title: Stokes matrices of hypergeometric integrals

abstract:
  In this work we compute the Stokes matrices of the ordinary differential
equation satisfied by the hypergeometric integrals associated to an arrangement
of hyperplanes in generic position. This generalizes the computation done by
Ramis and Duval for confluent hypergeometric functions, which correspond to the
arrangement of two points on the line. The proof is based on an explicit
description of a base of canonical solutions as integrals on the cones of the
arrangement, and combinatorial relations between integrals on cones and on
domains.


title:
Search for Bs --> mu+mu- and Bd --> mu+mu- Decays with 2fb-1 of ppbar
  Collisions

abstract:
  We have performed a search for Bs-->mu+mu- and Bd-->mu+mu- decays in ppbar
collisions at sqrt(s) = 1.96TeV using 2fb-1 of integrated luminosity collected
by the CDF II detector at the Fermilab Tevatron Collider. The observed number
of Bs and Bd candidates is consistent with background expectations. The
resulting upper limits on the branching fractions are B(Bs-->mu+mu-) <
5.8X10^-8 and B(Bd-->mu+mu-) < 1.8X10^-8 at 95% C.L.


title:
Explicit construction of manifolds realizing the prescribed homology
  classes

abstract:
  We consider a classical N. Steenrod's problem on realization of homology
classes by images of the fundamental classes of manifolds. It is well-known
that each integral homology class can be realized with some multiplicity as an
image of the fundamental class of a manifold. Our main result is an explicit
purely combinatorial construction that for a given integral cycle provides a
combinatorial manifold realizing a multiple of the homology class of this
cycle. The construction is based on a local procedure for resolving
singularities of a pseudo-manifold. We give an application of our result to the
problem of constructing a combinatorial manifold with the prescribed set of
links of vertices.


title: A simple method for timing an XFEL source to high-power lasers

abstract:
  We propose a technique, to be used for time-resolved pump-probe experiments,
for timing an x-ray free electron laser (XFEL) to a high-power conventional
laser with femtosecond accuracy. Our method takes advantage of the same
electron bunch to produce both an XFEL pulse and an ultrashort optical pulse
with the help of an optical radiator downstream of the x-ray undulator. Since
both pulses are produced by the same electron bunch, they are perfectly
synchronized. Application of cross-correlation techniques will allow to
determine relative jitter between the optical pulse (and, thus, the XFEL pulse)
and a pulse from an external pump-laser with femtosecond resolution. Technical
realization of the proposed timing scheme uses an optical replica synthesizer
(ORS) setup to be installed after the final bunch-compression stage of the
XFEL. The electron bunch is modulated in the ORS setup by an external optical
laser. Subsequently, it travels through the main undulator, and produces the
XFEL pulse. Finally, a powerful optical pulse of coherent edge radiation is
generated as the bunch passes through a long straight section and a separation
magnet downstream of the main undulator. Our study shows that at a moderate
(about 10%) density modulation of the electron bunch at the location of the
optical radiator allows production of high power x-ray and optical pulses.
Relative synchronization of these pulses is preserved by using the same
mechanical support for both x-ray and optical elements transporting radiation
down to the experimental area, where single-shot cross-correlation between
optical pulse and pump-laser pulse is performed. We illustrate the potential of
the proposed timing technique with numerical examples referring to the European
XFEL facility.


title:
Probing a Supersymmetric Model for Neutrino Masses at Ultrahigh Energy
  Neutrino Telescopes

abstract:
  A bilinear R-Parity breaking SUSY model for neutrino mass and mixing predicts
the lightest superparticle to decay mainly into a pair of tau leptons or b
quarks along with a neutrino for relatively light SUSY spectra. This leads to a
distinctive triple bang signature of SUSY events at ultrahigh energy neutrino
telescopes like IceCube or Antares. While the expected signal size is only
marginal at IceCube, it will be promising for a future multi-km^3 size neutrino
telescope.


title: The \'Etale Homology and The Cycle Maps in Adic Coefficients

abstract:
  In this article, we define the l-adic homology for a morphism of schemes
satisfying certain finiteness conditions. This homology has these functors
similar to the Chow groups: proper push-forward, flat pull-back, base change,
cap-product, etc. In particular on singular varieties, this kind of l-adic
homology behaves much better that the classical l-adic cohomology. As an
application, we give an much easier approach to construct the cycle maps for
arbitrary algebraic schemes over fields of finite cohomology dimension. And we
prove these cycle maps kill the algebraic equivalences and commute with the
Chern action of locally free sheaves.


title:
Detectable primordial non-gaussianities and gravitational waves in
  k-inflation

abstract:
  An inflationary single field model with a non-trivial kinetic term for the
inflaton is discussed. It is shown that it is possible to have large primordial
non-gaussianities and large tensor-to-scalar ratio in a simple concrete model
with just a scalar field and a generalized kinetic term for the inflaton field.
This is potentially interesting in the prospect of new forthcoming
observations.


title:
Molecular shells in IRC+10216: Evidence for non-isotropic and episodic
  mass loss enhancement

abstract:
  We report high angular-resolution VLA observations of cyanopolyyne molecules
HC$_3$N and HC$_5$N from the carbon rich circumstellar envelope of IRC+10216.
The observed low-lying rotational transitions trace a much more extended
emitting region than seen in previous observations at higher frequency
transitions. We resolve the hollow quasi-spherical distribution of the
molecular emissions into a number of clumpy shells. These molecular shells
coincide spatially with dust arcs seen in deep optical images of the IRC+10216
envelope, allowing us to study for the first time the kinematics of these
features. We find that the molecular and dust shells represent the same density
enhancements in the envelope separated in time by $\sim$120 to $\sim$360 yrs.
From the angular size and velocity spread of the shells, we estimate that each
shell typically covers about 10% of the stellar surface at the time of
ejection. The distribution of the shells seems to be random in space. The good
spatial correspondance between HC$_3$N and HC$_5$N emissions is in qualitative
agreement with a recent chemical model that takes into account the presence of
density-enhanced shells. The broad spatial distribution of the cyanopolyyne
molecules, however, would necessitate further study on their formation.


title: Shell-model structure of exotic nuclei beyond 132Sn

abstract:
  We report on a study of exotic nuclei around doubly magic 132Sn in terms of
the shell model employing a realistic effective interaction derived from the
CD-Bonn nucleon-nucleon potential. The short-range repulsion of the bare
potential is renormalized by constructing a smooth low-momentum potential,
V-low-k, that is used directly as input for the calculation of the effective
interaction. In this paper we focus attention on the nuclei 134Sn and 135Sb
which, with an N/Z ratio of 1.68 and 1.65, respectively, are at present the
most exotic nuclei beyond 132Sn for which information exists on excited states.
Comparison shows that the calculated results for both nuclei are in very good
agreement with the experimental data. We present our predictions of the
hitherto unknown spectrum of 136Sn.


title:
The next-to-leading order gravitational spin(1)-spin(2) dynamics in
  Hamiltonian form

abstract:
  Based on recent developments by the authors a next-to-leading order
spin(1)-spin(2) Hamiltonian is derived for the first time. The result is
obtained within the canonical formalism of Arnowitt, Deser, and Misner (ADM)
utilizing their generalized isotropic coordinates. A comparison with other
methods is given.


title:
The Einsteinian T(3)-Gauge Approach and the Stress Tensor of the Screw
  Dislocation in the Second Order: Avoiding the Cut-off at the Core

abstract:
  A translational gauge approach of the Einstein type is proposed for obtaining
the stresses that are due to non-singular screw dislocation. The stress
distribution of second order around the screw dislocation is classically known
for the hollow circular cylinder with traction-free external and internal
boundaries. The inner boundary surrounds the dislocation's core, which is not
captured by the conventional solution. The present gauge approach enables us to
continue the classically known quadratic stresses inside the core. The gauge
equation is chosen in the Hilbert--Einstein form, and it plays the role of
non-conventional incompatibility law. The stress function method is used, and
it leads to the modified stress potential given by two constituents: the
conventional one, say, the `background' and a short-ranged gauge contribution.
The latter just causes additional stresses, which are localized. The asymptotic
properties of the resulting stresses are studied. Since the gauge contributions
are short-ranged, the background stress field dominates sufficiently far from
the core. The outer cylinder's boundary is traction-free. At sufficiently
moderate distances, the second order stresses acquire regular continuation
within the core region, and the cut-off at the core does not occur. Expressions
for the asymptotically far stresses provide self-consistently new length scales
dependent on the elastic parameters. These lengths could characterize an
exteriority of the dislocation core region.


title:
Rapidities and Observable 3-Velocities in the Flat Finslerian Event
  Space with Entirely Broken 3D Isotropy

abstract:
  We study the geometric phase transitions that accompany the dynamic
rearrangement of vacuum under spontaneous violation of initial gauge symmetry.
The rearrangement may give rise to condensates of three types, namely the
scalar, axially symmetric, and entirely anisotropic condensates. The flat
space-time keeps being the Minkowski space in the only case of scalar
condensate. The anisotropic condensate having arisen, the respective anisotropy
occurs also in space-time. In this case the space-time filled with axially
symmetric condensate proves to be a flat relativistically invariant Finslerian
space with partially broken 3D isotropy, while the space-time filled with
entirely anisotropic condensate proves to be a flat relativistically invariant
Finslerian space with entirely broken 3D isotropy. The two Finslerian space
types are described briefly in the extended introduction to the work, while the
original part of the latter is devoted to determining observable 3-velocities
in the entirely anisotropic Finslerian event space. The main difficulties that
are overcome in solving that problem arose from the nonstandard form of the
light cone equation and from the necessity of correct introducing of a norm in
the linear vector space of rapidities.


title: Coset decomposition for semisimple Hopf algebras

abstract:
  The notion of double coset for semisimple finite dimensional Hopf algebras is
introduced. This is done by considering an equivalence relation on the set of
irreducible characters of the dual Hopf algebra. As an application formulae for
the restriction of the irreducible characters to normal Hopf subalgebras are
given.


title:
Application of the Generalized Alignment Index (GALI) method to the
  dynamics of multi--dimensional symplectic maps

abstract:
  We study the phase space dynamics of multi--dimensional symplectic maps,
using the method of the Generalized Alignment Index (GALI). In particular, we
investigate the behavior of the GALI for a system of N=3 coupled standard maps
and show that it provides an efficient criterion for rapidly distinguishing
between regular and chaotic motion.


title:
Higher derivative regularization and quantum corrections in N=1
  supersymmetric theories

abstract:
  We review some results of applying the higher covariant derivative
regularization to the investigation of quantum corrections structure in N=1
supersymmetric theories. In particular, we demonstrate that all integrals,
defining the Gell-Mann--Low function in supersymmetric theories, are integrals
of total derivatives. As a consequence, there is an identity for Green
functions, which does not follow from any known symmetry of the theory, in N=1
supersymmetric theories. We also discuss how to derive the exact
$\beta$-function by methods of the perturbation theory.


title: On small homotopies of loops

abstract:
  Two natural questions are answered in the negative:
  (1) If a space has the property that small nulhomotopic loops bound small
nulhomotopies, then are loops which are limits of nulhomotopic loops themselves
nulhomotopic?
  (2) Can adding arcs to a space cause an essential curve to become
nulhomotopic?
  The answer to the first question clarifies the relationship between the
notions of a space being homotopically Hausdorff and $\pi_1$-shape injective.


title: Diophantine Networks

abstract:
  We introduce a new class of deterministic networks by associating networks
with Diophantine equations, thus relating network topology to algebraic
properties. The network is formed by representing integers as vertices and by
drawing cliques between M vertices every time that M distinct integers satisfy
the equation. We analyse the network generated by the Pythagorean equation
$x^{2}+y^{2}= z^{2}$ showing that its degree distribution is well approximated
by a power law with exponential cut-off. We also show that the properties of
this network differ considerably from the features of scale-free networks
generated through preferential attachment. Remarkably we also recover a power
law for the clustering coefficient.


title: Coherent states for the quantum mechanics on a torus

abstract:
  The coherent states for the quantum mechanics on a torus and their basic
properties are discussed.


title:
Vinberg's \theta-groups in positive characteristic and
  Kostant-Weierstrass slices

abstract:
  We generalize the basic results of Vinberg's \theta-groups, or periodically
graded reductive Lie algebras, to fields of good positive characteristic. To
this end we clarify the relationship between the little Weyl group and the
(standard) Weyl group. We deduce that the ring of invariants associated to the
grading is a polynomial ring. This approach allows us to prove the existence of
a KW-section for a classical graded Lie algebra (in zero or good
characteristic), confirming a conjecture of Popov in this case.


title:
Quantum interference spectroscopy of RbHe exciplexes formed on helium
  nanodroplets

abstract:
  Femtosecond multiphoton pump-probe photoionization is applied to helium
nanodroplets doped with rubidium (Rb). The yield of Rb+ ions features
pronounced quantum interference (QI) fringes demonstrating the coherence of a
superposition of electronic states on a time scale of tens of picoseconds.
Furthermore, we observe QI in the yield of formed RbHe exciplex molecules. The
quantum interferogram allows to determine the vibrational structure of these
unstable molecules. From a sliced Fourier analysis one can not only extract the
population dynamics of vibrational states but also follow their energetic
evolution during the RbHe formation.


title: Scalar wormholes in cosmological setting and their instability

abstract:
  We construct exact nonstatic nonhomogeneous spherically symmetric solutions
in the theory of gravity with a scalar field possessing the exponential
potential. The solution of particular interest corresponds to the scalar field
with negative kinetic energy, i.e. a ghost, and represents two asymptotically
homogeneous spatially flat universes connected by a throat. We interpret this
solution as a wormhole in cosmological setting. Both the universes and the
wormhole throat are simultaneously expanding with acceleration. The character
of expansion qualitatively depends on the wormhole's mass $m$. For $m=0$ the
expansion goes exponentially, so that the corresponding spacetime configuration
represents two de Sitter universes joining by the throat. For $m>0$ the
expansion has the power character, so that one has the inflating wormhole
connecting two homogeneous spatially flat universes expanding according to the
power law into the final singularity. The stability analysis of the non-static
wormholes reveals their instability against linear spherically symmetric
perturbations.


title:
Features of Traffic Congestion caused by bad Weather Conditions or
  Accident

abstract:
  Spatiotemporal features and physics of vehicular traffic congestion occurring
due to heavy freeway bottlenecks caused by bad weather conditions or accidents
are found based on simulations in the framework of three-phase traffic theory.
A model of a heavy bottleneck is presented. Under a continuous non-limited
increase in bottleneck strength, i.e., when the average flow rate within a
congested pattern allowed by the heavy bottleneck decreases continuously up to
zero, the evolution of the traffic phases in congested traffic, synchronized
flow and wide moving jams, is studied.


title:
Dynamical Symmetry Breaking of a Relativistic Model in
  Quasi-(1+1)-Dimensions. I. Formulation

abstract:
  The dynamical symmetry breaking in a quasi-(1+1)-dimensional relativistic
model is investigated. The motions of particles in intrachain are described as
a relativistic electron-hole gas, while the interchain hopping term is
introduced as a 0th-component of vector in (1+1)-dimensions, a kind of chemical
potential of the system. The gauge symmetry of the model is chosen as U(1)
suitable for a possible situation of a real substance in condensed matter
physics. We consider the BCS-type contact interactions for the s-wave
fermion-pair condensates, while employ the nonlocal interactions of the
generalized BCS framework to generate the $p$-, $d$- and $f$-wave condensations
in the system. Especially we examine the dynamical generation of a Dirac mass
term and superconductivity in the model. The phenomenon is interpreted as
metal-insulator/metal-superconductor phase transitions.

categories: cond-mat.supr-con cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.str-el hep-ph


title:
Quantum Monte Carlo study of small pure and mixed spin-polarized tritium
  clusters

abstract:
  We have investigated the stability limits of small spin-polarized clusters
consisting of up to ten spin-polarized tritium T$\downarrow$ atoms and the
mixtures of T$\downarrow$ with spin-polarized deuterium D$\downarrow$ and
hydrogen H$\downarrow$ atoms. All of our calculations have been performed using
the variational and diffusion Monte Carlo methods. For clusters with
D$\downarrow$ atoms, the released node procedure is used in cases where the
wave function has nodes. In addition to the energy, we have also calculated the
structure of small clusters using unbiased estimators. Results obtained for
pure T$\downarrow$ clusters are in good accordance with previous calculations,
confirming that the trimer is the smallest spin-polarized tritium cluster. Our
results show that mixed T$\downarrow$-H$\downarrow$ clusters having up to ten
atoms are unstable and that it takes at least three tritium atoms to bind one,
two or three D$\downarrow$ atoms. Among all the considered clusters, we have
found no other Borromean states except the ground state of the T$\downarrow$
trimer.


title:
3D MHD Coronal Oscillations About a Magnetic Null Point: Application of
  WKB Theory

abstract:
  This paper is a demonstration of how the WKB approximation can be used to
help solve the linearised 3D MHD equations. Using Charpit's Method and a
Runge-Kutta numerical scheme, we have demonstrated this technique for a
potential 3D magnetic null point, ${\bf{B}}=(x,\epsilon y -(\epsilon +1)z)$.
Under our cold plasma assumption, we have considered two types of wave
propagation: fast magnetoacoustic and Alfv\'en waves. We find that the fast
magnetoacoustic wave experiences refraction towards the magnetic null point,
and that the effect of this refraction depends upon the Alfv\'en speed profile.
The wave, and thus the wave energy, accumulates at the null point. We have
found that current build up is exponential and the exponent is dependent upon
$\epsilon$. Thus, for the fast wave there is preferential heating at the null
point. For the Alfv\'en wave, we find that the wave propagates along the
fieldlines. For an Alfv\'en wave generated along the fan-plane, the wave
accumulates along the spine. For an Alfv\'en wave generated across the spine,
the value of $\epsilon$ determines where the wave accumulation will occur:
fan-plane ($\epsilon=1$), along the $x-$axis ($0<\epsilon <1$) or along the
$y-$axis ($\epsilon>1$). We have shown analytically that currents build up
exponentially, leading to preferential heating in these areas. The work
described here highlights the importance of understanding the magnetic topology
of the coronal magnetic field for the location of wave heating.


title:
The Theory of Uncertainty for Derived Results: Properties of Equations
  Representing Physicochemical Evaluation Systems

abstract:
  Any physiochemical variable (Ym) is always determined from certain measured
variables {Xi}. The uncertainties {ui} of measuring {Xi} are generally a priori
ensured as acceptable. However, there is no general method for assessing
uncertainty (em) in the desired Ym, i.e. irrespective of whatever might be its
system-specific-relationship (SSR) with {Xi}, and/ or be the causes of {ui}. We
here therefore study the behaviors of different typical SSRs. The study shows
that any SSR is characterized by a set of parameters, which govern em. That is,
em is shown to represent a net SSR-driven (purely systematic) change in ui(s);
and it cannot vary for whether ui(s) be caused by either or both statistical
and systematic reasons. We thus present the general relationship of em with
ui(s), and discuss how it can be used to predict a priori the requirements for
an evaluated Ym to be representative, and hence to set the guidelines for
designing experiments and also really appropriate evaluation models. Say: Y_m=
f_m ({X_i}_(i=1)^N), then, although: e_m= g_m ({u_i}_(i=1)^N), "N" is not a key
factor in governing em. However, simply for varying "fm", the em is
demonstrated to be either equaling a ui, or >ui, or even <ui. Further, the
limiting error (d_m^(Lim.)) in determining an Ym is also shown to be decided by
"fm" (SSR). Thus, all SSRs are classified into two groups: (I) the SSRs that
can never lead "d_m^(Lim.)" to be zero; and (II) the SSRs that enable
"d_m^(Lim.)" to be zero. In fact, the theoretical-tool (SSR) is by pros and
cons no different from any discrete experimental-means of a study, and has
resemblance with chemical reactions as well.


title:
Frequency- and electric-field-dependent conductivity of single-walled
  carbon nanotube networks of varying density

abstract:
  We present measurements of the frequency and electric field dependent
conductivity of single walled carbon nanotube(SWCNT) networks of various
densities. The ac conductivity as a function of frequency is consistent with
the extended pair approximation model and increases with frequency above an
onset frequency $\omega_0$ which varies over seven decades with a range of film
thickness from sub-monolayer to 200 nm. The nonlinear electric field-dependent
DC conductivity shows strong dependence on film thickness as well. Measurement
of the electric field dependence of the resistance R(E) allows for the
determination of a length scale $L_{E}$ possibly characterizing the distance
between tube contacts, which is found to systematically decrease with
increasing film thickness. The onset frequency $\omega_0$ of ac conductivity
and the length scale $L_{E}$ of SWCNT networks are found to be correlated, and
a physically reasonable empirical formula relating them has been proposed. Such
studies will help the understanding of transport properties and benefit the
applications of this material system.


title:
Chiral Extrapolation of light resonances from Unitarized Chiral
  Perturbation Theory

abstract:
  Both scalar and vector light resonances can be generated from the
unitarization of one-loop chiral perturbation theory. This amounts to using in
a dispersion relation the chiral expansion, which incorporates the correct QCD
quark mass dependence. We can thus predict the quark mass dependence of the
poles associated to those light resonances. Our results compare well with some
recent lattice results for the rho(770) mass and can be used as a benchmark for
future lattice results on the rho(770) or the f0(600) also known as the sigma.


title:
Systematic investigation of the structure of the Si(553)-Au surface from
  first principles

abstract:
  We present here a comprehensive search for the structure of the Si(553)-Au
reconstruction. More than two hundred different trial structures have been
studied using first-principles density-functional calculations with the SIESTA
code. An iterative procedure, with a step-by-step increase of the accuracy and
computational cost of the calculations, was used to allow for the study of this
large number of configurations. We have considered reconstructions restricted
to the topmost bilayer and studied two types: i) "flat" surface-bilayer models,
where atoms at the topmost bilayer present different coordinations and
registries with the underlying bulk, and ii) nine different models based on the
substitution of a silicon atom by a gold atom in different positions of a
$\pi$-bonded chain reconstruction of the Si(553) surface. We have developed a
compact notation that allows us to label and identify all these structures.
This is very useful for the automatic generation of trial geometries and
counting the number of inequivalent structures, i.e., structures having
different bonding topologies. The most stable models are those that exhibit a
honeycomb-chain structure at the step edge. One of our models (model f2)
reproduces the main features of the room temperature photoemission and
scanning-tunneling microscopy data. Thus we conclude that f2 structure is a
good candidate for the high temperature structure of the Si(553)-Au surface.


title:
Quantum dot defined in two-dimensional electron gas at n-AlGaAs/GaAs
  heterojunction: simulation of electrostatic potential and charging properties

abstract:
  We present a self-consistent Schroedinger-Poisson scheme for simulation of
electrostatic quantum dots defined in gated two-dimensional electron gas formed
at n-AlGaAs/GaAs heterojunction. The computational method is applied to a
quantitative description of transport properties studied experimentally by
Elzermann et al. [Appl. Phys. Lett. {\bf 84}, 4617 (2004)]. The
three-dimensional model describes the electrostatics of the entire device with
a quantum dot that changes shape and floats inside a gated region when the
applied voltages are varied. Our approach accounts for the metal electrodes of
arbitrary geometry and configuration, includes magnetic field applied
perpendicular to the growth direction, electron-electron correlation in the
confined electron system and its interaction with the electron reservoir
surrounding the quantum dot. We calculate the electric field, the space charge
distribution as well as energies and wave functions of confined electrons to
describe opening of two transport channels between the reservoir and the
confined charge puddle. We determine the voltages for charging the dot with up
to 4 electrons.
  The results are in a qualitative and quantitative agreement with the
experimental data.


title:
Ultra-High Energy Cosmic Ray production in the polar cap regions of
  black hole magnetospheres

abstract:
  We develop a model of ultra-high energy cosmic ray (UHECR) production via
acceleration in a rotation-induced electric field in vacuum gaps in the
magnetospheres of supermassive black holes (BH). We show that if the poloidal
magnetic field near the BH horizon is misaligned with the BH rotation axis,
charged particles, which initially spiral into the BH hole along the equatorial
plane, penetrate into the regions above the BH "polar caps" and are ejected
with high energies to infinity. We show that in such a model acceleration of
protons near a BH of typical mass 3e8 solar masses is possible only if the
magnetic field is almost aligned with the BH rotation axis. We find that the
power of anisotropic electromagnetic emission from an UHECR source near a
supermassive BH should be at least 10-100 times larger then UHECR power of the
source. This implies that if the number of UHECR sources within the 100 Mpc
sphere is ~100, the power of electromagnetic emission which accompanies proton
acceleration in each source, $10^{42-43}$ erg/s, is comparable to the typical
luminosities of active galactic nuclei (AGN) in the local Universe. We also
explore the acceleration of heavy nuclei, for which the constraints on the
electromagnetic luminosity and on the alignment of magnetic field in the gap
are relaxed.


title:
Characterizing and tracking single colloidal particles with video
  holographic microscopy

abstract:
  We use digital holographic microscopy and Mie scattering theory to
simultaneously characterize and track individual colloidal particles. Each
holographic snapshot provides enough information to measure a colloidal
sphere's radius and refractive index to within 1%, and simultaneously to
measure its three-dimensional position with nanometer in-plane precision and 10
nanometer axial resolution.


title:
Dimensions, Maximal Growth Sites and Optimization in the Dielectric
  Breakdown Model

abstract:
  We study the growth of fractal clusters in the Dielectric Breakdown Model
(DBM) by means of iterated conformal mappings. In particular we investigate the
fractal dimension and the maximal growth site (measured by the Hoelder exponent
$\alpha_{min}$) as a function of the growth exponent $\eta$ of the DBM model.
We do not find evidence for a phase transition from fractal to non-fractal
growth for a finite $\eta$-value. Simultaneously, we observe that the limit of
non-fractal growth ($D\to 1$) is consistent with $\alpha_{min} \to 1/2$.
Finally, using an optimization principle, we give a recipe on how to estimate
the effective value of $\eta$ from temporal growth data of fractal aggregates.


title:
Uniform existence of the integrated density of states for combinatorial
  and metric graphs over Z^d

abstract:
  We give an overview and extension of recent results on ergodic random
Schr\"odinger operators for models on $\mathbb{Z}^d$. The operators we consider
are defined on combinatorial or metric graphs, with random potentials, random
boundary conditions and random metrics taking values in a finite set. We show
that normalized finite volume eigenvalue counting functions converge to a limit
uniformly in the energy variable, at least locally. This limit, the integrated
density of states (IDS), can be expressed by a closed Shubin-Pastur type trace
formula. The set of points of increase of the IDS supports the spectrum and its
points of discontinuity are characterized by existence of compactly supported
eigenfunctions. This applies to several examples, including various periodic
operators and percolation models.


title:
A numerical study of Penrose-like inequalities in a family of axially
  symmetric initial data

abstract:
  Our current picture of black hole gravitational collapse relies on two
assumptions: i) the resulting singularity is hidden behind an event horizon --
weak cosmic censorship conjecture -- and ii) spacetime eventually settles down
to a stationarity state. In this setting, it follows that the minimal area
containing an apparent horizon is bound by the square of the total ADM mass
(Penrose inequality conjecture). Following Dain et al. 2002, we construct
numerically a family of axisymmetric initial data with one or several
marginally trapped surfaces. Penrose and related geometric inequalities are
discused for these data. As a by-product, it is shown how Penrose inequality
can be used as a diagnosis for an apparent horizon finder numerical routine.


title: Magnetic Doppler Imaging of Ap stars

abstract:
  Historically, the magnetic field geometries of the chemically peculiar Ap
stars were modelled in the context of a simple dipole field. However, with the
acquisition of increasingly sophisticated diagnostic data, it has become clear
that the large-scale field topologies exhibit important departures from this
simple model. Recently, new high-resolution circular and linear polarisation
spectroscopy has even hinted at the presence of strong, small-scale field
structures, which were completely unexpected based on earlier modelling. This
project investigates the detailed structure of these strong fossil magnetic
fields, in particular the large-scale field geometry, as well as small scale
magnetic structures, by mapping the magnetic and chemical surface structure of
a selected sample of Ap stars. These maps will be used to investigate the
relationship between the local field vector and local surface chemistry,
looking for the influence the field may have on the various chemical transport
mechanisms (i.e., diffusion, convection and mass loss). This will lead to
better constraints on the origin and evolution, as well as refining the
magnetic field model for Ap stars. Mapping will be performed using high
resolution and signal-to-noise ratio time-series of spectra in both circular
and linear polarisation obtained using the new-generation ESPaDOnS and NARVAL
spectropolarimeters at the CFHT and Pic du Midi Observatory. With these data we
will perform tomographic inversion of Doppler-broadened Stokes IQUV Zeeman
profiles of a large variety of spectral lines using the INVERS10 magnetic
Doppler imaging code, simultaneously recovering the detailed surface maps of
the vector magnetic field and chemical abundances.


title:
The Spectral Energy Distribution of Self-gravitating Interstellar Clouds
  I. Spheres

abstract:
  We derive the spectral energy distribution (SED) of dusty, isothermal, self
gravitating, stable and spherical clouds externally heated by the ambient
interstellar radiation field. For a given radiation field and dust properties,
the radiative transfer problem is determined by the pressure of the surrounding
medium and the cloud mass expressed as a fraction of the maximum stable cloud
mass above which the clouds become gravitational unstable.
  To solve the radiative transfer problem a ray-tracing code is used to
accurately derive the light distribution inside the cloud. This code considers
both non isotropic scattering on dust grains and multiple scattering events.
The dust properties inside the clouds are assumed to be the same as in the
diffuse interstellar medium in our galaxy. We analyse the effect of the
pressure, the critical mass fraction, and the ISRF on the SED and present
brightness profiles in the visible, the IR/FIR and the submm/mm regime with the
focus on the scattered emission and the thermal emission from PAH-molecules and
dust grains.


title:
Measurement of the KS->gg branching ratio using a pure KS beam with the
  KLOE detector

abstract:
  We have searched for the decay KS->gg in a sample of 2x10^9 phi->KS KL decays
collected at DAPHNE with an integrated luminosity of 1.9 fb^{-1}. KS are tagged
by the KL interaction in the calorimeter. Two prompt photons must also be
detected. Kinematic constraints reduce the initial 6x10^5 events to 2740
candidates, from which a signal of 711\pm 35 events is extracted. By
normalizing to the KS->2pi^0 decays counted in the same sample, we measure
BR(KS->gg)= (2.26\pm0.12_{stat}\pm0.06_{syst})x10^{-6}, in agreement with
O(p^4) Chiral Perturbation Theory predictions.


title:
Phenomenological description of the vortex density in rotating BEC
  superfluids

abstract:
  We propose a phenomenological equation for the vortex line density in
rotating Bose-Einstein condensates as a function of the angular speed. This
equation provides a simple description of the gross features of the increase in
vortex number from the appearance of the first vortex to the theoretical
rigid-body result for high vortex density, and allows one to compare with
analogous situations in superfluid helium, after the suitable changes in the
relevant parameters are made.


title: Characterisation of the magnetic field of the Herbig Be star HD 200775

abstract:
  After our recent discovery of four magnetic Herbig stars, we have decided to
study in detail one of them, HD 200775, to determine if its magnetic topology
is similar to that of the main sequence magnetic stars. With this aim, we
monitored this star in Stokes I and V over more than two years, using the new
spectropolarimeters ESPaDOnS at CFHT, and Narval at TBL. Using our data, we
find that HD 200775 is a double-lined spectroscopic binary system, whose
secondary seems similar, in temperature, to the primary. We determine the
luminosity ratio of the system, and using the luminosity of the system found in
literature, we derive the luminosity of both stars. From our measurements of
the radial velocities of both stars we determine the ephemeris and the orbital
parameters of the system. We have fitted 30 Stokes V profiles simultaneously,
using a chi2 minimisation method, with a decentered-dipole model. The best-fit
model provides a rotation period of 4.3281 d an inclination angle of 60
degrees, and a magnetic obliquity angle of 125 degrees. The polar strength of
the magnetic dipole field is 1000 G, which is decentered by 0.05 R* from the
center of the star. The derived magnetic field model is qualitatively identical
to those commonly observed in the Ap/Bp stars, which bring strong argument in
favour of the fossil field hypothesis, to explain the origin of the magnetic
fields in the main sequence Ap/Bp stars. Our determination of the inclination
of the rotation axis leads to a radius of the primary which is smaller than
that derived from the HR diagram position. This can be explained by a larger
intrinsic luminosity of the secondary relative to the primary, due to a larger
circumstellar extinction of the secondary relative to the primary.


title:
Effects of self-interaction corrections on the transport properties of
  phenyl-based molecular junctions

abstract:
  In transport calculations for molecular junctions based on density functional
theory the choice of exchange and correlation functional may dramatically
affect the results. In particular local and semi-local functionals tend to
over-delocalize the molecular levels thus artificially increasing their
broadening. In addition the same molecular levels are usually misplaced with
respect to the Fermi level of the electrodes. These shortfalls are reminiscent
of the inability of local functionals to describe Mott-Hubbard insulators, but
they can be corrected with a simple and computationally undemanding
self-interaction correction scheme. We apply such a scheme, as implemented in
our transport code Smeagol, to a variety of phenyl-based molecular junctions
attached to gold electrodes. In general the corrections reduce the current,
since the resonant Kohn-Sham states of the molecule are shifted away from the
contact Fermi level. In contrast, when the junction is already described as
insulating by local exchange and correlation potentials, the corrections are
minimal and the I-V is only weakly modified.


title:
The effect of binaries on the dynamical mass determination of star
  clusters

abstract:
  The total mass of distant star clusters is often derived from the virial
theorem, using line-of-sight velocity dispersion measurements and half-light
radii. Although most stars form in binary systems, this is mostly ignored when
interpreting the observations. The components of binary stars exhibit orbital
motion, which may increase the measured velocity dispersion, and may therefore
result in a dynamical mass overestimation. In this paper we quantify the effect
of neglecting the binary population on the derivation of the dynamical mass of
a star cluster. We simulate star clusters numerically, and study the dependence
of the derived dynamical mass on the properties of the binary population. We
find that the presence of binaries plays a crucial role for very sparse
clusters with a stellar density comparable to that of the field star population
(~0.1 stars/pc3), as the velocity dispersion is fully dominated by the binary
orbital motion. For such clusters, the dynamical mass may overestimate the true
mass by up to an order of magnitude. For very dense clusters (>10^7 stars/pc3),
binaries do not affect the dynamical mass estimation significantly. For
clusters of intermediate density (0.1-10^7 stars/pc3), the dynamical mass can
be overestimated by 10-100%, depending on the properties of the binary
population.


title: Particle acceleration by multiple parallel shocks

abstract:
  We present both numerical and semi-analytical results on test-particle
acceleration in multiple parallel shocks. We apply a kinetic Monte Carlo code
and an eigenfunction expansion method to calculate the distribution functions
for electron populations accelerated in subsequent parallel shocks with speeds
ranging from non- to fully-relativistic. We examine the levels of particle
anisotropy at the shocks and discuss the implications for AGN and microquasar
jets.


title: Neutrino Mass

abstract:
  This is a review article about the most recent developments on the field of
neutrino mass. The first part of the review introduces the idea of neutrino
masses and mixing angles, summarizes the most recent experimental data then
discusses the experimental prospects and challenges in this area. The second
part of the review discusses the implications of these results for particle
physics and cosmology, including the origin of neutrino mass, the see-saw
mechanism and sequential dominance, and large extra dimensions and cosmology.


title:
Hard particle spectra from parallel shocks due to turbulence
  transmission

abstract:
  If taken into account, the transmission of the particle-scattering turbulence
--in addition to just the particles-- through the shock front can change the
effective compression ratio felt by the accelerating particles significantly
from the compression of the underlying plasma. This can lead to significantly
harder energy spectra than what are traditionally predicted assuming frozen-in
turbulence. I consider the applicability and limitations of turbulence
transmission scenario in parallel shock waves of different thickness, its
consequences in AGN and microquasar environments, and discuss the possible
effects to the spectrum of the accelerated particles.


title: Out of equilibrium phase transitions in mean field Hamiltonian dynamics

abstract:
  Systems with long-range interactions display a short-time relaxation towards
Quasi-Stationary States (QSSs), whose lifetime increases with system size. With
reference to the Hamiltonian Mean Field (HMF) model, we here review
Lynden-Bell's theory of ``violent relaxation''. The latter results in a maximum
entropy scheme for a water-bag initial profile which predicts the presence of
out-of-equilibrium phase transitions} separating homogeneous (zero
magnetization) from inhomogeneous (non-zero magnetization) QSSs. Two different
parametric representations of the initial condition are analyzed and the
features of the phase diagram are discussed. In both representations we find a
second order and a first order line of phase transitions that merge at a
tricritical point. Particular attention is payed to the condition of existence
and stability of the homogenous phase.


title:
Generalizations of Swierczkowski's lemma and the arity gap of finite
  functions

abstract:
  Swierczkowski's Lemma - as it is usually formulated - asserts that if f is an
at least quaternary operation on a finite set A and every operation obtained
from f by identifying a pair of variables is a projection, then f is a
semiprojection. We generalize this lemma in various ways. First, it is extended
to B-valued functions on A instead of operations on A and to essentially at
most unary functions instead of projections. Then we characterize the arity gap
of functions of small arities in terms of quasi-arity, which in turn provides a
further generalization of Swierczkowski's Lemma. Moreover, we explicitly
classify all pseudo-Boolean functions according to their arity gap. Finally, we
present a general characterization of the arity gaps of B-valued functions on
arbitrary finite sets A.


title:
An Upper Bound of the Total Q-Curvature and Its Isoperimetric Deficit
  for Higher-dimensional Conformal Euclidean Metrics

abstract:
  The aim of this paper is to give not only an explicit upper bound of the
total Q-curvature but also an induced isoperimetric deficit formula for the
complete conformal metrics on $\mathbb R^n$, $n\ge 3$ with scalar curvature
being nonnegative near infinity and Q-curvature being absolutely convergent.


title: Euler-Mahonian Statistics On Ordered Set Partitions (II)

abstract:
  We study statistics on ordered set partitions whose generating functions are
related to $p,q$-Stirling numbers of the second kind. The main purpose of this
paper is to provide bijective proofs of all the conjectures of \stein
(Arxiv:math.CO/0605670). Our basic idea is to encode ordered partitions by a
kind of path diagrams and explore the rich combinatorial properties of the
latter structure. We also give a partition version of MacMahon's theorem on the
equidistribution of the statistics inversion number and major index on words.


title: Integrability of Vortex Equations on Riemann Surfaces

abstract:
  The Abelian Higgs model on a compact Riemann surface \Sigma of genus g is
considered. We show that for g > 1 the Bogomolny equations for multi-vortices
at critical coupling can be obtained as compatibility conditions of two linear
equations (Lax pair) which are written down explicitly. These vortices
correspond precisely to SO(3)-symmetric Yang-Mills instantons on the
(conformal) gravitational instanton \Sigma\times S^2 with a scalar-flat Kahler
metric. Thus, the standard methods of constructing solutions and studying their
properties by using Lax pairs (twistor approach, dressing method etc.) can be
applied to the vortex equations on \Sigma. In the twistor description,
solutions of the integrable vortex equations correspond to rank-2 holomorphic
vector bundles over the complex 3-dimensional twistor space of \Sigma\times
S^2. We show that in the general (nonintegrable) case there is a bijection
between the moduli spaces of solutions to vortex equations on \Sigma and of
pseudo-holomorphic bundles over the almost complex twistor space.


title:
On the nature of outflows in intermediate-mass protostars: a case study
  of IRAS 20050+2720

abstract:
  Context. This is the third of a series of papers devoted to study in detail
and with high-angular resolution intermediate-mass molecular outflows and their
powering sources. Aims. The aim of this paper is to study the intermediate-mass
YSO IRAS 20050+2720 and its molecular outflow, and put the results of this and
the previous studied sources in the context of intermediate-mass star
formation. Methods. We carried out VLA observations of the 7 mm continuum
emission, and OVRO observations of the 2.7 mm continuum emission, CO(1-0),
C18O(1-0), and HC3N(12-11) to map the core towards IRAS 20050+2720. The
high-angular resolution of the observations allowed us to derive the properties
of the dust emission, the molecular outflow, and the dense protostellar
envelope. By adding this source to the sample of intermediate-mass protostars
with outflows, we compare their properties and evolution with those of lower
mass counterparts. Results. The 2.7mm continuum emission has been resolved into
three sources, labeled OVRO 1, OVRO 2, and OVRO 3. Two of them, OVRO 1 and OVRO
2, have also been detected at 7 mm. OVRO 3, which is located close to the C18O
emission peak, could be associated with IRAS 20050+2720. The mass of the
sources, estimated from the dust continuum emission, is 6.5 Msun for OVRO 1,
1.8 Msun for OVRO 2, and 1.3 Msun for OVRO 3. The CO(1-0) emission traces two
bipolar outflows within the OVRO field of view, a roughly east-west bipolar
outflow, labeled A, driven by the intermediate-mass source OVRO 1, and a
northeast-southwest bipolar outflow, labeled B, probably powered by a YSO
engulfed in the circumstellar envelope surrounding OVRO 1.


title:
Hidden charm dynamically generated resonances and the $e^+e^-\to J/\psi
  D \bar D$, $J/\psi D\bar D^*$ reactions

abstract:
  We analyze two recent reactions of Belle, producing $D\bar D$ and $D\bar D^*$
states that have an enhancement of the invariant $D\bar D$, $D\bar D^*$ mass
distribution close to threshold, from the point of view that they might be
indicative of the existence of a hidden charm scalar and an axial vector meson
states below $D\bar D$ or $D\bar D^*$ thresholds, respectively. We conclude
that the data is compatible with the existing prediction of a hidden charm
scalar meson with mass around 3700 MeV, though other possibilities cannot be
discarded. The peak seen in the $D\bar D^*$ spectrum above threshold is,
however, unlikely to be due to a threshold enhancement produced by the
presence, below threshold, of the hidden charm axial vector meson X(3872).


title:
A Web-based System for Observing and Analyzing Computer Mediated
  Communications

abstract:
  Tracking data of user's activities resulting from Computer Mediated
Communication (CMC) tools (forum, chat, etc.) is often carried out in an ad-hoc
manner, which either confines the reusability of data in different purposes or
makes data exploitation difficult. Our research works are biased toward
methodological challenges involved in designing and developing a generic system
for tracking user's activities while interacting with asynchronous
communication tools like discussion forums. We present in this paper, an
approach for building a Web-based system for observing and analyzing user
activity on any type of discussion forums.


title: Lagrangian Klein bottles in R^{2n}

abstract:
  It is shown that the n-dimensional Klein bottle admits a Lagrangian embedding
into R^{2n} if and only if n is odd.


title: Effect of inhomogeneous coupling on superconductivity

abstract:
  We investigate the influence of inhomogeneity in the pairing coupling
constant $U(\vec r)$ on dirty BCS superconductors, focusing on $T_c$, the order
parameter $\Delta(\vec r)$, and the energy gap $E_g(\vec r)$. Within mean-field
theory, we find that when the length-scale of the inhomogeneity is comparable
to, or larger than the coherence length, the ratio $2E_g/T_c$ is significantly
reduced from that of a homogeneous superconductor, while in the opposite limit
this ratio stays unmodified. In two dimensions, when strong phase fluctuations
are included, the Kosterlitz-Thouless temperature $T_{KT}$ is also studied. We
find that when the inhomogeneity length scale is much larger than the coherence
length, $2E_g/T_{KT}$ can be larger than the usual BCS value. We use our
results to qualitatively explain recent experimental observation of a
surprisingly low value of $2E_g/T_c$ in thin films.


title:
Irrationalit\'e aux entiers impairs positifs d'un q-analogue de la
  fonction zeta de Riemann

abstract:
  In this paper, we focus on a q-analogue of the Riemann zeta function at
positive integers, which can be written for s\in\N^* by \zeta_q(s)=\sum_{k\geq
1}q^k\sum_{d|k}d^{s-1}. We give a new lower bound for the dimension of the
vector space over \Q spanned, for 1/q\in\Z\setminus\{-1;1\} and an even integer
A, by 1,\zeta_q(3),\zeta_q(5),...,\zeta_q(A-1). This improves a recent result
of Krattenthaler, Rivoal and Zudilin (\emph{S\'eries hyperg\'eom\'etriques
basiques, q-analogues des valeurs de la fonction zeta et s\'eries
d'Eisenstein}, J. Inst. Jussieu {\bf 5}.1 (2006), 53-79). In particular, a
consequence of our result is that for 1/q\in\Z\setminus\{-1;1\}, at least one
of the numbers \zeta_q(3),\zeta_q(5),\zeta_q(7),\zeta_q(9) is irrational.


title: The Sun in Time: Activity and Environment

abstract:
  (abridged) The Sun's magnetic activity has steadily declined during its
main-sequence life. While the solar photospheric luminosity was about 30% lower
4.6 Gyr ago when the Sun arrived on the main sequence compared to present-day
levels, its faster rotation generated enhanced magnetic activity; magnetic
heating processes in the chromosphere, the transition region, and the corona
induced ultraviolet, extreme-ultraviolet, and X-ray emission about 10, 100, and
1000 times, respectively, the present-day levels, as inferred from young
solar-analog stars. Also, the production rate of accelerated, high-energy
particles was orders of magnitude higher than in present-day solar flares, and
a much stronger wind escaped from the Sun, permeating the entire solar system.
The consequences of the enhanced radiation and particle fluxes from the young
Sun were potentially severe for the evolution of solar-system planets and
moons. Interactions of high-energy radiation and the solar wind with upper
planetary atmospheres may have led to the escape of important amounts of
atmospheric constituents. The present dry atmosphere of Venus and the thin
atmosphere of Mars may be a product of early irradiation and heating by solar
high-energy radiation. High levels of magnetic activity are also inferred for
the pre-main sequence Sun. At those stages, interactions of high-energy
radiation and particles with the circumsolar disk in which planets eventually
formed were important. Traces left in meteorites by energetic particles and
anomalous isotopic abundance ratios in meteoritic inclusions may provide
evidence for a highly active pre-main sequence Sun. The present article reviews
these various issues related to the magnetic activity of the young Sun and the
consequent interactions with its environment.


title:
Proper Motions of the LMC and SMC: Reanalysis of Hubble Space Telescope
  Data

abstract:
  Kallivayalil et al. have used the \textit{Hubble Space Telescope} to measure
proper motions of the LMC and SMC using images in 21 and five fields,
respectively, all centered on known QSOs. These results are more precise than
previous measurements, but have surprising and important physical implications:
for example, the LMC and SMC may be approaching the Milky Way for the first
time; they might not have been in a binary system; and the origin of the
Magellanic Stream needs to be re-examined. Motivated by these implications, we
have reanalyzed the original data in order to check the validity of these
measurements. Our work has produced a proper motion for the LMC that is in
excellent agreement with that of Kallivayalil et al., and for the SMC that is
in acceptable agreement.
  We have detected a dependence between the brightness of stars and their mean
measured motion in a majority of the fields in both our reduction and that of
Kallivayalil et al. Correcting for this systematic error and for the errors
caused by the decreasing charge transfer efficiency of the detector produces
better agreement between the measurements from different fields. With our
improved reduction, we do not need to exclude any fields from the final
averages and, for the first time using proper motions, we are able to detect
the rotation of the LMC. The best-fit amplitude of the rotation curve at a
radius of 275 arcmin in the disk plane is $120 \pm 15$ km s$^{-1}$. This value
is larger than the 60--70 km s$^{-1}$ derived from the radial velocities of HI
and carbon stars, but in agreement with the value of 107 km s$^{-1}$ derived
from the radial velocities of red supergiants.


title: Solving Simple Stochastic Games with Few Random Vertices

abstract:
  Simple stochastic games are two-player zero-sum stochastic games with
turn-based moves, perfect information, and reachability winning conditions. We
present two new algorithms computing the values of simple stochastic games.
Both of them rely on the existence of optimal permutation strategies, a class
of positional strategies derived from permutations of the random vertices. The
"permutation-enumeration" algorithm performs an exhaustive search among these
strategies, while the "permutation-improvement" algorithm is based on
successive improvements, \`a la Hoffman-Karp. Our algorithms improve previously
known algorithms in several aspects. First they run in polynomial time when the
number of random vertices is fixed, so the problem of solving simple stochastic
games is fixed-parameter tractable when the parameter is the number of random
vertices. Furthermore, our algorithms do not require the input game to be
transformed into a stopping game. Finally, the permutation-enumeration
algorithm does not use linear programming, while the permutation-improvement
algorithm may run in polynomial time.


title: Sur le centralisateur d'une involution de 2E6(2)

abstract:
  In this paper we prove that $2^{20+1}.U_6(2)$, known as the centralizer of an
involution in the group $2E_6(2)$ is a quotient of a Coxeter group. We obtain a
presentation of $2^{20+1}.U_6(2)$ as a $Q_{222}$-group, which now resolve a
long pending question.


title: Quantitative non contact dynamic Casimir force measurements

abstract:
  We show that the Casimir force gradient can be quantitatively measured with
no contact involved. Results of the Casimir force measurement with systematic
uncertainty of 3% are presented for the distance range of 100-600 nm. The
statistical uncertainty is shown to be due to the thermal fluctuations of the
force probe. The corresponding signal to noise ratio equals unity at the
distance of 600 nm. Direct contact between surfaces used in most previous
studies to determine absolute distance separation is here precluded. Use of
direct contact to identify the origin of distances is a severe limitation for
studies of the Casimir forces on structured surfaces as it deteriorates
irreversibly the studied surface and the probe. This force machine uses a
dynamical method with an inserted gold sphere probe glued to a lever. The lever
is mechanically excited at resonant frequency in front of a chosen sample. The
absolute distance determination is achieved to be possible, without any direct
probe/sample contact, using an electrostatic method associated to a real time
correction of the mechanical drift. The positioning shift uncertainty is as low
as 2 nm.


title:
Conceptions et usages des plates-formes de formation, Revue Sciences et
  Technologies de l'Information et de la Communication pour l'\'Education et la
  Formation

abstract:
  Educative platforms are at the heart of the development of online education.
They can not only be reduced to technological aspects. Underlying models impact
teaching and learning from the preparing of lessons to the learning sessions.
Research related to these platforms are numerous and their stakes are
important. For these reasons, we launched a call to a special issue on "Designs
and uses of educative platforms" An educative platform is a computer system
designed to automate various functions relating to the organization of the
course, to the management of their content, to the monitoring of learners and
supervision of persons in charge of various formations (Office de la langue
fran\c{c}aise, 2005). So educative platforms are Learning Management Systems
(LMS) which are specific to education contexts.


title:
The Schrodinger model for the minimal representation of the indefinite
  orthogonal group O(p, q)

abstract:
  We introduce the `Fourier transform' F_C on the isotropic cone C associated
to an indefinite quadratic form of signature (n_1,n_2) on R^n (n=n_1+n_2:
even). This transform is in some sense the unique and natural unitary operator
on L^2(C), as is the case with the Euclidean Fourier transform.
  Inspired by recent developments of algebraic representation theory of
reductive groups, we shed new light on classical analysis on the one hand, and
give the global formulas for the L^2-model of the minimal representation of the
simple Lie group G=O(n_1+1,n_2+1) on the other hand.
  The transform F_C expands functions on C into joint eigenfunctions of the n
commuting, self-adjoint, second order differential operators. We decompose F_C
into the singular Radon transform and the Mellin--Barnes integral, find its
distribution kernel, and establish the inversion and the Plancherel formula.
F_C reduces to the Hankel transform if G is O(n,2) or O(3,3).
  The unitary operator F_C together with the simple action of the conformal
transformation group generates the minimal representation of the indefinite
orthogonal group G. Various different models of the same representation have
been constructed by Kazhdan, Kostant, Binegar-Zierau, Gross-Wallach, Zhu-Huang,
Torasso, Brylinski, and Kobayashi-Orsted, and others. Among them, our model
built on L^2(C) generalizes the classic Schrodinger model of the Weil
representation.
  Yet another motif is special functions. Large group symmetries in the minimal
representation yield functional equations of various special functions. We find
explicit K-finite vectors on L^2(C), and give a new proof of the Plancherel
formula for Meijer's G-transforms.


title: Cosmological horizons and reconstruction of quantum field theories

abstract:
  As a starting point, we state some relevant geometrical properties enjoyed by
the cosmological horizon of a certain class of Friedmann-Robertson-Walker
backgrounds. Those properties are generalised to a larger class of expanding
spacetimes $M$ admitting a geodesically complete cosmological horizon $\scrim$
common to all co-moving observers. This structure is later exploited in order
to recast, in a cosmological background, some recent results for a linear
scalar quantum field theory in spacetimes asymptotically flat at null infinity.
Under suitable hypotheses on $M$, encompassing both the cosmological de Sitter
background and a large class of other FRW spacetimes, the algebra of
observables for a Klein-Gordon field is mapped into a subalgebra of the algebra
of observables $\cW(\scrim)$ constructed on the cosmological horizon. There is
exactly one pure quasifree state $\lambda$ on $\cW(\scrim)$ which fulfils a
suitable energy-positivity condition with respect to a generator related with
the cosmological time displacements. Furthermore $\lambda$ induces a preferred
physically meaningful quantum state $\lambda_M$ for the quantum theory in the
bulk. If $M$ admits a timelike Killing generator preserving $\scrim$, then the
associated self-adjoint generator in the GNS representation of $\lambda_M$ has
positive spectrum (i.e. energy). Moreover $\lambda_M$ turns out to be invariant
under every symmetry of the bulk metric which preserves the cosmological
horizon. In the case of an expanding de Sitter spacetime, $\lambda_M$ coincides
with the Euclidean (Bunch-Davies) vacuum state, hence being Hadamard in this
case. Remarks on the validity of the Hadamard property for $\lambda_M$ in more
general spacetimes are presented.


title: Deducing the string-breaking distance in strong production processes

abstract:
  We show that the string-breaking distance can be read from meson-production
data, by employing a previously derived expression for the production
amplitude. Accordingly, we find that the radii of 0.67, 0.34 and 0.20 fm for
the creation of non-strange q-qbar pairs obtained in the
Resonance-Spectrum-Expansion model, for light-quark, c-cbar, and b-bbar
environments, respectively, are in perfect agreement with S-wave di-pion
production data, upon employing an ansatz with no additional free parameters.


title: The dynamics of non-linear optical absorption

abstract:
  On traversing materials with absorbing dyes, weak optical beams develop a
Beer (exponential) profile, while intense beams develop a spatially initially
linear and then finally an exponential profile. This anomalous, deep
penetration due to photo-bleaching of surface layers is important for heavy
dye-loading and intense beams, for instance in photo-actuation. We address the
problem of the evolution in time from initial Beer's Law to deeply penetrating
optical profiles in dyes. Our solution of the coupled, non-linear, partial
differential equations governing the spatio-temporal decay of the Poynting flux
and the non-linear dynamics of the \textit{trans-cis} conversion is applicable
to general systems of photo-active molecules under intense irradiation, for
instance in biology, in spectroscopy and in opto-mechanical devices.


title: Collective Atomic Recoil Laser as a synchronization transition

abstract:
  We consider here a model previously introduced to describe the collective
behavior of an ensemble of cold atoms interacting with a coherent
electromagnetic field. The atomic motion along the self-generated
spatially-periodic force field can be interpreted as the rotation of a phase
oscillator. This suggests a relationship with synchronization transitions
occurring in globally coupled rotators. In fact, we show that whenever the
field dynamics can be adiabatically eliminated, the model reduces to a
self-consistent equation for the probability distribution of the atomic
"phases". In this limit, there exists a formal equivalence with the Kuramoto
model, though with important differences in the self-consistency conditions.
Depending on the field-cavity detuning, we show that the onset of synchronized
behavior may occur through either a first- or second-order phase transition.
Furthermore, we find a secondary threshold, above which a periodic self-pulsing
regime sets in, that is immediately followed by the unlocking of the
forward-field frequency. At yet higher, but still experimentally meaningful,
input intensities, irregular, chaotic oscillations may eventually appear.
Finally, we derive a simpler model, involving only five scalar variables, which
is able to reproduce the entire phenomenology exhibited by the original model.


title: Photon emission by an atom in a lossy cavity

abstract:
  The dynamics of an initially excited two-level atom in a lossy cavity is
studied by using the quantum trajectory method. Unwanted losses are included,
such as photon absorption and scattering by the cavity mirrors and spontaneous
emission of the atom. Based on the obtained analytical solutions, it is shown
that the shape of the extracted spatiotemporal radiation mode sensitively
depends on the atom-field interaction. In the case of a short-term atom-field
interaction we show how different pulse shapes for the field extracted from the
cavity can be controlled by the interaction time.


title: On Computation of Error Locations and Values in Hermitian Codes

abstract:
  We obtain a technique to reduce the computational complexity associated with
decoding of Hermitian codes. In particular, we propose a method to compute the
error locations and values using an uni-variate error locator and an
uni-variate error evaluator polynomial. To achieve this, we introduce the
notion of Semi-Erasure Decoding of Hermitian codes and prove that decoding of
Hermitian codes can always be performed using semi-erasure decoding. The
central results are:
  * Searching for error locations require evaluating an univariate error
locator polynomial over $q^2$ points as in Chien search for Reed-Solomon codes.
  * Forney's formula for error value computation in Reed-Solomon codes can
directly be applied to compute the error values in Hermitian codes.
  The approach develops from the idea that transmitting a modified form of the
information may be more efficient that the information itself.


title: Reflection-Free One-Way Edge Modes in a Gyromagnetic Photonic Crystal

abstract:
  We point out that electromagnetic one-way edge modes analogous to quantum
Hall edge states, originally predicted by Raghu and Haldane in 2D gyroelectric
photonic crystals possessing Dirac point-derived bandgaps, can appear in more
general settings. In particular, we show that the TM modes in a gyromagnetic
photonic crystal can be formally mapped to electronic wavefunctions in a
periodic electromagnetic field, so that the only requirement for the existence
of one-way edge modes is that the Chern number for all bands below a gap is
non-zero. In a square-lattice gyromagnetic Yttrium-Iron-Garnet photonic crystal
operating at microwave frequencies, which lacks Dirac points, time-reversal
breaking is strong enough that the effect should be easily observable. For
realistic material parameters, the edge modes occupy a 10% band gap. Numerical
simulations of a one-way waveguide incorporating this crystal show 100%
transmission across strong defects, such as perfect conductors several lattice
constants wide, larger than the width of the waveguide.


title: Finite Sections of Weighted Hardy's Inequality

abstract:
  We study finite sections of weighted Hardy's inequality following the
approach of De Bruijn. Similar to the unweighted case, we obtain an asymptotic
expression for the optimal constant.


title:
Isospin and isospin/strangeness correlations in relativistic heavy ion
  collisions

abstract:
  A fundamental symmetry of nuclear and particle physics is isospin whose third
component is the Gell-Mann/Nishijima expression I(z)=Q-(B+S)/2 . The role of
isospin symmetry in relativistic heavy ion collisions is studied. An isospin
I(z), strangeness S correlation is shown to be a direct and simple measure of
flavor correlations, vanishing in a Qg phase of uncorrelated flavors in both
symmetric N=Z and asymmetric N not equal to Z systems. By contrast, in a hadron
phase, a I(z)/S correlation exists as long as the electrostatic charge chemical
potential mu(Q)does not equal 0 as in N not equal to Z asymmetric systems. A
parallel is drawn with a Zeeman effect which breaks a spin degeneracy


title: Neutrino oscillations in a stochastic model for space-time foam

abstract:
  We study decoherence models for flavour oscillations in four-dimensional
stochastically fluctuating space times and discuss briefly the sensitivity of
current neutrino experiments to such models. We pay emphasis on demonstrating
the model dependence of the associated decoherence-induced damping coefficients
in front of the oscillatory terms in the respective transition probabilities
between flavours. Within the context of specific models of foam, involving
point-like D-branes and leading to decoherence-induced damping which is
inversely proportional to the neutrino energies, we also argue that future
limits on the relevant decoherence parameters coming from TeV astrophysical
neutrinos, to be observed in ICE-CUBE, are not far from theoretically expected
values with Planck mass suppression. Ultra high energy neutrinos from Gamma Ray
Bursts at cosmological distances can also exhibit in principle sensitivity to
such effects.


title:
Conformal symbols and the action of contact vector fields over the
  superline

abstract:
  Let K be the Lie superalgebra of contact vector fields on the supersymmetric
line. We compute the action of K on the modules of differential and
pseudodifferential operators between spaces of tensor densities, in terms of
their conformal symbols. As applications we deduce the geometric subsymbols,
1-cohomology, and various uniserial subquotients of these modules. We also
outline the computation of the K-equivalences and symmetries of their
subquotients.


title: Homogenization of variational problems in manifold valued Sobolev spaces

abstract:
  Homogenization of integral functionals is studied under the constraint that
admissible maps have to take their values into a given smooth manifold. The
notion of tangential homogenization is defined by analogy with the tangential
quasiconvexity introduced by Dacorogna, Fonseca, Maly and Trivisa \cite{DFMT}.
For energies with superlinear or linear growth, a $\Gamma$-convergence result
is established in Sobolev spaces, the homogenization problem in the space of
functions of bounded variation being the object of \cite{BM}.


title: Existence of rational points on smooth projective varieties

abstract:
  Fix a number field k. We prove that if there is an algorithm for deciding
whether a smooth projective geometrically integral k-variety has a k-point,
then there is an algorithm for deciding whether an arbitrary k-variety has a
k-point and also an algorithm for computing X(k) for any k-variety X for which
X(k) is finite. The proof involves the construction of a one-parameter
algebraic family of Chatelet surfaces such that exactly one of the surfaces
fails to have a k-point.


title:
A patient-specific respiratory model of anatomical motion for radiation
  treatment planning

abstract:
  Modeling of respiratory motion is important for a more accurate understanding
and accounting of its effect on dose to cancers in the thorax and abdomen by
radiotherapy. We have developed a model of respiration-induced organ motion in
the thorax, without the commonly adopted assumption of repeatable breath
cycles. The model describes the motion of a volume of interest within the
patient, based on a reference 3-dimensional image (at end-expiration), and the
diaphragm positions at different time points. The input data are
respiration-correlated CT images of patients treated for nonsmall cell lung
cancer, consisting of 3D images, including the diaphragm positions, at 10
phases of the respiratory cycle. A deformable image registration algorithm
calculates the deformation field that maps each 3D image to the reference 3D
image. A principle component analysis is performed to parameterize the 3D
deformation field in terms of the diaphragm motion. We show that the first two
principal components are adequate to accurately and completely describe the
organ motion in the data of 4 patients. Artifacts in the RCCT images that
commonly occur at the mid-respiration states are reduced in the model-generated
images. Further validation of the model is demonstrated in the successful
application of the parameterized 3D deformation field to RCCT data of the same
patient but acquired several days later. We have developed a method for
predicting respiration-induced organ motion in patients that has potential for
improving the accuracy of dose calculation in radiotherapy.


title:
Magnetohydrodynamics wave propagation in the neighbourhood of two
  dipoles

abstract:
  This paper is the third in a series of investigations by the authors. The
nature of fast magnetoacoustic and Alfv\'en waves is investigated in a 2D
$\beta=0$ plasma in the neighbourhood of two dipoles. We use both numerical
simulations (two-step Lax-Wendroff scheme) and analytical techniques (WKB
approximation). It is found that the propagation of the linear fast wave is
dictated by the Alfv\'en speed profile and that close to the null, the wave is
attracted to the neutral point. However, it is also found that in this magnetic
configuration some of the wave can escape the refraction effect; this had not
been seen in previous investigations by the authors. The wave split occurs near
the regions of very high Alfv\'en speed (found near the loci of the two
dipoles). Also, for the set-up investigated it was found that 40% of the wave
energy accumulates at the null. Ohmic dissipation will then extract the wave
energy at this point. The Alfv\'en wave behaves in a different manner in that
part of the wave accumulates along the separatrices and part escapes. Hence,
the current density will accumulate at this part of the topology and this is
where wave heating will occur. The phenomenon of wave accumulation at a
specific place is a feature of both wave types, as is the result that a
fraction of the wave can now escape the numerical box when propagating in this
magnetic configuration.


title: The set of non-squares in a number field is diophantine

abstract:
  Fix a number field k. We prove that k* - k*^2 is diophantine over k. This is
deduced from a theorem that for a nonconstant separable polynomial P(x) in
k[x], there are at most finitely many a in k* modulo squares such that there is
a Brauer-Manin obstruction to the Hasse principle for the conic bundle X given
by y^2 - az^2 = P(x).


title: Unparticle-Enhanced Black Holes at the LHC

abstract:
  Based on the idea that tensor unparticles can enhance the gravitational
interactions between standard model particles, potential black hole formation
in high energy collisions is examined. Modifications to the horizon radius
$r_H$ are derived, and the corresponding geometric cross-sections of such
objects are calculated. It is shown that $r_H$ increases dramatically to the
electroweak scale for masses $M_{BH} \sim 1-10 $TeV, yielding a geometric
cross-section $\sigma_{BH}=\pi r_H^2$ on the order of $leq 50$ pb. This
suggests that unparticle physics provides a mechanism for black hole formation
in future accelerators, without the requirement of extra spatial dimensions.


title: Different approaches to the second order Klein-Gordon equation

abstract:
  We derive the Klein--Gordon equation for a single scalar field coupled to
gravity at second order in perturbation theory and leading order in slow-roll.
This is done in two ways: we derive the Klein--Gordon equation first using the
Einstein field equations, and then directly from the action after integrating
out the constraint equations. We also point out an unexpected result regarding
the treatment of the field equations.


title:
Pressure dependence of the optical properties of the charge-density-wave
  compound LaTe$_2$

abstract:
  We report the pressure dependence of the optical response of LaTe$_2$, which
is deep in the charge-density-wave (CDW) ground state even at 300 K. The
reflectivity spectrum is collected in the mid-infrared spectral range at room
temperature and at pressures between 0 and 7 GPa. We extract the energy scale
due to the single particle excitation across the CDW gap and the Drude weight.
We establish that the gap decreases upon compressing the lattice, while the
Drude weight increases. This signals a reduction in the quality of nesting upon
applying pressure, therefore inducing a lesser impact of the CDW condensate on
the electronic properties of LaTe$_2$. The consequent suppression of the CDW
gap leads to a release of additional charge carriers, manifested by the shift
of weight from the gap feature into the metallic component of the optical
response. On the contrary, the power-law behavior, seen in the optical
conductivity at energies above the gap excitation and indicating a weakly
interacting limit within the Tomonaga-Luttinger liquid scenario, seems to be
only moderately dependent on pressure.


title: Bianchi VII_h models and the cold spot texture

abstract:
  We have returned to our previous Bianchi VII_h analysis in light of the Cruz
et al. 2007 suggestion that the cold spot observed near the southern Galactic
pole may be a remnant temperature perturbation of a cosmic texture. In Bridges
et al. 2006b we found two favoured left handed Bianchi VII_h templates with
restricted prior probabilities so that the template was centred close to the
cold spot. Using WMAP data `corrected' for the texture fit we have now
reexamined both models to assess any changes to these conclusions. We find that
both models are left almost entirely unconstrained by the data and consequently
exhibit significantly reduced Bayesian evidences. Both models are now
disfavoured by the data. This result reinforces our previous suggestion that
the cold spot was likely to be driving any Bianchi VII_h detection.


title:
Hydrodynamic model for relaxation of optically injected currents in
  quantum wells

abstract:
  We use a hydrodynamic model to describe the relaxation of optically injected
currents in quantum wells on a picosecond time scale, numerically solving the
continuity and velocity evolution equations with the Hermite-Gaussian functions
employed as a basis. The interplay of the long-range Coulomb forces and
nonlinearity in the equations of motion leads to rather complex patterns of the
calculated charge and current densities. We find that the time dependence of
even the first moment of the electron density is sensitive to this complex
evolution.


title: Unique Identification of Lee-Wick Gauge Bosons at Linear Colliders

abstract:
  Grinstein, O'Connell and Wise have recently presented an extension of the
Standard Model (SM), based on the ideas of Lee and Wick (LW), which
demonstrates an interesting way to remove the quadratically divergent
contributions to the Higgs mass induced by radiative corrections. This model
predicts the existence of negative-norm copies of the usual SM fields at the
TeV scale with ghost-like propagators and negative decay widths, but with
otherwise SM-like couplings. In earlier work, it was demonstrated that the LW
states in the gauge boson sector of these models, though easy to observe,
cannot be uniquely identified as such at the LHC. In this paper, we address the
issue of whether or not this problem can be resolved at an $e^+e^-$ collider
with a suitable center of mass energy range. We find that measurements of the
cross section and the left-right polarization asymmetry associated with Bhabha
scattering can lead to a unique identification of the neutral electroweak gauge
bosons of the Lee-Wick type.


title:
MHD wave propagation in the neighbourhood of a two-dimensional null
  point

abstract:
  The nature of fast magnetoacoustic and Alfv\'en waves is investigated in a
zero $\beta$ plasma. This gives an indication of wave propagation in the low
$\beta$ solar corona. It is found that for a two-dimensional null point, the
fast wave is attracted to that point and the front of the wave slows down as it
approaches the null point, causing the current density to accumulate there and
rise rapidly. Ohmic dissipation will extract the energy in the wave at this
point. This illustrates that null points play an important role in the rapid
dissipation of fast magnetoacoustic waves and suggests the location where wave
heating will occur in the corona. The Alfv\'en wave behaves in a different
manner in that the wave energy is dissipated along the separatrices. For
Alfv\'en waves that are decoupled from fast waves, the value of the plasma
$\beta$ is unimportant. However, the phenomenon of dissipating the majority of
the wave energy at a specific place is a feature of both wave types.


title: D-brane Instantons in 4D Supersymmetric String Vacua

abstract:
  We review some aspects of D-instantons in intersecting D-brane models. In
particular, we present applications of the recently proposed instanton calculus
to computations of charged matter superpotential couplings and corrections to
the gauge kinetic function in the low energy effective action of type IIA
orientifolds. As an interesting byway, we also discuss how one-loop corrections
to the gauge kinetic function can be deduced from gauge threshold corrections
in the type IIA setting.


title: On deep Frobenius descent and flat bundles

abstract:
  Let R be an integral domain of finite type over Z and let f:X --> Spec R be a
smooth projective morphism of relative dimension d >= 1. We investigate, for a
vector bundle E on the total space X, under what arithmetical properties of a
sequence (p_n, e_n)_{n \in \NN}, consisting of closed points p_n in Spec R and
Frobenius descent data E_{p_n} \cong F^{e_n}^*(F) on the closed fibers X_{p_n},
the bundle E_0 on the generic fiber X_0 is semistable.


title: E(11)-extended spacetime and gauged supergravities

abstract:
  We formulate all the five dimensional gauged maximal supergravity theories as
non-linear realisations of the semi-direct product of E_{11} and a set of
generators which transform according to the first fundamental representation l
of E_{11}. The latter introduces a generalised space-time which plays a crucial
role for these theories. We derive the E_{11} and l transformations of all the
form fields and their dynamics. We also formulate the five dimensional gauged
supergravity theories using the closure of the supersymmetry algebra. We show
that this closes on the bosonic field content predicted by E_{11} and we derive
the field transformations and the dynamics of this theory. The results are in
precise agreement with those found from the E_{11} formulation. This provides a
very detailed check of E_{11} and also the first substantial evidence for the
generalised space-time. The results can be generalised to all gauged maximal
supergravities, thus providing a unified framework of all these theories as
part of E_{11}.


title: Fundamental parameter-free solutions in Modified Gravity

abstract:
  Modified Gravity (MOG) has been used successfully to explain the rotation
curves of galaxies, the motion of galaxy clusters, the Bullet Cluster, and
cosmological observations without the use of dark matter or Einstein's
cosmological constant. We now have the ability to demonstrate how these
solutions can be obtained directly from the action principle, without resorting
to the use of fitted parameters or empirical formulae. We obtain numerical
solutions to the theory's field equations that are exact in the sense that no
terms are omitted, in two important cases: the spherically symmetric, static
vacuum solution and the cosmological case of an homogeneous, isotropic
universe. We compare these results to selected astrophysical and cosmological
observations.


title: Lens space surgeries & primitive/Seifert type constructions

abstract:
  We show that lens space surgeries on knots in $S^3$ which arise from the
primitive/Seifert type construction also arise from the primitive/primitive
construction. This is the first step of a three step program to prove the Berge
conjecture for tunnel number one knots.


title: Fractional generalization of Fick's law: a microscopic approach

abstract:
  In the study of transport in inhomogeneous systems it is common to construct
transport equations invoking the inhomogeneous Fick law. The validity of this
approach requires that at least two ingredients be present in the system.
First, finite characteristic length and time scales associated to the dominant
transport process must exist. Secondly, the transport mechanism must satisfy a
microscopic symmetry: global reversibility. Global reversibility is often
satisfied in nature. However, many complex systems exhibit a lack of finite
characteristic scales. In this Letter we show how to construct a generalization
of the inhomogeneous Fick law that does not require the existence of
characteristic scales while still satisfying global reversibility.


title: S_3-flavour symmetry as realized in lepton flavour violating processes

abstract:
  A variety of lepton flavour violating effects related to the recent discovery
of neutrino oscillations and mixings is here systematically discussed in terms
of an S_3-flavour permutational symmetry. After a brief review of some relevant
results on lepton masses and mixings, that had been derived in the framework of
a Minimal S_3-Invariant Extension of the Standard Model, we derive explicit
analytical expressions for the matrices of the Yukawa couplings and compute the
branching ratios of some selected flavour changing neutral current (FCNC)
processes, as well as, the contribution of the exchange of neutral flavour
changing scalars to the anomaly of the muon's magnetic moment as functions of
the masses of the charged leptons and the neutral Higgs bosons. We find that
the S_3 x Z_2 flavour symmetry and the strong mass hierarchy of the charged
leptons strongly suppress the FCNC processes in the leptonic sector well below
the present experimental upper bounds by many orders of magnitude. The
contribution of FCNC to the anomaly of the muon's magnetic moment is small but
non-negligible.


title:
Conception d'outils de communication sp\'ecifiques au contexte
  \'educatif

abstract:
  In a distance learning context, providing usual communication tools (forum,
chat, ...) is not always enough to create efficient interactions between
learners and to favour collective knowledge building. A solution consists in
setting-up collective activities which encourage learners to communicate. But,
even in that case, tools can sometimes become a barrier to communication. We
present in this paper examples of specific tools that are designed in order to
favour and to guide communications in an educational context, but also to
foster interactions during learning activities that are not inherently
collaborative. We describe synchronous communication tools (semi-structured
chat), asynchronous tools (temporally structured forum, contextual forum) and a
system which promotes mutual aid between learners.


title:
Metallicity distribution of red giants in the Inner Galaxy from Near
  Infrared spectra

abstract:
  In this paper, we present low resolution (R=500) near-infrared spectra for
selected and serendipitous sources in six inner in-plane Galactic fields, with
the aim of analysing the stellar content present. From the equivalent widths of
the main features of the K band spectra (the NaI, CaI and CO bandheads) we have
derived the metallicities of the sources by means of the empirical scale
obtained by Ram\'irez et al. (2000) and Frogel et al. (2001) for luminous red
giants.
  Our results show how the mean metallicity of the sample varies with Galactic
longitude. We find two groups of stars, one whose [Fe/H] is similar to the
values obtained for the bulge in other studies (Molla et al. 2000; Schultehis
et al 2003), and a second one with a metallicity similar to that of the inner
parts of the disc (Rocha-Pinto et al. 2006). The relative density of both
groups of stars in our sample varies in a continuous way from the bulge to the
disc. This could hint at the existence of a single component apart from the
disc and bulge, running from l=7 to l=27 and able to transport disc stars
inwards and bulge stars outwards, which could be the Galactic bar that has been
detected in previous works as an overdensity of stars located at those galactic
coordinates (Hammersley et al. 1994, 2000; Picaud et al. 2003).


title:
Existence de points fixes enlac\'es \`a une orbite p\'eriodique d'un
  hom\'eomorphisme du plan

abstract:
  Let f be an orientation-preserving homeomorphism of the plane such that f-Id
is contracting. Under these hypotheses, we establish the existence, for every
periodic orbit, of a fixed point which has nonzero linking number with this
periodic orbit.


title: Tournament MAC with Constant Size Congestion Window for WLAN

abstract:
  In the context of radio distributed networks, we present a generalized
approach for the Medium Access Control (MAC) with fixed congestion window. Our
protocol is quite simple to analyze and can be used in a lot of different
situations. We give mathematical evidence showing that our performance is
tight, in the sense that no protocol with fixed congestion window can do
better. We also place ourselves in the WiFi/WiMAX framework, and show
experimental results enlightening collision reduction of 14% to 21% compared to
the best known other methods. We show channel capacity improvement, and
fairness considerations.


title: Simplicial complexes and Macaulay's inverse systems

abstract:
  Let $\Delta$ be a simplicial complex on $V = \{x_1,...,x_n\}$, with
Stanley-Reisner ideal $I_{\Delta}\subseteq R = k[x_1,...,x_n]$. The goal of
this paper is to investigate the class of artinian algebras
$A=A(\Delta,a_1,...,a_n)= R/(I_{\Delta},x_1^{a_1},...,x_n^{a_n})$, where each
$a_i \geq 2$. By utilizing the technique of Macaulay's inverse systems, we can
explicitly describe the socle of $A$ in terms of $\Delta$. As a consequence, we
determine the simplicial complexes, that we will call {\em levelable}, for
which there exists a tuple $(a_1,...,a_n)$ such that $A(\Delta,a_1,...,a_n)$ is
a level algebra.


title:
Quantum control of EIT dispersion via atomic tunneling in a double-well
  Bose-Einstein condensate

abstract:
  Electromagnetically induced transparency (EIT) is an important tool for
controlling light propagation and nonlinear wave mixing in atomic gases with
potential applications ranging from quantum computing to table top tests of
general relativity. Here we consider EIT in an atomic Bose-Einstein Condensate
(BEC) trapped in a double well potential. A weak probe laser propagates through
one of the wells and interacts with atoms in a three-level $\Lambda$
configuration. The well through which the probe propagates is dressed by a
strong control laser with Rabi frequency $\Omega_{\mu}$, as in standard EIT
systems. Tunneling between the wells at the frequency $g$ provides a coherent
coupling between identical electronic states in the two wells, which leads to
the formation of inter-well dressed states. The tunneling in conjunction with
the macroscopic interwell coherence of the BEC wave function, results in the
formation of two ultra-narrow absorption resonances for the probe field that
are inside of the ordinary EIT transparency window. We show that these new
resonances can be interpreted in terms of the inter-well dressed states and the
formation of a novel type of dark state involving the control laser and the
inter-well tunneling. To either side of these ultra-narrow resonances there is
normal dispersion with very large slope controlled by $g$. For realistic values
of $g$, the large slope of this dispersion yields group velocities for the
probe field that are two orders of magnitude slower than standard EIT systems.
We discuss prospects for observing these ultra-narrow resonances and the
corresponding regions of high dispersion experimentally.


title:
The Quantum Transverse Field Ising Model on an Infinite Tree from Matrix
  Product States

abstract:
  We give a generalization to an infinite tree geometry of Vidal's infinite
time-evolving block decimation (iTEBD) algorithm for simulating an infinite
line of quantum spins. We numerically investigate the quantum Ising model in a
transverse field on the Bethe lattice using the Matrix Product State ansatz. We
observe a second order phase transition, with certain key differences from the
transverse field Ising model on an infinite spin chain. We also investigate a
transverse field Ising model with a specific longitudinal field. When the
transverse field is turned off, this model has a highly degenerate ground state
as opposed to the pure Ising model whose ground state is only doubly
degenerate.


title:
Intrinsic Formulation of Geometric Integrability and Generation of
  Conservation Laws

abstract:
  An intrinsic version of the integrability theorem for the classical Backlund
theorem is presented. It is characterized by a one-form which can be put in the
form of a Riccati system. It is shown how this system can be linearized. Based
on this, a procedure for generating an infinite number of conservation laws is
given.


title:
The FFLO state in the one-dimensional attractive Hubbard model and its
  fingerprint in the spatial noise correlations

abstract:
  We explore the pairing properties of the one-dimensional attractive Hubbard
model in the presence of finite spin polarization. The correlation exponents
for the most important fluctuations are determined as a function of the density
and the polarization. We find that in a system with spin population imbalance,
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-type pairing at wavevector
Q=|k_{F,\uparrow}-k_{F,\downarrow}| is always dominant and there is no
Chandrasekhar-Clogston limit. We then investigate the case of weakly coupled 1D
systems and determine the region of stability of the 1D FFLO phase. This
picture is corroborated by density-matrix-renormalization-group (DMRG)
simulations of the spatial noise correlations in uniform and trapped systems,
unambiguously revealing the presence of fermion pairs with nonzero momentum Q.
This opens up an interesting possibility for experimental studies of FFLO
states.


title: MHD Wave Propagation in the Neighbourhood of Two Null Points

abstract:
  The nature of fast magnetoacoustic and Alfv\'en waves is investigated in a
zero $\beta$ plasma in the neighbourhood of a pair of two-dimensional null
points. This gives an indication of wave propagation in the low $\beta$ solar
corona, for a more complicated magnetic configuration than that looked at by
McLaughlin & Hood (2004). It is found that the fast wave is attracted to the
null points and that the front of the wave slows down as it approaches the null
point pair, with the wave splitting and part of the wave accumulating at one
null and the rest at the other. Current density will then accumulate at these
points and ohmic dissipation will then extract the energy in the wave at these
points. This suggests locations where wave heating will occur in the corona.
The Alfv\'en wave behaves in a different manner in that the wave accumulates
along the separatrices. Hence, the current density will accumulate at this part
of the topology and this is where wave heating will occur. However, the
phenomenon of wave accumulation at a specific place is a feature of both wave
types, and illustrates the importance of studying the topology of the corona
when considering MHD wave propagation.


title:
Flux tubes and the type-I/type-II transition in a superconductor coupled
  to a superfluid

abstract:
  We analyze magnetic flux tubes at zero temperature in a superconductor that
is coupled to a superfluid via both density and gradient (``entrainment'')
interactions. The example we have in mind is high-density nuclear matter, which
is a proton superconductor and a neutron superfluid, but our treatment is
general and simple, modeling the interactions as a Ginzburg-Landau effective
theory with four-fermion couplings, including only s-wave pairing. We
numerically solve the field equations for flux tubes with an arbitrary number
of flux quanta, and compare their energies. This allows us to map the
type-I/type-II transition in the superconductor, which occurs at the
conventional kappa = 1/sqrt(2) if the condensates are uncoupled.
  We find that a density coupling between the condensates raises the critical
kappa and, for a sufficiently high neutron density, resolves the type-I/type-II
transition line into an infinite number of bands corresponding to
``type-II(n)'' phases, in which n, the number of quanta in the favored flux
tube, steps from 1 to infinity. For lower neutron density, the coupling creates
spinodal regions around the type-I/type-II boundary, in which metastable flux
configurations are possible. We find that a gradient coupling between the
condensates lowers the critical kappa and creates spinodal regions. These
exotic phenomena may not occur in nuclear matter, which is thought to be deep
in the type-II region, but might be observed in condensed matter systems.