Particle production in Pb-Pb collisions at √ s NN = 2.76 TeV is studied in the (3 + 1)-dimensional viscous hydrodynamic model. The shapes of the calculated transverse momentum spectra of π + , K + , protons, − , and − are in satisfactory agreement with preliminary data of the ALICE Collaboration, while the particle ratio p/π + is slightly overpredicted, and the strange baryon yields are underpredicted.
The directed flow of particles emitted from the fireball created in a
heavy-ion collision is shown to be a very sensitive measure of the pressure
equilibration in the first 1 fm/c of the evolution. Performing a 3+1
dimensional relativistic hydrodynamic calculation with nonequilibrated
longitudinal and transverse pressures, we show that the directed flow is
strongly reduced if the pressure imbalance survives for even a short time.
Transverse momentum spectra, elliptic flow and interferometry correlation radii
are not very sensitive to this early pressure anisotropy. Comparison with the
data points toward a short equilibration time of the order of $0.25$fm/c or
less
DOI: will be assignedA new method of measuring the pressure anisotropy in the early stage of heavy-ion collisions is discussed. It is shown that the transverse momentum spectra, elliptic flow and interferometry radii are not sensitive to the early anisotropy. We propose the directed flow as a measure of the early transverse and longitudinal pressures. Calculations indicate that the transverse and longitudinal pressures equilibrate in less than 0.25 fm/c.
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