining action for
this diffeomorphism tensor field and establish a principle for how it interacts
with other fields and with point particles in any dimension. All interactions
are related to the action of the diffeomorphism group. We discuss possible
interpretations of this field.Comment: 12 pages, more readable, references adde
Coulomb-gas formulation of SU(2) branes and chiral blocks
We construct boundary states in WZNW models using the bosonized
Wakimoto free-field representation and study their properties. We introduce a
Fock space representation of Ishibashi states which are coherent states of
bosons with zero-mode momenta (boundary Coulomb-gas charges) summed over
certain lattices according to Fock space resolution of . The Virasoro
invariance of the coherent states leads to families of boundary states
including the B-type D-branes found by Maldacena, Moore and Seiberg, as well as
the A-type corresponding to trivial current gluing conditions. We then use the
Coulomb-gas technique to compute exact correlation functions of WZNW primary
fields on the disk topology with A- and B-type Cardy states on the boundary. We
check that the obtained chiral blocks for A-branes are solutions of the
Knizhnik-Zamolodchikov equations.Comment: 14 pages, 3 figures, revtex4. Essentially the published versio
Aspects of Open-Closed Duality in a Background B-Field
We study closed string exchanges in background -field. By analysing the
two point one loop amplitude in bosonic string theory, we show that tree-level
exchange of lowest lying, tachyonic and massless closed string modes, have IR
singularities similar to those of the nonplanar sector in noncommutative gauge
theories. We further isolate the contributions from each of the massless modes.
We interpret these results as the manifestation of open/closed string duality,
where the IR behaviour of the boundary noncommutative gauge theory is
reconstructed from the bulk theory of closed strings.Comment: 33 pages, 4 figures, v2:references added, v3: minor changes, typos
corrected, references adde
Two-loop Yang-Mills diagrams from superstring amplitudes
Starting from the superstring amplitude describing interactions among
D-branes with a constant world-volume field strength, we present a detailed
analysis of how the open string degeneration limits reproduce the corresponding
field theory Feynman diagrams. A key ingredient in the string construction is
represented by the twisted (Prym) super differentials, as their periods encode
the information about the background field. We provide an efficient method to
calculate perturbatively the determinant of the twisted period matrix in terms
of sets of super-moduli appropriate to the degeneration limits. Using this
result we show that there is a precise one-to-one correspondence between the
degeneration of different factors in the superstring amplitudes and
one-particle irreducible Feynman diagrams capturing the gauge theory effective
action at the two-loop level.Comment: 42 pages plus appendices, 10 figure
Nonperturbative SUSY Correlators at Finite Temperature
We calculate finite temperature effects on a correlation function in the two
dimensional supersymmetric nonlinear O(3) sigma model. The correlation function
violates chiral symmetry and at zero temperature it has been shown to be a
constant, which gives rise to a double-valued condensate. Within the bilinear
approximation we find an exact result in a one-instanton background at finite
temperature. In contrast to the result at zero temperature we find that the
correlation function decays exponentially at large distances.Comment: Latex, 27 pages, 1 Postscript figur
Noise, emissions and costs trade factors for regional jet platforms using a new software for aircraft preliminary design
A multidisciplinary analysis approach plays a very important role in the development of future transport aircraft, being able to interconnect all aircraft-related subjects and suppliers. A major issue, which has prevented aircraft manufacturers from implementing efficient and cost-effective design processes, is the loose integration of engine models into iterative aircraft design workflows. The continuous improvement of computer calculation capabilities over years has allowed the growth of a large family of software dedicated to aircraft preliminary design activities concerning also multi-disciplinary analyses, and optimizations. In this context, a new software for aircraft preliminary design, multi-disciplinary analyses and optimizations named JPAD (Java toolchain of Programs for Aircraft Design) has been developed at the University of Naples Federico II. The main purpose of this paper is to show the capabilities of the JPAD software applied to typical preliminary design problems. Thus, results of the activities carried out by means of JPAD in the scope of the Work Package 2 (WP2) of the European CleanSky2 project ADORNO will be shown. Those will concern trade factors and response surfaces related to environmental noise, DOC, and pollutant emissions (linked to the design mission block fuel) for a Rear-Mounted engines (RM) reference 2014 aircraft configuration
Spacelike Branes
Scalar field theories with appropriate potentials in Minkowski space can have
time-dependent classical solutions containing topological defects which
correspond to S-branes - i.e. branes all of whose tangential dimensions are
spacelike. It is argued that such S-branes arise in string theory as
time-dependent solutions of the worldvolume tachyon field of an unstable
D-brane or D-brane-anti-D-brane pair. Using the known coupling of the spacetime
RR fields to the worldvolume tachyon it is shown that these S-branes carry a
charge, defined as the integral of a RR field strength over a sphere
(containing a time as well as spatial dimensions) surrounding the S-brane. This
same charge is carried by SD-branes, i.e. Dirichlet branes arising from open
string worldsheet conformal field theories with a Dirichlet boundary condition
on the timelike dimension. The corresponding SD-brane boundary state is
constructed. Supergravity solutions carrying the same charges are also found
for a few cases.Comment: 23 pages, harvmac(b), no figures, v2 references added and minor
changes, v4: more references adde
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