1,272 research outputs found
Generation of vortices and observation of Quantum Turbulence in an oscillating Bose-Einstein Condensate
We report on the experimental observation of vortex formation and production
of tangled vortex distribution in an atomic BEC of Rb-87 atoms submitted to an
external oscillatory perturbation. The oscillatory perturbations start by
exciting quadrupolar and scissors modes of the condensate. Then regular
vortices are observed finally evolving to a vortex tangle configuration. The
vortex tangle is a signature of the presence of a turbulent regime in the
cloud. We also show that this turbulent cloud has suppression of the aspect
ratio inversion typically observed in quantum degenerate bosonic gases during
free expansion.Comment: to appear in JLTP - QFS 200
The one-loop six-dimensional hexagon integral and its relation to MHV amplitudes in N=4 SYM
We provide an analytic formula for the (rescaled) one-loop scalar hexagon
integral with all external legs massless, in terms of classical
polylogarithms. We show that this integral is closely connected to two
integrals appearing in one- and two-loop amplitudes in planar
super-Yang-Mills theory, and . The derivative of
with respect to one of the conformal invariants yields
, while another first-order differential operator applied to
yields . We also introduce some kinematic
variables that rationalize the arguments of the polylogarithms, making it easy
to verify the latter differential equation. We also give a further example of a
six-dimensional integral relevant for amplitudes in
super-Yang-Mills.Comment: 18 pages, 2 figure
Three-vortex configurations in trapped Bose-Einstein condensates
We report on the creation of three-vortex clusters in a
Bose-Einstein condensate by oscillatory excitation of the condensate. This
procedure can create vortices of both circulation, so that we are able to
create several types of vortex clusters using the same mechanism. The
three-vortex configurations are dominated by two types, namely, an
equilateral-triangle arrangement and a linear arrangement. We interpret these
most stable configurations respectively as three vortices with the same
circulation, and as a vortex-antivortex-vortex cluster. The linear
configurations are very likely the first experimental signatures of predicted
stationary vortex clusters.Comment: 4 pages, 4 figure
Amplitudes at Weak Coupling as Polytopes in AdS_5
We show that one-loop scalar box functions can be interpreted as volumes of
geodesic tetrahedra embedded in a copy of AdS_5 that has dual conformal
space-time as boundary. When the tetrahedron is space-like, it lies in a
totally geodesic hyperbolic three-space inside AdS_5, with its four vertices on
the boundary. It is a classical result that the volume of such a tetrahedron is
given by the Bloch-Wigner dilogarithm and this agrees with the standard physics
formulae for such box functions. The combinations of box functions that arise
in the n-particle one-loop MHV amplitude in N=4 super Yang-Mills correspond to
the volume of a three-dimensional polytope without boundary, all of whose
vertices are attached to a null polygon (which in other formulations is
interpreted as a Wilson loop) at infinity.Comment: 16 pages, 5 figure
Self-similar expansion of the density profile in a turbulent Bose-Einstein condensate
In a recent study we demonstrated the emergence of turbulence in a trapped
Bose-Einstein condensate of Rb-87 atoms. An intriguing observation in such a
system is the behavior of the turbulent cloud during free expansion.The aspect
ratio of the cloud size does not change in the way one would expect for an
ordinary non-rotating (vortex-free) condensate. Here we show that the anomalous
expansion can be understood, at least qualitatively, in terms of the presence
of vorticity distributed throughout the cloud, effectively counteracting the
usual reversal of the aspect ratio seen in free time-of-flight expansion of
non-rotating condensates.Comment: 8 pages, 4 figure
Hidden Simplicity of Gauge Theory Amplitudes
These notes were given as lectures at the CERN Winter School on Supergravity,
Strings and Gauge Theory 2010. We describe the structure of scattering
amplitudes in gauge theories, focussing on the maximally supersymmetric theory
to highlight the hidden symmetries which appear. Using the BCFW recursion
relations we solve for the tree-level S-matrix in N=4 super Yang-Mills theory,
and describe how it produces a sum of invariants of a large symmetry algebra.
We review amplitudes in the planar theory beyond tree-level, describing the
connection between amplitudes and Wilson loops, and discuss the implications of
the hidden symmetries.Comment: 46 pages, 15 figures. v2 ref added, typos fixe
Synaptic abnormalities in the infralimbic cortex of a model of congenital depression
Multiple lines of evidence suggest that disturbances in excitatory transmission contribute to depression. Whether these defects involve the number, size, or composition of glutamatergic contacts is unclear. This study used recently introduced procedures for fluorescence deconvolution tomography in a well-studied rat model of congenital depression to characterize excitatory synapses in layer I of infralimbic cortex, a region involved in mood disorders, and of primary somatosensory cortex. Three groups were studied: (1) rats bred for learned helplessness (cLH); (2) rats resistant to learned helplessness (cNLH); and (3) control Sprague Dawley rats. In fields within infralimbic cortex, cLH rats had the same numerical density of synapses, immunolabeled for either the postsynaptic density (PSD) marker PSD95 or the presynaptic protein synaptophysin, as controls. However, PSD95 immunolabeling intensities were substantially lower in cLH rats, as were numerical densities of synapse-sized clusters of the AMPA receptor subunit GluA1. Similar but less pronounced differences (comparable numerical densities but reduced immunolabeling intensity for PSD95) were found in the somatosensory cortex. In contrast, non-helpless rats had 25% more PSDs than either cLH or control rats without any increase in synaptophysin-labeled terminal frequency. Compared with controls, both cLH and cNLH rats had fewer GABAergic contacts. These results indicate that congenital tendencies that increase or decrease depression-like behavior differentially affect excitatory synapses
Basics of Generalized Unitarity
We review generalized unitarity as a means for obtaining loop amplitudes from
on-shell tree amplitudes. The method is generally applicable to both
supersymmetric and non-supersymmetric amplitudes, including non-planar
contributions. Here we focus mainly on N=4 Yang-Mills theory, in the context of
on-shell superspaces. Given the need for regularization at loop level, we also
review a six-dimensional helicity-based superspace formalism and its
application to dimensional and massive regularizations. An important feature of
the unitarity method is that it offers a means for carrying over any identified
tree-level property of on-shell amplitudes to loop level, though sometimes in a
modified form. We illustrate this with examples of dual conformal symmetry and
a recently discovered duality between color and kinematics.Comment: 37 pages, 10 figures. Invited review for a special issue of Journal
of Physics A devoted to "Scattering Amplitudes in Gauge Theories", R.
Roiban(ed), M. Spradlin(ed), A. Volovich(ed
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