2,431 research outputs found
Instantons and SL(2,R) Symmetry in Type IIB Supergravity
We discuss the relation between the dual formulations of Type IIB
supergravity emphasizing the differences between Lorentz and Euclidean
signature. We demonstrate how the SL(2,R) symmetry of the usual action is
manifested in the solution of the equations of motion with Euclidean signature
for the dual theory.Comment: 4 pages, no figure
Instantons of Type IIB Supergravity in Ten Dimensions
A family of SO(10) symmetric instanton solutions in Type IIB supergravity is
developed. The instanton of least action is a candidate for the low-energy,
semiclassical approximation to the {D=--1} brane. Unlike a previously published
solution,[GGP] this admits an interpretation as a tunneling amplitude between
perturbatively degenerate asymptotic states, but with action twice that found
previously. A number of associated issues are discussed such as the relation
between the magnetic and electric pictures, an inversion symmetry of the
dilaton and the metric, the topology of the background, and some
properties of the solution in an "instanton frame" corresponding to a
Lagrangian in which the dilaton's kinetic energy vanishes.Comment: 15 pages, no figures; Version 2 has revised sections IV and V.
Earlier equations are essentially unchanged, but interpretation changed, on
advice of counse
Nonlinear paramagnetic magnetization in the mixed state of CeCoIn_5
Torque and magnetization measurements in magnetic fields up to 14 T were
performed on CeCoIn single crystals. The amplitude of the paramagnetic
torque shows an dependence in the mixed state and an
dependence in the normal state. In addition, the mixed-state magnetizations for
both and axes show anomalous behavior after the
subtraction of the corresponding paramagnetic contributions as linear
extrapolations of the normal-state magnetization. These experimental results
point towards a nonlinear paramagnetic magnetization in the mixed state of
CeCoIn, which is a result of the fact that both orbital and Pauli limiting
effects dominate in the mixed state.Comment: 2 pages, 2 figures, conferenc
Seeking the Shadowing in eA Processes
We consider the behavior of the slope of the nuclear structure function
in the kinematic region which will be explored in the eA HERA collider.
We demonstrate that, similarly to the nucleon case, a `turn over' is predicted
in this observable. Moreover, we predict that the presence of the shadowing
corrections implies that the maximum value of the slope is dependent of the
number of nucleons A, differently from the DGLAP predictions. Our conclusion is
that the measurement of this observable will allow to evidentiate the presence
of the shadowing corrections.Comment: 10 pages, 1 figur
The Effective Potential, the Renormalisation Group and Vacuum Stability
We review the calculation of the the effective potential with particular
emphasis on cases when the tree potential or the
renormalisation-group-improved, radiatively corrected potential exhibits
non-convex behaviour. We illustrate this in a simple Yukawa model which
exhibits a novel kind of dimensional transmutation. We also review briefly
earlier work on the Standard Model. We conclude that, despite some recent
claims to the contrary, it can be possible to infer reliably that the tree
vacuum does not represent the true ground state of the theory.Comment: 23 pages; 5 figures; v2 includes minor changes in text and additional
reference
Planar diagrams in light-cone gauge
We consider the open string vacuum amplitude determining the interaction
between a stack of N D3-branes and a single probe brane. When using light cone
gauge, it is clear that the sum of planar diagrams (relevant in the large-N
limit) is described by the free propagation of a closed string. A naive
calculation suggests that the Hamiltonian of the closed string is of the form H
= H0 - (g_s N) P. The same form of the Hamiltonian follows from considering the
bosonic part of the closed string action propagating in the full D3-brane
background suggesting the naive calculation captures the correct information.
Further, we compute explicitly P from the open string side in the bosonic
sector and show that, in a certain limit, the result agrees with the closed
string expectations up to extra terms due to the fact that we ignored the
fermionic sector. We briefly discuss extensions of the results to the
superstring and to the sum of planar diagrams in field theory. In particular we
argue that the calculations seem valid whenever one can define a sigma tau
dual Hamiltonian in the world-sheet which in principle does not require the
existence of a string action. This seems more generic than the existence of a
string dual in the large-N limit.Comment: 42 pages, 7 figures, LaTeX. v2: references added. v3: typos and minor
corrections. Small change of conventions in fermionic secto
Electrifying catheters with light
Smart minimally invasive devices face a connectivity challenge. An example is found in intracardiac echocardiography where the signal transmission and supply of power at the distal end require many thin and fragile wires in order to keep the catheter slim and flexible. We have built a fully functional bench-top prototype to demonstrate that electrical wires may be replaced by optical fibers. The prototype is immediately scalable to catheter dimensions. The absence of conductors will provide intrinsic galvanic isolation as well as radio frequency (RF) and magnetic resonance imaging (MRI) compatibility. Using optical fibers, we show signal transfer of synthetic aperture ultrasound images as well as photo-voltaic conversion to supply all electronics. The simple design utilizes only off the shelf components and holds a promise of cost effectiveness which may be pivotal for translation of these advanced devices into the clinic
Unitarity Corrections to the Proton Structure Functions through the Dipole Picture
We study the dipole picture for the description of the deep inelastic
scattering, focusing on the structure functions which are driven directly by
the gluon distribution. One performs estimates using the effective dipole cross
section given by the Glauber-Mueller approach in QCD, which encodes the
corrections due to the unitarity effects associated with the saturation
phenomenon. We also address issues about frame invariance of the calculations
when analysing the observables.Comment: 16 pages, 8 figures. Version to be published in Phys. Rev.
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