1,835 research outputs found
Two-point function for the Maxwell field in flat Robertson-Walker spacetimes
We obtain an explicit two-point function for the Maxwell field in flat
Roberson-Walker spaces, thanks to a new gauge condition which takes the scale
factor into account and assume a simple form. The two-point function is found
to have the short distance Hadamard behavior.Comment: 4 pages, Revte
Massive scalar field on (A)dS space from a massless conformal field in
We show how the equations for the scalar field (including the massive,
massless, minimally and conformally coupled cases) on de Sitter and Anti-de
Sitter spaces can be obtained from both the SO-invariant equation
in and two geometrical constraints defining
the (A)dS space. Apart from the equation in , the results only
follow from the geometry.Comment: Revtex 4.1, 6 pages. In v3: New material added (references, relation
with mass ladder operator), accepted in JM
An improvement on codebook search for vector quantization
Presents a simple but effective algorithm to speed up the codebook search in a vector quantization scheme when a MSE criterion is used. A considerable reduction in the number of operations is achieved. This algorithm was originally designed for image vector quantization in which the samples of the image signal (pixels) are positive, although it can be used with any positive-negative signal with only minor modifications.Peer ReviewedPostprint (published version
Saturation properties of nuclear matter in a relativistic mean field model constrained by quark dynamics
We have built an effective Walecka-type hadronic Lagrangian in which the
hadron masses and the density dependence of the coupling constants are deduced
from the quark dynamics using a Nambu-Jona-Lasinio model. The parameters of
this Nambu-Jona-Lasinio model have been determined using the meson properties
in the vacuum but also in the medium through the omega meson mass in nuclei
measured by the TAPS collaboration. Realistic properties of nuclear matter have
been obtained.Comment: 4 pages, 1 figure, to appear in the Proceedings of "Quark Confinement
and the Hadron Spectrum VII
A toy model of a fake inflation
Discontinuities in non linear field theories propagate through null geodesics
in an effective metric that depends on its dynamics and on the background
geometry. Once information of the geometry of the universe comes mostly from
photons, one should carefully analyze the effects of possible nonlinearities on
Electrodynamics in the cosmic geometry. Such phenomenon of induced metric is
rather general and may occurs for any nonlinear theory independently of its
spin properties. We limit our analysis here to the simplest case of non linear
scalar field. We show that a class of theories that have been analyzed in the
literature, having regular configuration in the Minkowski space-time background
is such that the field propagates like free waves in an effective deSitter
geometry. The observation of these waves would led us to infer, erroneously,
that we live in a deSitter universe
Conformally covariant quantization of Maxwell field in de Sitter space
In this article, we quantize the Maxwell ("massless spin one") de Sitter
field in a conformally invariant gauge. This quantization is invariant under
the SO group and consequently under the de Sitter group. We obtain a
new de Sitter invariant two-points function which is very simple. Our method
relies on the one hand on a geometrical point of view which uses the
realization of Minkowski, de Sitter and anti-de Sitter spaces as intersections
of the null cone in \setR^6 and a moving plane, and on the other hand on a
canonical quantization scheme of the Gupta-Bleuler type.Comment: v2 is is the definitive (improved compare to v1) versio
Statistical properties of metastable intermediates in DNA unzipping
We unzip DNA molecules using optical tweezers and determine the sizes of the
cooperatively unzipping and zipping regions separating consecutive metastable
intermediates along the unzipping pathway. Sizes are found to be distributed
following a power law, ranging from one base pair up to more than a hundred
base pairs. We find that a large fraction of unzipping regions smaller than 10
bp are seldom detected because of the high compliance of the released single
stranded DNA. We show how the compliance of a single nucleotide sets a limit
value around 0.1 N/m for the stiffness of any local force probe aiming to
discriminate one base pair at a time in DNA unzipping experiments.Comment: Main text: 4 pages, 3 figures. Supplementary Information: 18 pages,
15 figure
Work distributions in the T=0 Random Field Ising Model
We perform a numerical study of the three-dimensional Random Field
Ising Model at T=0. We compare work distributions along metastable
trajectories obtained with the single-spin flip dynamics with the distribution
of the internal energy change along equilibrium trajectories. The goal is to
investigate the possibility of extending the Crooks fluctuation theorem to zero
temperature when, instead of the standard ensemble statistics, one considers
the ensemble generated by the quenched disorder. We show that a simple
extension of Crooks fails close to the disordered induced equilibrium phase
transition due to the fact that work and internal energy distributions are very
asymmetric
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