Viscosity in the excluded volume hadron gas model

The shear viscosity $\eta$ 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 $\eta$ 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 $\eta/s\approx 0.3$ near $r \approx 0.5$ fm is found at high collision energies. For the charge-neutral system at $T=T_c=180$ MeV, a minimum of the ratio $\eta/s\cong 0.24$ is reached for $r\cong 0.53$ fm. To justify a hydrodynamic approach to nucleus-nucleus collisions within the hadron phase the restriction from below, $r~ \ge ~0.2$ 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

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

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

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 ($T$) and vanishing baryon chemical potential ($\mu_{B}$). These calculations are of limited use at finite $\mu_{B}$ 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 $T$ and $\mu_{B}=0$. We test our hybrid model by reproducing the entire data for strongly interacting matter and predict our results at finite $\mu_{B}$ 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