833 research outputs found
NS5-branes on an ellipsis and novel marginal deformations with parafermions
We consider NS5-branes distributed along the circumference of an ellipsis and
explicitly construct the corresponding gravitational background. This provides
a continuous line of deformations between the limiting cases, considered
before, in which the ellipsis degenerates into a circle or into a bar. We show
that a slight deformation of the background corresponding to a circle
distribution into an ellipsoidal one is described by a novel non-factorizable
marginal perturbation of bilinears of dressed parafermions. The latter are
naturally defined for the circle case since, as it was shown in the past, the
background corresponds to an orbifold of the exact conformal field theory coset
model SU(2)/U(1) times SL(2,R)/U(1). We explore the possibility to define
parafermionic objects at generic points of the ellipsoidal families of
backgrounds away from the circle point. We also discuss a new limiting case in
which the ellipsis degenerates into two infinitely stretched parallel bars and
show that the background is related to the Eguchi-Hanson metric, via T-duality.Comment: 24 page
G3-homogeneous gravitational instantons
We provide an exhaustive classification of self-dual four-dimensional
gravitational instantons foliated with three-dimensional homogeneous spaces,
i.e. homogeneous self-dual metrics on four-dimensional Euclidean spaces
admitting a Bianchi simply transitive isometry group. The classification
pattern is based on the algebra homomorphisms relating the Bianchi group and
the duality group SO(3). New and general solutions are found for Bianchi III.Comment: 24 pages, few correction
Comment on superluminality in general relativity
General relativity provides an appropriate framework for addressing the issue
of sub- or superluminality as an apparent effect. Even though a massless
particle travels on the light cone, its average velocity over a finite path
measured by different observers is not necessarily equal to the velocity of
light, as a consequence of the time dilation or contraction in gravitational
fields. This phenomenon occurs in either direction (increase or depletion)
irrespectively of the details and strength of the gravitational interaction.
Hence, it does not intrinsically guarantee superluminality, even when the
gravitational field is reinforced.Comment: 6 page
Three-dimensional black holes from deformed anti-de Sitter
We present new exact three-dimensional black-string backgrounds, which
contain both NS--NS and electromagnetic fields, and generalize the BTZ black
holes and the black string studied by Horne and Horowitz. They are obtained as
deformations of the Sl(2,R) WZW model. Black holes resulting from purely
continuous deformations possess true curvature singularities. When discrete
identifications are introduced, extra chronological singularities appear, which
under certain circumstances turn out to be naked. The backgrounds at hand
appear in the moduli space of the Sl(2,R) WZW model. Hence, they provide exact
string backgrounds and allow for a more algebraical CFT description. This makes
possible the determination of the spectrum of primaries.Comment: JHEP style, 33 pages, 1 figur
The role of electromagnetic trapped modes in extraordinary transmission in nanostructured materials
We assert that the physics underlying the extraordinary light transmission
(reflection) in nanostructured materials can be understood from rather general
principles based on the formal scattering theory developed in quantum
mechanics. The Maxwell equations in passive (dispersive and absorptive) linear
media are written in the form of the Schr\"{o}dinger equation to which the
quantum mechanical resonant scattering theory (the Lippmann-Schwinger
formalism) is applied. It is demonstrated that the existence of long-lived
quasistationary eigenstates of the effective Hamiltonian for the Maxwell theory
naturally explains the extraordinary transmission properties observed in
various nanostructured materials. Such states correspond to quasistationary
electromagnetic modes trapped in the scattering structure. Our general approach
is also illustrated with an example of the zero-order transmission of the
TE-polarized light through a metal-dielectric grating structure. Here a direct
on-the-grid solution of the time-dependent Maxwell equations demonstrates the
significance of resonances (or trapped modes) for extraordinary light
transmissioComment: 14 pages, 6 figures; Discussion in Section 4 expanded; typos
corrected; a reference added; Figure 4 revise
Boundary States for D-branes in
We construct boundary states representing D-strings in . These wrap
twisted conjugacy classes of SL(2,R), and the boundary states are therefore
based on continuous representations only. We check Cardy's condition and find a
consistent open string spectrum. The open string spectrum on all the D-branes
is the same.Comment: 23 pages, late
The BRST quantization and the no-ghost theorem for AdS_3
In our previous papers, we prove the no-ghost theorem without light-cone
directions (hep-th/0005002, hep-th/0303051). We point out that our results are
valid for more general backgrounds. In particular, we prove the no-ghost
theorem for AdS_3 in the context of the BRST quantization (with the standard
restriction on the spin). We compare our BRST proof with the OCQ proof and
establish the BRST-OCQ equivalence for AdS_3. The key in both approaches lies
in the certain structure of the matter Hilbert space as a product of two Verma
modules. We also present the no-ghost theorem in the most general form.Comment: 22 pages, JHEP and AMS-LaTeX; v2 & 3: minor improvement
Reduction of Low-Thrust Continuous Controls for Trajectory Dynamics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76670/1/AIAA-40619-128.pd
Non-local string theories on AdS_3 times S^3 and stable non-supersymmetric backgrounds
We exhibit a simple class of exactly marginal "double-trace" deformations of
two dimensional CFTs which have AdS_3 duals, in which the deformation is given
by a product of left and right-moving U(1) currents. In this special case the
deformation on AdS_3 is generated by a local boundary term in three dimensions,
which changes the physics also in the bulk via bulk-boundary propagators.
However, the deformation is non-local in six dimensions and on the string
worldsheet, like generic non-local string theories (NLSTs). Due to the
simplicity of the deformation we can explicitly make computations in the
non-local string theory and compare them to CFT computations, and we obtain
precise agreement. We discuss the effect of the deformation on closed strings
and on D-branes. The examples we analyze include a supersymmetry-breaking but
exactly marginal "double-trace" deformation, which is dual to a string theory
in which no destabilizing tadpoles are generated for moduli nonperturbatively
in all couplings, despite the absence of supersymmetry. We explain how this
cancellation works on the gravity side in string perturbation theory, and also
non-perturbatively at leading order in the deformation parameter. We also
discuss possible flat space limits of our construction.Comment: 40 pages, 6 figures, harvma
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