22,167 research outputs found
BMN operators with vector impurities, Z_2 symmetry and pp-waves
We calculate the coefficients of three-point functions of BMN operators with
two vector impurities. We find that these coefficients can be obtained from
those of the three-point functions of scalar BMN operators by interchanging the
coefficient for the symmetric-traceless representation with the coefficient for
the singlet. We conclude that the Z_2 symmetry of the pp-wave string theory is
not manifest at the level of field theory three-point correlators.Comment: 25 pages, 7 figures. v1: A reference and a footnote added; v2: New
contributions found, Z_2 symmetry lost in 3-point function
Local Commutativity and Causality in Interacting PP-wave String Field Theory
In this paper, we extend our previous study of causality and local
commutativity of string fields in the pp-wave lightcone string field theory to
include interaction. Contrary to the flat space case result of Lowe,
Polchinski, Susskind, Thorlacius and Uglum, we found that the pp-wave
interaction does not affect the local commutativity condition. Our results show
that the pp-wave lightcone string field theory is not continuously connected
with the flat space one. We also discuss the relation between the condition of
local commutativity and causality. While the two notions are closely related in
a point particle theory, their relation is less clear in string theory. We
suggest that string local commutativity may be relevant for an operational
defintion of causality using strings as probes.Comment: Latex, JHEP3.cls, 18 pages, no figures. v2: add comments about the
UV-IR mixing effect displayed in our result. version to appear in JHE
Conformal Dimensions of Two-Derivative BMN Operators
We compute the anomalous dimensions of BMN operators with two covariant
derivative impurities at the planar level up to first order in the effective
coupling lambda'. The result equals those for two scalar impurities as well as
for mixed scalar and vector impurities given in the literature. Though the
results are the same, the computation is very different from the scalar case.
This is basically due to the existence of a non-vanishing overlap between the
derivative impurity and the ``background'' field Z. We present details of these
differences and their consequences.Comment: 27 pages, v2: references added, minor change
D-branes with Lorentzian signature in the Nappi-Witten model
Lorentzian signature D-branes of all dimensions for the Nappi-Witten string
are constructed. This is done by rewriting the gluing condition for
the model chiral currents on the brane as a well posed first order differential
problem and by solving it for Lie algebra isometries other than Lie algebra
automorphisms. By construction, these D-branes are not twined conjugacy
classes. Metrically degenerate D-branes are also obtained.Comment: 22 page
Operator mixing and three-point functions in N=4 SYM
We study the three-point functions between two BPS and one non-BPS local
gauge invariant operators in N=4 Super Yang-Mills theory. In particular we
show, in explicit 1-loop examples, that the operator mixing discussed in
arXiv:0810.0499 plays an important role in the computations of the correlators
and is necessary to cancel contributions that would violate the constraints
following from the superconformal and the bonus U(1)_Y symmetries. We analyse
the same type of correlators also at strong coupling by using the BMN limit of
the AdS_5xS^5 string theory. Again the mixing between states with different
types of impurities is crucial to ensure the cancellation of various amplitudes
that would violate the constraints mentioned above. However, on the string
side, we also find some examples of interactions between one non-BPS and two
BPS states that do not satisfy expectations based on the superconformal and the
bonus U(1)_Y symmetries.Comment: 28 pages, 5 figure
Chiral primary cubic interactions from pp-wave supergravity
We explicitly construct cubic interaction light-cone Hamiltonian for the
chiral primary system involving the metric fields and the self-dual four-form
fields in the IIB pp-wave supergravity. The background fields representing
pp-waves exhibit SO(4)*SO(4)*Z_2 invariance. It turns out that the interaction
Hamiltonian is precisely the same as that for the dilaton-axion system, except
for the fact that the chiral primary system fields have the opposite parity to
that of the dilaton-axion fields under the Z_2 transformation that exchanges
two SO(4)'s.Comment: 14 pages, A few comments are adde
Raman scattering study of (KSr)FeAs ( = 0.0, 0.4)
Polarized Raman spectra of non-superconducting SrFeAs and
superconducting KSrFeAs ( K) are reported.
All four phonon modes (A + B + 2E) allowed by symmetry, are
found and identified. Shell model gives reasonable description of the spectra.
No detectable anomalies are observed near the tetragonal-to-orthorhombic
transition in SrFeAs or the superconducting transition in
KSrFeAs.Comment: 4 pages, 4 figures, 2 table
Geometric construction of D-branes in WZW models
The geometric description of D-branes in WZW models is pushed forward. Our
starting point is a gluing condition\, that matches the model's
chiral currents at the worldsheet boundary through a linear map acting on
the WZW Lie algebra. The equivalence of boundary and gluing conditions of this
type is studied in detail. The analysis involves a thorough discussion of
Frobenius integrability, shows that must be an isometry, and applies to
both metrically degenerate and nondegenerate D-branes. The isometry need
not be a Lie algebra automorphism nor constantly defined over the brane. This
approach, when applied to isometries of the form with a constant Lie
algebra automorphism, validates metrically degenerate -twined conjugacy
classes as D-branes. It also shows that no D-branes exist in semisimple WZW
models for constant\, .Comment: 23 pages, discussion of limitations of the gluing condition approach
adde
Random on-board pixel sampling (ROPS) X-ray Camera
Recent advances in compressed sensing theory and algorithms offer new
possibilities for high-speed X-ray camera design. In many CMOS cameras, each
pixel has an independent on-board circuit that includes an amplifier, noise
rejection, signal shaper, an analog-to-digital converter (ADC), and optional
in-pixel storage. When X-ray images are sparse, i.e., when one of the following
cases is true: (a.) The number of pixels with true X-ray hits is much smaller
than the total number of pixels; (b.) The X-ray information is redundant; or
(c.) Some prior knowledge about the X-ray images exists, sparse sampling may be
allowed. Here we first illustrate the feasibility of random on-board pixel
sampling (ROPS) using an existing set of X-ray images, followed by a discussion
about signal to noise as a function of pixel size. Next, we describe a possible
circuit architecture to achieve random pixel access and in-pixel storage. The
combination of a multilayer architecture, sparse on-chip sampling, and
computational image techniques, is expected to facilitate the development and
applications of high-speed X-ray camera technology.Comment: 9 pages, 6 figures, Presented in 19th iWoRI
UV-divergences of Wilson Loops for Gauge/Gravity Duality
We analyze the structure of the UV divergences of the Wilson loop for a
general gauge/gravity duality. We find that, due to the presence of a
nontrivial NSNS B-field and metric, new divergences that cannot be subtracted
out by the conventional Legendre transform may arise. We also derive conditions
on the B-field and the metric, which when satisfied, the leading UV divergence
will become linear, and can be cancelled out by choosing the boundary condition
of the string appropriately. Our results, together with the recent result of
arXiv:0807.5127, where the effect of a nontrivial dilaton on the structure of
UV divergences in Wilson loop is analysed, allow us to conclude that Legendre
transform is at best capable of cancelling the linear UV divergences arising
from the area of the worldsheet, but is incapable to handle the divergences
associated with the dilaton or the B-field in general. We also solve the
conditions for the cancellation of the leading linear divergences generally and
find that many well-known supergravity backgrounds are of these kinds,
including examples such as the Sakai-Sugimoto QCD model or N=1 duality with
Sasaki-Einstein spaces. We also point out that Wilson loop in the
Klebanov-Strassler background have a divergence associated with the B-field
which cannot be cancelled away with the Legendre transform. Finally we end with
some comments on the form of the Wilson loop operator in the ABJM
superconformal Chern-Simons theory.Comment: 26 pages. LaTeX. v2: reference added. version to appear in JHE
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