515 research outputs found
Viscosity in the excluded volume hadron gas model
The shear viscosity in the van der Waals excluded volume
hadron-resonance gas model is considered. For the shear viscosity the result of
the non-relativistic gas of hard-core particles is extended to the mixture of
particles with different masses, but equal values of hard-core radius r. The
relativistic corrections to hadron average momenta in thermal equilibrium are
also taken into account. The ratio of the viscosity to the entropy
density s is studied. It monotonously decreases along the chemical freeze-out
line in nucleus-nucleus collisions with increasing collision energy. As a
function of hard-core radius r, a broad minimum of the ratio near fm is found at high collision energies. For the
charge-neutral system at MeV, a minimum of the ratio is reached for fm. To justify a hydrodynamic approach to
nucleus-nucleus collisions within the hadron phase the restriction from below,
fm, on the hard-core hadron radius should be fulfilled in the
excluded volume hadron-resonance gas.Comment: 12 pages, 3 figure
Turbulence without pressure
We develop exact field theoretic methods to treat turbulence when the effect
of pressure is negligible. We find explicit forms of certain probability
distributions, demonstrate that the breakdown of Galilean invariance is
responsible for intermittency and establish the operator product expansion. We
also indicate how the effects of pressure can be turned on perturbatively.Comment: 12 page
Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis
A sufficiently large unpolarized current can cause a spin-wave instability in
thin nanomagnets with asymmetric contacts. The dynamics beyond the instability
is understood in the perturbative regime of small spin-wave amplitudes, as well
as by numerically solving a discretized model. In the absence of an applied
magnetic field, our numerical simulations reveal a hierarchy of instabilities,
leading to chaotic magnetization dynamics for the largest current densities we
consider.Comment: 14 pages, 10 figures; revtex
Unique Fock quantization of scalar cosmological perturbations
We investigate the ambiguities in the Fock quantization of the scalar
perturbations of a Friedmann-Lema\^{i}tre-Robertson-Walker model with a massive
scalar field as matter content. We consider the case of compact spatial
sections (thus avoiding infrared divergences), with the topology of a
three-sphere. After expanding the perturbations in series of eigenfunctions of
the Laplace-Beltrami operator, the Hamiltonian of the system is written up to
quadratic order in them. We fix the gauge of the local degrees of freedom in
two different ways, reaching in both cases the same qualitative results. A
canonical transformation, which includes the scaling of the matter field
perturbations by the scale factor of the geometry, is performed in order to
arrive at a convenient formulation of the system. We then study the
quantization of these perturbations in the classical background determined by
the homogeneous variables. Based on previous work, we introduce a Fock
representation for the perturbations in which: (a) the complex structure is
invariant under the isometries of the spatial sections and (b) the field
dynamics is implemented as a unitary operator. These two properties select not
only a unique unitary equivalence class of representations, but also a
preferred field description, picking up a canonical pair of field variables
among all those that can be obtained by means of a time-dependent scaling of
the matter field (completed into a linear canonical transformation). Finally,
we present an equivalent quantization constructed in terms of gauge-invariant
quantities. We prove that this quantization can be attained by a mode-by-mode
time-dependent linear canonical transformation which admits a unitary
implementation, so that it is also uniquely determined.Comment: 19 pages, minor impovementes included, typos correcte
Early out-of-equilibrium beam-plasma evolution
We solve analytically the out-of-equilibrium initial stage that follows the
injection of a radially finite electron beam into a plasma at rest and test it
against particle-in-cell simulations. For initial large beam edge gradients and
not too large beam radius, compared to the electron skin depth, the electron
beam is shown to evolve into a ring structure. For low enough transverse
temperatures, the filamentation instability eventually proceeds and saturates
when transverse isotropy is reached. The analysis accounts for the variety of
very recent experimental beam transverse observations.Comment: to appear in Phys. Rev. Letter
Anticorrelation between Ion Acceleration and Nonlinear Coherent Structures from Laser-Underdense Plasma Interaction
In laser-plasma experiments, we observed that ion acceleration from the
Coulomb explosion of the plasma channel bored by the laser, is prevented when
multiple plasma instabilities such as filamentation and hosing, and nonlinear
coherent structures (vortices/post-solitons) appear in the wake of an
ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp
allows us to control the onset of these insabilities. We deduced that the laser
pulse is depleted into these structures in our conditions, when a plasma at
about 10% of the critical density exhibits a gradient on the order of 250
{\mu}m (gaussian fit), thus hindering the acceleration. A promising
experimental setup with a long pulse is demonstrated enabling the excitation of
an isolated coherent structure for polarimetric measurements and, in further
perspectives, parametric studies of ion plasma acceleration efficiency.Comment: 4 pages, 5 figure
A Hybrid Model for QCD Deconfining Phase Boundary
Intensive search for a proper and realistic equations of state (EOS) is still
continued for studying the phase diagram existing between quark gluon plasma
(QGP) and hadron gas (HG) phases. Lattice calculations provide such EOS for the
strongly interacting matter at finite temperature () and vanishing baryon
chemical potential (). These calculations are of limited use at finite
due to the appearance of notorious sign problem. In the recent past,
we had constructed a hybrid model description for the QGP as well as HG phases
where we make use of a new excluded-volume model for HG and a
thermodynamically-consistent quasiparticle model for the QGP phase and used
them further to get QCD phase boundary and a critical point. Since then many
lattice calculations have appeared showing various thermal and transport
properties of QCD matter at finite and . We test our hybrid
model by reproducing the entire data for strongly interacting matter and
predict our results at finite so that they can be tested in future.
Finally we demonstrate the utility of the model in fixing the precise location,
the order of the phase transition and the nature of CP existing on the QCD
phase diagram. We thus emphasize the suitability of the hybrid model as
formulated here in providing a realistic EOS for the strongly interacting
matter.Comment: 22 pages, 10 figures. corrected version published in Physical Review
D. arXiv admin note: substantial text overlap with arXiv:1201.044
Short Intense Laser Pulse Collapse in Near-Critical Plasma
It is observed that the interaction of an intense ultra-short laser pulse
with an overdense gas jet results in the pulse collapse and the deposition of a
significant part of energy in a small and well localized volume in the rising
part of the gas jet, where the electrons are efficiently accelerated and
heated. A collisionless plasma expansion over 150 microns at a sub-relativistic
velocity (~c/3) has been optically monitored in time and space, and attributed
to the quasistatic field ionization of the gas associated to the hot electron
current. Numerical simulations in good agreement with the observations suggest
the acceleration in the collapse region of relativistic electrons, along with
the excitation of a sizeable magnetic dipole that sustains the electron current
over several picoseconds. Perspectives of ion beam generation at high
repetition rate directly from gas jets are discussed
Correlated two-particle scattering on finite cavities
The correlated two-particle problem is solved analytically in the presence of
a finite cavity. The method is demonstrated here in terms of exactly solvable
models for both the cavity as well as the two-particle correlation where the
two-particle potential is chosen in separable form. The two-particle phase
shift is calculated and compared to the single-particle one. The two-particle
bound state behavior is discussed and the influence of the cavity on the
binding properties is calculated.Comment: Derivation shortened and corrected, 14 pages 10 figure
Shape oscillations in non-degenerate Bose gases - transition from the collisionless to the hydrodynamic regime
We investigate collective oscillations of non-degenerate clouds of Rb-87
atoms as a function of density in an elongated magnetic trap. For the low-lying
M=0 monopole-quadrupole shape oscillation we measure the oscillation
frequencies and damping rates. At the highest densities the mean-free-path is
smaller than the axial dimension of the sample, which corresponds to
collisionally hydrodynamic conditions. This allows us to cover the cross-over
from the collisionless to the hydrodynamic regime. The experimental results
show good agreement with theory. We also analyze the influence of trap
anharmonicities on the oscillations in relation to observed temperature
dependencies of the dipole and quadrupole oscillation frequencies. We present
convenient expressions to quantify these effects.Comment: 10 pages, 5 figure
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