2,108 research outputs found
Current-density functional theory of time-dependent linear response in quantal fluids: recent progress
Vignale and Kohn have recently formulated a local density approximation to
the time-dependent linear response of an inhomogeneous electron system in terms
of a vector potential for exchange and correlation. The vector potential
depends on the induced current density through spectral kernels to be evaluated
on the homogeneous electron-gas. After a brief review of their theory, the case
of inhomogeneous Bose superfluids is considered, with main focus on dynamic
Kohn-Sham equations for the condensate in the linear response regime and on
quantal generalized hydrodynamic equations in the weak inhomogeneity limit. We
also present the results of calculations of the exchange-correlation spectra in
both electron and superfluid boson systems.Comment: 12 pages, 2 figures, Postscript fil
Gluon Scattering Amplitudes in Finite Temperature Gauge/Gravity Dualities
We examine the gluon scattering amplitude in N=4 super Yang-Mills at finite
temperature with nonzero R-charge densities, and in Non-Commutative gauge
theory at finite temperature. The gluon scattering amplitude is defined as a
light-like Wilson loop which lives at the horizon of the T-dual black holes of
the backgrounds we consider. We study in detail a special amplitude, which
corresponds to forward scattering of a low energy gluon off a high energy one.
For this kinematic configuration in the considered backgrounds, we find the
corresponding minimal surface which is directly related to the gluon scattering
amplitude. We find that for increasing the chemical potential or the
non-commutative parameter, the on-shell action corresponding to our Wilson loop
in the T-dual space decreases. For all of our solutions the length of the short
side of the Wilson loop is constrained by an upper bound which depends on the
temperature, the R-charge density and the non-commutative parameter. Due to
this constraint, in the limit of zeroth temperature our approach breaks down
since the upper bound goes to zero, while by keeping the temperature finite and
letting the chemical potential or the non-commutative parameter to approach to
zero the limit is smooth.Comment: 30 pages, 16 figures, minor corrections (plus improved numerical
computation for the non-commutative case
The Effects of Twitter Sentiment on Stock Price Returns
Social media are increasingly reflecting and influencing behavior of other
complex systems. In this paper we investigate the relations between a well-know
micro-blogging platform Twitter and financial markets. In particular, we
consider, in a period of 15 months, the Twitter volume and sentiment about the
30 stock companies that form the Dow Jones Industrial Average (DJIA) index. We
find a relatively low Pearson correlation and Granger causality between the
corresponding time series over the entire time period. However, we find a
significant dependence between the Twitter sentiment and abnormal returns
during the peaks of Twitter volume. This is valid not only for the expected
Twitter volume peaks (e.g., quarterly announcements), but also for peaks
corresponding to less obvious events. We formalize the procedure by adapting
the well-known "event study" from economics and finance to the analysis of
Twitter data. The procedure allows to automatically identify events as Twitter
volume peaks, to compute the prevailing sentiment (positive or negative)
expressed in tweets at these peaks, and finally to apply the "event study"
methodology to relate them to stock returns. We show that sentiment polarity of
Twitter peaks implies the direction of cumulative abnormal returns. The amount
of cumulative abnormal returns is relatively low (about 1-2%), but the
dependence is statistically significant for several days after the events
Wilson loops stability in the gauge/string correspondence
We study the stability of some classical string worldsheet solutions employed
for computing the potential energy between two static fundamental quarks in
confining and non-confining gravity duals. We discuss the fixing of the
diffeomorphism invariance of the string action, its relation with the
fluctuation orientation and the interpretation of the quark mass substraction
worldsheet needed for computing the potential energy in smooth (confining)
gravity background. We consider various dual gravity backgrounds and show by a
numerical analysis the existence of instabilities under linear fluctuations for
classical string embedding solutions having positive length function derivative
. Finally we make a brief discussion of 't Hooft loops in
non-conformal backgrounds.Comment: 34 pages, 36 figures. Reference added. Final version JHEP accepte
Exact Results and Holography of Wilson Loops in N=2 Superconformal (Quiver) Gauge Theories
Using localization, matrix model and saddle-point techniques, we determine
exact behavior of circular Wilson loop in N=2 superconformal (quiver) gauge
theories. Focusing at planar and large `t Hooft couling limits, we compare its
asymptotic behavior with well-known exponential growth of Wilson loop in N=4
super Yang-Mills theory. For theory with gauge group SU(N) coupled to 2N
fundamental hypermultiplets, we find that Wilson loop exhibits non-exponential
growth -- at most, it can grow a power of `t Hooft coupling. For theory with
gauge group SU(N) x SU(N) and bifundamental hypermultiplets, there are two
Wilson loops associated with two gauge groups. We find Wilson loop in untwisted
sector grows exponentially large as in N=4 super Yang-Mills theory. We then
find Wilson loop in twisted sector exhibits non-analytic behavior with respect
to difference of two `t Hooft coupling constants. By letting one gauge coupling
constant hierarchically larger/smaller than the other, we show that Wilson
loops in the second type theory interpolate to Wilson loop in the first type
theory. We infer implications of these findings from holographic dual
description in terms of minimal surface of dual string worldsheet. We suggest
intuitive interpretation that in both type theories holographic dual background
must involve string scale geometry even at planar and large `t Hooft coupling
limit and that new results found in the gauge theory side are attributable to
worldsheet instantons and infinite resummation therein. Our interpretation also
indicate that holographic dual of these gauge theories is provided by certain
non-critical string theories.Comment: 52 pages, 7 figures v2. more figures embedded v3. minor stylistic
changes, v4. published versio
Revealed Preference Dimension via Matrix Sign Rank
Given a data-set of consumer behaviour, the Revealed Preference Graph
succinctly encodes inferred relative preferences between observed outcomes as a
directed graph. Not all graphs can be constructed as revealed preference graphs
when the market dimension is fixed. This paper solves the open problem of
determining exactly which graphs are attainable as revealed preference graphs
in -dimensional markets. This is achieved via an exact characterization
which closely ties the feasibility of the graph to the Matrix Sign Rank of its
signed adjacency matrix. The paper also shows that when the preference
relations form a partially ordered set with order-dimension , the graph is
attainable as a revealed preference graph in a -dimensional market.Comment: Submitted to WINE `1
BPS States in Omega Background and Integrability
We reconsider string and domain wall central charges in N=2 supersymmetric
gauge theories in four dimensions in presence of the Omega background in the
Nekrasov-Shatashvili (NS) limit. Existence of these charges entails presence of
the corresponding topological defects in the theory - vortices and domain
walls. In spirit of the 4d/2d duality we discuss the worldsheet low energy
effective theory living on the BPS vortex in N=2 Supersymmetric Quantum
Chromodynamics (SQCD). We discuss some aspects of the brane realization of the
dualities between various quantum integrable models. A chain of such dualities
enables us to check the AGT correspondence in the NS limit.Comment: 48 pages, 10 figures, minor changes, references added, typos
correcte
Causality violation, gravitational shockwaves and UV completion
The effective actions describing the low-energy dynamics of QFTs involving
gravity generically exhibit causality violations. These may take the form of
superluminal propagation or Shapiro time advances and allow the construction of
"time machines", i.e. spacetimes admitting closed non-spacelike curves. Here,
we discuss critically whether such causality violations may be used as a
criterion to identify unphysical effective actions or whether, and how,
causality problems may be resolved by embedding the action in a fundamental, UV
complete QFT. We study in detail the case of photon scattering in an
Aichelburg-Sexl gravitational shockwave background and calculate the phase
shifts in QED for all energies, demonstrating their smooth interpolation from
the causality-violating effective action values at low-energy to their
manifestly causal high-energy limits. At low energies, these phase shifts may
be interpreted as backwards-in-time coordinate jumps as the photon encounters
the shock wavefront, and we illustrate how the resulting causality problems
emerge and are resolved in a two-shockwave time machine scenario. The
implications of our results for ultra-high (Planck) energy scattering, in which
graviton exchange is modelled by the shockwave background, are highlighted.Comment: 42 pages, 15 figures, updated reference
Automatic Network Fingerprinting through Single-Node Motifs
Complex networks have been characterised by their specific connectivity
patterns (network motifs), but their building blocks can also be identified and
described by node-motifs---a combination of local network features. One
technique to identify single node-motifs has been presented by Costa et al. (L.
D. F. Costa, F. A. Rodrigues, C. C. Hilgetag, and M. Kaiser, Europhys. Lett.,
87, 1, 2009). Here, we first suggest improvements to the method including how
its parameters can be determined automatically. Such automatic routines make
high-throughput studies of many networks feasible. Second, the new routines are
validated in different network-series. Third, we provide an example of how the
method can be used to analyse network time-series. In conclusion, we provide a
robust method for systematically discovering and classifying characteristic
nodes of a network. In contrast to classical motif analysis, our approach can
identify individual components (here: nodes) that are specific to a network.
Such special nodes, as hubs before, might be found to play critical roles in
real-world networks.Comment: 16 pages (4 figures) plus supporting information 8 pages (5 figures
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