10,145 research outputs found
Bulk viscosity of a gas of neutrinos and coupled scalar particles, in the era of recombination
Bulk viscosity may serve to damp sound waves in a system of neutrinos coupled
to very light scalar particles, in the era after normal neutrino decoupling but
before recombination. We calculate the bulk viscosity parameter in a minimal
scheme involving the coupling of the two systems. We add some remarks on the
bulk viscosity of a system of fully ionized hydrogen plus photons.Comment: 6 pages, 4 figure
Scaling of in heavy ion collisions
We interpret the scaling of the corrected elliptic flow parameter w.r.t. the
corrected multiplicity, observed to hold in heavy ion collisions for a wide
variety of energies and system sizes. We use dimensional analysis and
power-counting arguments to place constraints on the changes of initial
conditions in systems with different center of mass energy .
Specifically, we show that a large class of changes in the (initial) equation
of state, mean free path, and longitudinal geometry over the observed
are likely to spoil the scaling in observed experimentally. We
therefore argue that the system produced at most Super Proton Synchrotron (SPS)
and Relativistic Heavy Ion Collider (RHIC) energies is fundamentally the same
as far as the soft and approximately thermalized degrees of freedom are
considered. The ``sQGP'' (Strongly interacting Quark-Gluon Plasma) phase, if it
is there, is therefore not exclusive to RHIC. We suggest, as a goal for further
low-energy heavy ion experiments, to search for a ``transition''
where the observed scaling breaks.Comment: Accepted for publication by Phys. Rev. C Based on presentation in
mini-symposium on QGP collective properties, Frankfurt. Discussion expanded,
results adde
Dangerous Angular KK/Glueball Relics in String Theory Cosmology
The presence of Kaluza-Klein particles in the universe is a potential
manifestation of string theory cosmology. In general, they can be present in
the high temperature bath of the early universe. In particular examples, string
theory inflation often ends with brane-antibrane annihilation followed by the
energy cascading through massive closed string loops to KK modes which then
decay into lighter standard model particles. However, massive KK modes in the
early universe may become dangerous cosmological relics if the inner manifold
contains warped throat(s) with approximate isometries. In the complimentary
picture, in the AdS/CFT dual gauge theory with extra symmetries, massive
glueballs of various spins become the dangerous cosmological relics. The decay
of these angular KK modes/glueballs, located around the tip of the throat, is
caused by isometry breaking which results from gluing the throat to the compact
CY manifold. We address the problem of these angular KK particles/glueballs,
studying their interactions and decay channels, from the theory side, and the
resulting cosmological constraints on the warped compactification parameters,
from the phenomenology side. The abundance and decay time of the long-lived
non-relativistic angular KK modes depend strongly on the parameters of the
warped geometry, so that observational constraints rule out a significant
fraction of the parameter space. In particular, the coupling of the angular KK
particles can be weaker than gravitational.Comment: 58 pages, 11 figures, published versio
3+1D hydrodynamic simulation of relativistic heavy-ion collisions
We present MUSIC, an implementation of the Kurganov-Tadmor algorithm for
relativistic 3+1 dimensional fluid dynamics in heavy-ion collision scenarios.
This Riemann-solver-free, second-order, high-resolution scheme is characterized
by a very small numerical viscosity and its ability to treat shocks and
discontinuities very well. We also incorporate a sophisticated algorithm for
the determination of the freeze-out surface using a three dimensional
triangulation of the hyper-surface. Implementing a recent lattice based
equation of state, we compute p_T-spectra and pseudorapidity distributions for
Au+Au collisions at root s = 200 GeV and present results for the anisotropic
flow coefficients v_2 and v_4 as a function of both p_T and pseudorapidity. We
were able to determine v_4 with high numerical precision, finding that it does
not strongly depend on the choice of initial condition or equation of state.Comment: 16 pages, 11 figures, version accepted for publication in PRC,
references added, minor typos corrected, more detailed discussion of
freeze-out routine adde
Dissipative hydrodynamics in 2+1 dimension
In 2+1 dimension, we have simulated the hydrodynamic evolution of QGP fluid
with dissipation due to shear viscosity. Comparison of evolution of ideal and
viscous fluid, both initialised under the same conditions e.g. same
equilibration time, energy density and velocity profile, reveal that the
dissipative fluid evolves slowly, cooling at a slower rate. Cooling get still
slower for higher viscosity. The fluid velocities on the otherhand evolve
faster in a dissipative fluid than in an ideal fluid. The transverse expansion
is also enhanced in dissipative evolution. For the same decoupling temperature,
freeze-out surface for a dissipative fluid is more extended than an ideal
fluid. Dissipation produces entropy as a result of which particle production is
increased. Particle production is increased due to (i) extension of the
freeze-out surface and (ii) change of the equilibrium distribution function to
a non-equilibrium one, the last effect being prominent at large transverse
momentum. Compared to ideal fluid, transverse momentum distribution of pion
production is considerably enhanced. Enhancement is more at high than at
low . Pion production also increases with viscosity, larger the viscosity,
more is the pion production. Dissipation also modifies the elliptic flow.
Elliptic flow is reduced in viscous dynamics. Also, contrary to ideal dynamics
where elliptic flow continues to increase with transverse momentum, in viscous
dynamics, elliptic flow tends to saturate at large transverse momentum. The
analysis suggest that initial conditions of the hot, dense matter produced in
Au+Au collisions at RHIC, as extracted from ideal fluid analysis can be changed
significantly if the QGP fluid is viscous.Comment: 11 pages, 10 figures (revised). In the revised version, calculations
are redone with ADS/CFT and perurbative estimate of viscosity. Comments on
the unphysical effects like early reheating of the fluid, in 1st order
dissipative theories are added. The particle spectra calculations are redone
with modified programm
How much entropy is produced in strongly coupled Quark-Gluon Plasma (sQGP) by dissipative effects?
We argue that estimates of dissipative effects based on the first-order
hydrodynamics with shear viscosity are potentially misleading because higher
order terms in the gradient expansion of the dissipative part of the stress
tensor tend to reduce them. Using recently obtained sound dispersion relation
in thermal =4 supersymmetric plasma, we calculate the effect
of these high order terms for Bjorken expansion appropriate to RHIC/LHC
collisions. A reduction of entropy production is found to be substantial, up to
an order of magnitude.Comment: 4 pages, 4 figur
Dissipative hydrodynamics for viscous relativistic fluids
Explicit equations are given for describing the space-time evolution of
non-ideal (viscous) relativistic fluids undergoing boost-invariant longitudinal
and arbitrary transverse expansion. The equations are derived from the
second-order Israel-Stewart approach which ensures causal evolution. Both
azimuthally symmetric (1+1)-dimensional and non-symmetric (2+1)-dimensional
transverse expansion are discussed. The latter provides the formal basis for
the hydrodynamic computation of elliptic flow in relativistic heavy-ion
collisions including dissipative effects.Comment: 12 pages, no figures. Submitted to Physical Review
Some Remarks on Oscillating Inflation
In a recent paper Damour and Mukhanov describe a scenario where inflation may
continue during the oscillatory phase. This effect is possible because the
scalar field spends a significant fraction of each period of oscillation on the
upper part of the potential. Such additional period of inflation could push
perturbations after the slow roll regime to observable scales. Although in this
work we show that the small region of the Damour-Mukhanov parameter q gives the
main contribution to oscillating inflation, it was not satisfactory understood
until now. Furthermore, it gives an expression for the energy density spectrum
of perturbations, which is well behaved in the whole physical range of q .Comment: 4 pages including figures caption, 3 ps-figures. To appear in Phys.
Rev.
Relativistic hydrodynamics for heavy-ion collisions
Relativistic hydrodynamics is essential to our current understanding of
nucleus-nucleus collisions at ultrarelativistic energies (current experiments
at the Relativistic Heavy Ion Collider, forthcoming experiments at the CERN
Large Hadron Collider). This is an introduction to relativistic hydrodynamics
for graduate students. It includes a detailed derivation of the equations, and
a description of the hydrodynamical evolution of a heavy-ion collisions. Some
knowledge of thermodynamics and special relativity is assumed.Comment: Lectures given at the Advanced School on Quark-Gluon Plasma, Indian
Institute of Technology, Bombay, 3-13 July, 200
Dual Interpretations of Pion Clouds at RHIC
A gauge theory of pions interacting with rho-mesons at elevated temperatures
is used to calculate the pressure in a hot pion gas. No reference is made to
the pion's status as a QCD Goldstone boson. The role of the pion is merely that
of a carrier of an SU(2) symmetry, gauged to create a vector-meson interaction,
the rho playing the role of the interacting vector particle. The results are in
rough agreement with much more elaborate calculations, both of the purely
hadronic variety, and those that invoke quark-gluon degrees of freedom. The
quark-gluon and purely hadronic calculations seemingly lead to very similar
predictions which are in accord with receent data from RHIC. The results
motivate the question as to whether the two descriptions are dual to each other
in the sense of being alternate models, each sufficient to explain the observed
data.Comment: 11 pages, 4 figures (2 eps files, 1 ps file) + a figure that uses
metafont package feynmf, also forwarded. Open with "latex feynmf.ins". See
http://www.ctan.org/tex-archive/help/Catalogue/entries/feynmf.html 9/6/06
replaced figure 2 with scaled version of sam
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