177 research outputs found
Accretion Disc Theory: From the Standard Model Until Advection
Accretion disc theory was first developed as a theory with the local heat
balance, where the whole energy produced by a viscous heating was emitted to
the sides of the disc. One of the most important new invention of this theory
was a phenomenological treatment of the turbulent viscosity, known as ''alpha''
prescription, when the (r) component of the stress tensor was
approximated by ( P) with a unknown constant . This
prescription played the role in the accretion disc theory as well important as
the mixing-length theory of convection for stellar evolution. Sources of
turbulence in the accretion disc are discussed, including nonlinear
hydrodynamical turbulence, convection and magnetic field role. In parallel to
the optically thick geometrically thin accretion disc models, a new branch of
the optically thin accretion disc models was discovered, with a larger
thickness for the same total luminosity. The choice between these solutions
should be done of the base of a stability analysis. The ideas underlying the
necessity to include advection into the accretion disc theory are presented and
first models with advection are reviewed. The present status of the solution
for a low-luminous optically thin accretion disc model with advection is
discussed and the limits for an advection dominated accretion flows (ADAF)
imposed by the presence of magnetic field are analysed.Comment: Roceeding of the Int. Workshop "Observational Evidence for Black
Holes in the Universe". Calcutta, 11-17 January 1998. Kluwer Acad. Pu
A slice of AdS_5 as the large N limit of Seiberg duality
A slice of AdS_5 is used to provide a 5D gravitational description of 4D
strongly-coupled Seiberg dual gauge theories. An (electric) SU(N) gauge theory
in the conformal window at large N is described by the 5D bulk, while its
weakly coupled (magnetic) dual is confined to the IR brane. This framework can
be used to construct an N = 1 MSSM on the IR brane, reminiscent of the original
Randall-Sundrum model. In addition, we use our framework to study
strongly-coupled scenarios of supersymmetry breaking mediated by gauge forces.
This leads to a unified scenario that connects the extra-ordinary gauge
mediation limit to the gaugino mediation limit in warped space.Comment: 47 Pages, axodraw4j.st
Finite-Temperature Fractional D2-Branes and the Deconfinement Transition in 2+1 Dimensions
The supergravity dual to N regular and M fractional D2-branes on the cone
over \mathbb{CP}^3 has a naked singularity in the infrared. One can resolve
this singularity and obtain a regular fractional D2-brane solution dual to a
confining 2+1 dimensional N = 1 supersymmetric field theory. The confining
vacuum of this theory is described by the solution of Cvetic, Gibbons, Lu and
Pope. In this paper, we explore the alternative possibility for resolving the
singularity - the creation of a regular horizon. The black-hole solution we
find corresponds to the deconfined phase of this dual gauge theory in three
dimensions. This solution is derived in perturbation theory in the number of
fractional branes. We argue that there is a first-order deconfinement
transition. Connections to Chern--Simons matter theories, the ABJM proposal and
fractional M2-branes are presented.Comment: v3: analytic solutions are expose
Theoretical and Phenomenological Constraints on Form Factors for Radiative and Semi-Leptonic B-Meson Decays
We study transition form factors for radiative and rare semi-leptonic B-meson
decays into light pseudoscalar or vector mesons, combining theoretical
constraints and phenomenological information from Lattice QCD, light-cone sum
rules, and dispersive bounds. We pay particular attention to form factor
parameterisations which are based on the so-called series expansion, and study
the related systematic uncertainties on a quantitative level. In this context,
we also provide the NLO corrections to the correlation function between two
flavour-changing tensor currents, which enters the unitarity constraints for
the coefficients in the series expansion.Comment: 52 pages; v2: normalization error in (29ff.) corrected, conclusion
about relevance of unitarity bounds modified; form factor fits unaffected;
references added; v3: discussion on truncation of series expansion added,
matches version to be published in JHEP; v4: corrected typos in Tables 5 and
Quantum Symmetries and Marginal Deformations
We study the symmetries of the N=1 exactly marginal deformations of N=4 Super
Yang-Mills theory. For generic values of the parameters, these deformations are
known to break the SU(3) part of the R-symmetry group down to a discrete
subgroup. However, a closer look from the perspective of quantum groups reveals
that the Lagrangian is in fact invariant under a certain Hopf algebra which is
a non-standard quantum deformation of the algebra of functions on SU(3). Our
discussion is motivated by the desire to better understand why these theories
have significant differences from N=4 SYM regarding the planar integrability
(or rather lack thereof) of the spin chains encoding their spectrum. However,
our construction works at the level of the classical Lagrangian, without
relying on the language of spin chains. Our approach might eventually provide a
better understanding of the finiteness properties of these theories as well as
help in the construction of their AdS/CFT duals.Comment: 1+40 pages. v2: minor clarifications and references added. v3: Added
an appendix, fixed minor typo
Theory of disk accretion onto supermassive black holes
Accretion onto supermassive black holes produces both the dramatic phenomena
associated with active galactic nuclei and the underwhelming displays seen in
the Galactic Center and most other nearby galaxies. I review selected aspects
of the current theoretical understanding of black hole accretion, emphasizing
the role of magnetohydrodynamic turbulence and gravitational instabilities in
driving the actual accretion and the importance of the efficacy of cooling in
determining the structure and observational appearance of the accretion flow.
Ongoing investigations into the dynamics of the plunging region, the origin of
variability in the accretion process, and the evolution of warped, twisted, or
eccentric disks are summarized.Comment: Mostly introductory review, to appear in "Supermassive black holes in
the distant Universe", ed. A.J. Barger, Kluwer Academic Publishers, in pres
Foundations of Black Hole Accretion Disk Theory
This review covers the main aspects of black hole accretion disk theory. We
begin with the view that one of the main goals of the theory is to better
understand the nature of black holes themselves. In this light we discuss how
accretion disks might reveal some of the unique signatures of strong gravity:
the event horizon, the innermost stable circular orbit, and the ergosphere. We
then review, from a first-principles perspective, the physical processes at
play in accretion disks. This leads us to the four primary accretion disk
models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin)
disks, slim disks, and advection-dominated accretion flows (ADAFs). After
presenting the models we discuss issues of stability, oscillations, and jets.
Following our review of the analytic work, we take a parallel approach in
reviewing numerical studies of black hole accretion disks. We finish with a few
select applications that highlight particular astrophysical applications:
measurements of black hole mass and spin, black hole vs. neutron star accretion
disks, black hole accretion disk spectral states, and quasi-periodic
oscillations (QPOs).Comment: 91 pages, 23 figures, final published version available at
http://www.livingreviews.org/lrr-2013-
Exciton properties in zincblende InGaN-GaN quantum wells under the effects of intense laser fields
ABSTRACT: In this work, we study the exciton states in a zincblende InGaN/GaN quantum well using a variational technique. The system is considered under the action of intense laser fields with the incorporation of a direct current electric field as an additional external probe. The effects of these external influences as well as of the changes in the geometry of the heterostructure on the exciton binding energy are discussed in detail
Collaborative development of predictive toxicology applications
OpenTox provides an interoperable, standards-based Framework for the support of predictive toxicology data management, algorithms, modelling, validation and reporting. It is relevant to satisfying the chemical safety assessment requirements of the REACH legislation as it supports access to experimental data, (Quantitative) Structure-Activity Relationship models, and toxicological information through an integrating platform that adheres to regulatory requirements and OECD validation principles. Initial research defined the essential components of the Framework including the approach to data access, schema and management, use of controlled vocabularies and ontologies, architecture, web service and communications protocols, and selection and integration of algorithms for predictive modelling. OpenTox provides end-user oriented tools to non-computational specialists, risk assessors, and toxicological experts in addition to Application Programming Interfaces (APIs) for developers of new applications. OpenTox actively supports public standards for data representation, interfaces, vocabularies and ontologies, Open Source approaches to core platform components, and community-based collaboration approaches, so as to progress system interoperability goals
On the structure of quadrilateral brane tilings
Brane tilings provide the most general framework in string and M-theory for
matching toric Calabi-Yau singularities probed by branes with superconformal
fixed points of quiver gauge theories. The brane tiling data consists of a
bipartite tiling of the torus which encodes both the classical superpotential
and gauge-matter couplings for the quiver gauge theory. We consider the class
of tilings which contain only tiles bounded by exactly four edges and present a
method for generating any tiling within this class by iterating combinations of
certain graph-theoretic moves. In the context of D3-branes in IIB string
theory, we consider the effect of these generating moves within the
corresponding class of supersymmetric quiver gauge theories in four dimensions.
Of particular interest are their effect on the superpotential, the vacuum
moduli space and the conditions necessary for the theory to reach a
superconformal fixed point in the infrared. We discuss the general structure of
physically admissible quadrilateral brane tilings and Seiberg duality in terms
of certain composite moves within this class.Comment: 57 pages, 22 figure
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