586 research outputs found
Multiscale expansions of difference equations in the small lattice spacing regime, and a vicinity and integrability test. I
We propose an algorithmic procedure i) to study the ``distance'' between an
integrable PDE and any discretization of it, in the small lattice spacing
epsilon regime, and, at the same time, ii) to test the (asymptotic)
integrability properties of such discretization. This method should provide, in
particular, useful and concrete informations on how good is any numerical
scheme used to integrate a given integrable PDE. The procedure, illustrated on
a fairly general 10-parameter family of discretizations of the nonlinear
Schroedinger equation, consists of the following three steps: i) the
construction of the continuous multiscale expansion of a generic solution of
the discrete system at all orders in epsilon, following the Degasperis -
Manakov - Santini procedure; ii) the application, to such expansion, of the
Degasperis - Procesi (DP) integrability test, to test the asymptotic
integrability properties of the discrete system and its ``distance'' from its
continuous limit; iii) the use of the main output of the DP test to construct
infinitely many approximate symmetries and constants of motion of the discrete
system, through novel and simple formulas.Comment: 34 pages, no figur
Non-linear electromagnetic response of graphene
It is shown that the massless energy spectrum of electrons and holes in
graphene leads to the strongly non-linear electromagnetic response of this
system. We predict that the graphene layer, irradiated by electromagnetic
waves, emits radiation at higher frequency harmonics and can work as a
frequency multiplier. The operating frequency of the graphene frequency
multiplier can lie in a broad range from microwaves to the infrared.Comment: 5 pages, 3 figure
The GAMMA-400 space observatory: status and perspectives
The present design of the new space observatory GAMMA-400 is presented in
this paper. The instrument has been designed for the optimal detection of gamma
rays in a broad energy range (from ~100 MeV up to 3 TeV), with excellent
angular and energy resolution. The observatory will also allow precise and high
statistic studies of the electron component in the cosmic rays up to the multi
TeV region, as well as protons and nuclei spectra up to the knee region. The
GAMMA-400 observatory will allow to address a broad range of science topics,
like search for signatures of dark matter, studies of Galactic and
extragalactic gamma-ray sources, Galactic and extragalactic diffuse emission,
gamma-ray bursts and charged cosmic rays acceleration and diffusion mechanism
up to the knee
Statistical Mechanics of Canonical-Dissipative Systems and Applications to Swarm Dynamics
We develop the theory of canonical-dissipative systems, based on the
assumption that both the conservative and the dissipative elements of the
dynamics are determined by invariants of motion. In this case, known solutions
for conservative systems can be used for an extension of the dynamics, which
also includes elements such as the take-up/dissipation of energy. This way, a
rather complex dynamics can be mapped to an analytically tractable model, while
still covering important features of non-equilibrium systems. In our paper,
this approach is used to derive a rather general swarm model that considers (a)
the energetic conditions of swarming, i.e. for active motion, (b) interactions
between the particles based on global couplings. We derive analytical
expressions for the non-equilibrium velocity distribution and the mean squared
displacement of the swarm. Further, we investigate the influence of different
global couplings on the overall behavior of the swarm by means of
particle-based computer simulations and compare them with the analytical
estimations.Comment: 14 pages incl. 13 figures. v2: misprints in Eq. (40) corrected, ref.
updated. For related work see also:
http://summa.physik.hu-berlin.de/~frank/active.htm
Search for anisotropies in cosmic-ray positrons detected by the PAMELA experiment
The PAMELA detector was launched on board of the Russian Resurs-DK1 satellite
on June 15, 2006. Data collected during the first four years have been used to
search for large-scale anisotropies in the arrival directions of cosmic-ray
positrons. The PAMELA experiment allows for a full sky investigation, with
sensitivity to global anisotropies in any angular window of the celestial
sphere. Data samples of positrons in the rigidity range 10 GV R
200 GV were analyzed. This article discusses the method and the results of the
search for possible local sources through analysis of anisotropy in positron
data compared to the proton background. The resulting distributions of arrival
directions are found to be isotropic. Starting from the angular power spectrum,
a dipole anisotropy upper limit \delta = 0.166 at 95% C.L. is determined.
Additional search is carried out around the Sun. No evidence of an excess
correlated with that direction was found.Comment: The value of the dipole anisotropy upper limit has been changed. The
method is correct but there was a miscalculation in the relative formul
PAMELA's measurements of geomagnetic cutoff variations during solar energetic particle events
Data from the PAMELA satellite experiment were used to measure the
geomagnetic cutoff for high-energy ( 80 MeV) protons during the solar
particle events on 2006 December 13 and 14. The variations of the cutoff
latitude as a function of rigidity were studied on relatively short timescales,
corresponding to single spacecraft orbits (about 94 minutes). Estimated cutoff
values were cross-checked with those obtained by means of a trajectory tracing
approach based on dynamical empirical modeling of the Earth's magnetosphere. We
find significant variations in the cutoff latitude, with a maximum suppression
of about 6 deg for 80 MeV protons during the main phase of the storm. The
observed reduction in the geomagnetic shielding and its temporal evolution were
compared with the changes in the magnetosphere configuration, investigating the
role of IMF, solar wind and geomagnetic (Kp, Dst and Sym-H indexes) variables
and their correlation with PAMELA cutoff results.Comment: Conference: The 34th International Cosmic Ray Conference (ICRC2015),
30 July - 6 August, 2015, The Hague, The Netherlands, Volume:
PoS(ICRC2015)28
Solar energetic particle events: trajectory analysis and flux reconstruction with PAMELA
The PAMELA satellite experiment is providing first direct measurements of
Solar Energetic Particles (SEPs) with energies from about 80 MeV to several GeV
in near-Earth space, bridging the low energy data by other space-based
instruments and the Ground Level Enhancement (GLE) data by the worldwide
network of neutron monitors. Its unique observational capabilities include the
possibility of measuring the flux angular distribution and thus investigating
possible anisotropies. This work reports the analysis methods developed to
estimate the SEP energy spectra as a function of the particle pitch-angle with
respect to the Interplanetary Magnetic Field (IMF) direction. The crucial
ingredient is provided by an accurate simulation of the asymptotic exposition
of the PAMELA apparatus, based on a realistic reconstruction of particle
trajectories in the Earth's magnetosphere. As case study, the results for the
May 17, 2012 event are presented.Comment: Conference: The 34th International Cosmic Ray Conference (ICRC2015),
30 July - 6 August, 2015, The Hague, The Netherlands, Volume:
PoS(ICRC2015)08
Mid-infrared plasmons in scaled graphene nanostructures
Plasmonics takes advantage of the collective response of electrons to
electromagnetic waves, enabling dramatic scaling of optical devices beyond the
diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns)
plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100
times smaller than the on-resonance light wavelength in free space. We reveal,
for the first time, the crucial damping channels of graphene plasmons via its
intrinsic optical phonons and scattering from the edges. A plasmon lifetime of
20 femto-seconds and smaller is observed, when damping through the emission of
an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2
substrate underneath the graphene nanostructures lead to a significantly
modified plasmon dispersion and damping, in contrast to a non-polar
diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the
plasmon resonance frequencies are close to the polar phonon frequencies. Our
study paves the way for applications of graphene in plasmonic waveguides,
modulators and detectors in an unprecedentedly broad wavelength range from
sub-terahertz to mid-infrared.Comment: submitte
The Polarised Valence Quark Distribution from semi-inclusive DIS
The semi-inclusive difference asymmetry A^{h^{+}-h^{-}} for hadrons of
opposite charge has been measured by the COMPASS experiment at CERN. The data
were collected in the years 2002-2004 using a 160 GeV polarised muon beam
scattered off a large polarised ^6LiD target and cover the range 0.006 < x <
0.7 and 1 < Q^2 < 100 (GeV/c)^2. In leading order QCD (LO) the asymmetry
A_d^{h^{+}-h^{-}} measures the valence quark polarisation and provides an
evaluation of the first moment of Delta u_v + Delta d_v which is found to be
equal to 0.40 +- 0.07 (stat.) +- 0.05 (syst.) over the measured range of x at
Q^2 = 10 (GeV/c)^2. When combined with the first moment of g_1^d previously
measured on the same data, this result favours a non-symmetric polarisation of
light quarks Delta u-bar = - Delta d-bar at a confidence level of two standard
deviations, in contrast to the often assumed symmetric scenario Delta u-bar =
Delta d-bar = Delta s-bar = Delta s.Comment: 7 pages, 3 figures, COMPASS, revised: details added, author list
update
Multiplicities of charged pions and unidentified charged hadrons from deep-inelastic scattering of muons off an isoscalar target
Multiplicities of charged pions and unidentified hadrons produced in
deep-inelastic scattering were measured in bins of the Bjorken scaling variable
, the relative virtual-photon energy and the relative hadron energy .
Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam and
an isoscalar target (LiD). They cover the kinematic domain in the photon
virtuality > 1(GeV/c, , and . In addition, a leading-order pQCD analysis was performed using the
pion multiplicity results to extract quark fragmentation functions
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