97 research outputs found
Thermal nature of charmonium transverse momentum spectra from Au-Au collisions at the highest energies available at the BNL Relativistic Heavy Ion Collider (RHIC)
We analyze the transverse momentum distribution of mesons produced
in Au + Au collisions at the top RHIC energy within a blast-wave model that
accounts for a possible inhomogeneity of the charmonium distribution and/or
flow fluctuations. The results imply that the transverse momentum spectra
of, and hadrons measured at the RHIC can be described
well if kinetic freeze-out takes place just after chemical freeze-out for these
particles.Comment: 17 pages, 5 figures, minor corrections, to appear in Phys. Rev.
On freeze-out problem in hydro-kinetic approach to A+A collisions
A new method for evaluating spectra and correlations in the hydrodynamic
approach is proposed. It is based on an analysis of Boltzmann equations (BE) in
terms of probabilities for constituent particles to escape from the interacting
system. The conditions of applicability of Cooper-Frye freeze-out prescription
are considered within the method. The results are illustrated with a
non-relativistic exact solution of BE for expanding spherical fireball as well
as with approximate solutions for ellipsoidally expanding ones.Comment: 4 pages including 2 figures, RevTex, stylistic and clarifying
corrections are made, submitted to Phys. Rev. Let
Initial condition for hydrodynamics, partonic free streaming, and the uniform description of soft observables at RHIC
We investigate the role of the initial condition used for the hydrodynamic
evolution of the system formed in ultra-relativistic heavy-ion collisions and
find that an appropriate choice motivated by the models of early-stage
dynamics, specifically a simple two-dimensional Gaussian profile, leads to a
uniform description of soft observables measured in the Relativistic Heavy-Ion
Collider (RHIC). In particular, the transverse-momentum spectra, the
elliptic-flow, and the Hanbury-Brown--Twiss correlation radii, including the
ratio R_out/R_side as well as the dependence of the radii on the azimuthal
angle (azHBT), are properly described. We use the perfect-fluid hydrodynamics
with a realistic equation of state based on lattice calculations and the
hadronic gas at high and low temperatures, respectively. We also show that the
inclusion of the partonic free-streaming in the early stage allows to delay the
start of the hydrodynamical description to comfortable times of the order of 1
fm/c. Free streaming broadens the initial energy-density profile, but generates
the initial transverse and elliptic flow. The data may be described equally
well when the hydrodynamics is started early, or with a delay due to partonic
free-streaming.Comment: 4 pages, 4 figure
HBT search for new states of matter in A+A collisions
A method allowing studies of the hadronic matter at the early evolution stage
in A+A collisions is developed. It is based on an interferometry analysis of
approximately conserved values such as the averaged phase-space density (APSD)
and the specific entropy of thermal pions. The plateau found in the APSD
behavior vs collision energy at SPS is associated, apparently, with the
deconfinement phase transition at low SPS energies; a saturation of this
quantity at the RHIC energies indicates the limiting Hagedorn temperature for
hadronic matter. It is shown that if the cubic power of effective temperature
of pion transverse spectra grows with energy similarly to the rapidity density
(that is roughly consistent with experimental data), then the interferometry
volume is inverse proportional to the pion APSD that is about a constant
because of limiting Hagedorn temperature. This sheds light on the HBT puzzle.Comment: 23 pages, 8 figures, minor correction
Free-streaming approximation in early dynamics of relativistic heavy-ion collisions
We investigate an approximation to early dynamics in relativistic heavy-ion
collisions, where after formation the partons are free streaming and around the
proper time of 1 fm/c undergo a sudden equilibration described in terms of the
Landau matching condition. We discuss physical and formal aspects of this
approach. In particular, we show that initial azimuthally asymmetric transverse
flow develops for non-central collisions as a consequence of the sudden
equilibration. Moreover, the energy-momentum tensor from the free-streaming
stage matches very smoothly to the form used in the transverse hydrodynamics,
whereas matching to isotropic hydrodynamics requires a more pronounced change
in the energy-momentum tensor. After the hydrodynamic phase statistical
hadronization is carried out with the help of THERMINATOR. The physical results
for the transverse-momentum spectra, the elliptic-flow, and the
Hanbury-Brown--Twiss correlation radii, including the ratio R_out/R_side as
well as the dependence of the radii on the azimuthal angle (azHBT), are
properly described within our approach. The agreement is equally good for a
purely hydrodynamic evolution started at an early proper time of 0.25 fm/c, or
for the free streaming started at that time, followed by the sudden
equilibration at tau ~1 fm/c and then by perfect hydrodynamics. Thus, the
inclusion of free streaming allows us to delay the start of hydrodynamics to
more realistic times of the order of 1 fm/c.Comment: 10 pages, 12 figure
Early anisotropic hydrodynamics and the RHIC early-thermalization and HBT puzzles
We address the problem if the early thermalization and HBT puzzles in
relativistic heavy-ion collisions may be solved by the assumption that the
early dynamics of the produced matter is locally anisotropic. The hybrid model
describing the purely transverse hydrodynamic evolution followed by the
perfect-fluid hydrodynamic stage is constructed. The transition from the
transverse to perfect-fluid hydrodynamics is described by the Landau matching
conditions applied at a fixed proper time. The global fit to the RHIC data
reproduces the soft hadronic observables (the pion, kaon, and the proton
spectra, the pion and kaon elliptic flow, and the pion HBT radii) with the
accuracy of about 20%. These results indicate that the assumption of the very
fast thermalization may be relaxed. In addition, the presented model suggests
that a large part of the inconsistencies between the theoretical and
experimental HBT results may be removed.Comment: replaced with the version published in Phys.Rev.C 8
Particle Freeze-out and Discontinuities in Relativistic Hydrodynamics
Freeze-out of particles in relativistic hydrodynamics is considered across a
3-dimensional space-time hypersurface. The conservation laws for time-like
parts of the freeze-out hypersurface require different values of temperature,
baryonic chemical potential and flow velocity in the fluid and in the final
particle spectra. We analyze this freeze-out discontinuity and its connection
to the shock-wave phenomena in relativistic hydrodynamics.Comment: 6 figure
Small size boundary effects on two-pion interferometry
The Bose-Einstein correlations of two identically charged pions are derived
when these particles, the most abundantly produced in relativistic heavy ion
collisions, are confined in finite volumes. Boundary effects on single pion
spectrum are also studied. Numerical results emphasize that conventional
formulation usually adopted to describe two-pion interferometry should not be
used when the source size is small, since this is the most sensitive case to
boundary effects. Specific examples are considered for better illustration.Comment: more discussion on Figure4 and diffuse boundar
Feeding of the elliptic flow by hard partons
We propose that in nuclear collisions at the LHC the elliptic flow may get a
contribution from leading hard and semihard partons which deposit energy and
momentum into the hydrodynamic bulk medium. The crucial effect is that these
partons induce wakes which interact and merge if they come together. The
contribution to the integrated elliptic flow is estimated with the help of a
toy model to about 25% of the observed value and shows strong event-by-event
fluctuations.Comment: 9 pages, 3 figure
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