45 research outputs found
Effects of partial thermalization on HBT interferometry
Hydrodynamical models have generally failed to describe interferometry radii
measured at RHIC. In order to investigate this ``HBT puzzle'', we carry out a
systematic study of HBT radii in ultrarelativistic heavy-ion collisions within
a two-dimensional transport model. We compute the transverse radii and
as functions of for various values of the Knudsen number, which
measures the degree of thermalization in the system. For realistic values of
the Knudsen number estimated from data, we obtain ,
much closer to data than standard hydrodynamical results. Femtoscopic
observables vary little with the degree of thermalization. Azimuthal
oscillations of the radii in non central collisions do not provide a good probe
of thermalization.Comment: Proceedings for Quark Matter 2009, Knoxville, TN US
Dynamical evolution of heavy quarkonia in a deconfined medium
We investigate some consequences of the possibility that heavy quarkonia in a
quark-gluon plasma possess different (quasi-) bound states, between which
transitions are possible. In particular, we show that the time-evolution
eigenstates in the medium are mixtures of the vacuum eigenstates. This leads to
abundance ratios of quarkonia that differ from those predicted in statistical
models or in the sequential-melting picture.Comment: 5 pages, 3 figure
Triangular flow in hydrodynamics and transport theory
In ultrarelativistic heavy-ion collisions, the Fourier decomposition of the
relative azimuthal angle, \Delta \phi, distribution of particle pairs yields a
large cos(3\Delta \phi) component, extending out to large rapidity separations
\Delta \eta >1. This component captures a significant portion of the ridge and
shoulder structures in the \Delta \phi distribution, which have been observed
after contributions from elliptic flow are subtracted. An average finite
triangularity due to event-by-event fluctuations in the initial matter
distribution, followed by collective flow, naturally produces a cos(3\Delta
\phi) correlation. Using ideal and viscous hydrodynamics, and transport theory,
we study the physics of triangular (v_3) flow in comparison to elliptic (v_2),
quadrangular (v_4) and pentagonal (v_5) flow. We make quantitative predictions
for v_3 at RHIC and LHC as a function of centrality and transverse momentum.
Our results for the centrality dependence of v_3 show a quantitative agreement
with data extracted from previous correlation measurements by the STAR
collaboration. This study supports previous results on the importance of
triangular flow in the understanding of ridge and shoulder structures.
Triangular flow is found to be a sensitive probe of initial geometry
fluctuations and viscosity.Comment: 10 pages, 12 figures. minor changes, and results for added
(fig.12
The centrality dependence of elliptic flow, the hydrodynamic limit, and the viscosity of hot QCD
We show that the centrality and system-size dependence of elliptic flow
measured at RHIC are fully described by a simple model based on eccentricity
scaling and incomplete thermalization. We argue that the elliptic flow is at
least 25% below the (ideal) ``hydrodynamic limit'', even for the most central
Au-Au collisions. This lack of perfect equilibration allows for estimates of
the effective parton cross section in the Quark-Gluon Plasma and of its
viscosity to entropy density ratio. We also show how the initial conditions
affect the transport coefficients and thermodynamic quantities extracted from
the data, in particular the viscosity and the speed of sound.Comment: 5 pages, 2 figures. Extended discussion of the results, in particular
of lower viscosity and sound velocity required by CGC initial condition
Elliptic flow in transport theory and hydrodynamics
We present a new direct simulation Monte-Carlo method for solving the
relativistic Boltzmann equation. We solve numerically the 2-dimensional
Boltzmann equation using this new algorithm. We find that elliptic flow from
this transport calculation smoothly converges towards the value from ideal
hydrodynamics as the number of collisions per particle increases, as expected
on general theoretical grounds, but in contrast with previous transport
calculations.Comment: 5 pages, 3 figures, revise
Are eccentricity fluctuations able to explain the centrality dependence of ?
The fourth harmonic of the azimuthal distribution of particles has been
measured for Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC).
The centrality dependence of does not agree with the prediction from
hydrodynamics. In particular, the ratio , where denotes the
second harmonic of the azimuthal distribution of particles, is significantly
larger than predicted by hydrodynamics. We argue that this discrepancy is
mostly due to elliptic flow () fluctuations. We evaluate these
fluctuations on the basis of a Monte Carlo Glauber calculation. The effect of
deviations from local thermal equilibrium is also studied, but appears to be
only a small correction. Combining these two effects allows us to reproduce
experimental data for peripheral and midcentral collisions. However, we are
unable to explain the large magnitude of observed for the most
central collisions.Comment: talk presented at the Strangeness in Quark Matter Conference, Buzios,
Brazil, Sept. 27 - oct. 2, 200