492 research outputs found
The "Ridge" in Proton-Proton Scattering at 7 TeV
One of the most important experimental results for proton-proton scattering
at the LHC is the observation of a so-called "ridge" structure in the two
particle correlation function versus the pseudorapidity difference
and the azimuthal angle difference . One finds a strong correlation
around , extended over many units in . We show that a
hydrodynamical expansion based on flux tube initial conditions leads in a
natural way to the observed structure. To get this result, we have to perform
an event-by-event calculation, because the effect is due to statistical
fluctuations of the initial conditions, together with a subsequent collective
expansion. This is a strong point in favour of a fluid-like behavior even in
scattering, where we have to deal with length scales of the order of 0.1
fm.Comment: 5 pages, 4 figure
Dynamical freeze-out in event-by-event hydrodynamics
In hydrodynamical modeling of the ultrarelativistic heavy-ion collisions the
freeze-out is typically performed at a constant temperature or density. In this
work we apply a dynamical freeze-out criterion, which compares the
hydrodynamical expansion rate with the pion scattering rate. Recently many
calculations have been done using event-by-event hydrodynamics where the
initial density profile fluctuates from event to event. In these event-by-event
calculations the expansion rate fluctuates strongly as well, and thus it is
interesting to check how the dynamical freeze-out changes hadron distributions
with respect to the constant temperature freeze-out. We present hadron spectra
and elliptic flow calculated using (2+1)-dimensional ideal hydrodynamics, and
show the differences between constant temperature and dynamical freeze-out
criteria. We find that the differences caused by different freeze-out criteria
are small in all studied cases.Comment: 8 pages, 11 figures, to be published in the proceedings of the 28th
Winter Workshop on Nuclear Dynamics, Puerto Rico, April 7-14, 201
Sound Mode Hydrodynamics from Bulk Scalar Fields
We study the hydrodynamic sound mode using gauge/gravity correspondence by
examining a generic black brane background's response to perturbations. We
assume that the background is generated by a single scalar field, and then
generalize to the case of multiple scalar fields. The relevant differential
equations obeyed by the gauge invariant variables are presented in both cases.
Finally, we present an analytical solution to these equations in a special
case; this solution allows us to determine the speed of sound and bulk
viscosity for certain special metrics. These results may be useful in
determining sound mode transport coefficients in phenomenologically motivated
holographic models of strongly coupled systems.Comment: 17 pages. Corrections made to one of the gauge invariant equations
(66). This equation was not used in the other main conclusions of the paper,
so the rest of the results are unchange
Effects of fluctuations on the initial eccentricity from the Color Glass Condensate in heavy ion collisions
We introduce a modified form of the Kharzeev-Levin-Nardi (KLN) approach for
nuclear collisions. The new ansatz for the unintegrated gluon distribution
function preserves factorization, and the saturation scale is bound from below
by that for a single nucleon. It also reproduces the correct scaling with the
number of collisions at high transverse momentum. The corresponding Monte Carlo
implementation allows us to account for fluctuations of the hard sources
(nucleons) in the transverse plane. We compute various definitions of the
eccentricity within the new approach, which are relevant for the interpretation
of the elliptic flow. Our approach predicts breaking of the scaling of the
eccentricity with the Glauber eccentricity at the level of about 30%.Comment: 9 pages, 10 figures, Updated version as accepted by Phys.Rev.
Results on Identified Hadrons from the PHENIX Experiment at RHIC
Recent results on identified hadrons from the PHENIX experiment in Au+Au
collisions at mid-rapidity at = 200 GeV are presented. The
centrality dependence of transverse momentum distributions and particle ratios
for identified charged hadrons are studied. The transverse flow velocity and
freeze-out temperature are extracted from spectra within the framework
of a hydrodynamic collective flow model. Two-particle HBT correlations for
charged pions are measured in different centrality selections for a broad range
of transverse momentum of the pair. Results on elliptic flow measurements with
respect to the reaction plane for identified particles are also presented.Comment: 10 pages, 10 figures. Talk presented at Quark Matter 2002, Nantes,
France, July 18-24, 2002. To appear in the proceedings (Nucl. Phys. A
Chemical freeze-out temperature in hydrodynamical description of Au+Au collisions at sqrt(s_NN) = 200 GeV
We study the effect of separate chemical and kinetic freeze-outs to the ideal
hydrodynamical flow in Au+Au collisions at RHIC (sqrt(s_NN) = 200 GeV energy).
Unlike in earlier studies we explore how these effects can be counteracted by
changes in the initial state of the hydrodynamical evolution. We conclude that
the reproduction of pion, proton and antiproton yields necessitates a chemical
freeze-out temperature of T = 150 MeV instead of T = 160 - 170 MeV motivated by
thermal models. Unlike previously reported, this lower temperature makes it
possible to reproduce the p_T-spectra of hadrons if one assumes very small
initial time, tau_0 = 0.2 fm/c. However, the p_T-differential elliptic flow,
v_2(p_T) remains badly reproduced. This points to the need to include
dissipative effects (viscosity) or some other refinement to the model.Comment: 8 pages, 7 figures; Accepted for publication in European Physical
Journal A; Added discussion about the effect of weak decays to chemical
freeze-out temperature and a figure showing isentropic curves in T-mu plan
Second order hydrodynamics for a special class of gravity duals
The sound mode hydrodynamic dispersion relation is computed up to order
for a class of gravitational duals which includes both Schwarzschild and
Dp-Brane metrics. The implications for second order transport coefficients are
examined within the context of Israel-Stewart theory. These sound mode results
are compared with previously known results for the shear mode. This comparison
allows one to determine the third order hydrodynamic contributions to the shear
mode for the class of metrics considered here.Comment: 20 page
Influence of a temperature-dependent shear viscosity on the azimuthal asymmetries of transverse momentum spectra in ultrarelativistic heavy-ion collisions
We study the influence of a temperature-dependent shear viscosity over
entropy density ratio , different shear relaxation times , as
well as different initial conditions on the transverse momentum spectra of
charged hadrons and identified particles. We investigate the azimuthal flow
asymmetries as a function of both collision energy and centrality. The elliptic
flow coefficient turns out to be dominated by the hadronic viscosity at RHIC
energies. Only at higher collision energies the impact of the viscosity in the
QGP phase is visible in the flow asymmetries. Nevertheless, the shear viscosity
near the QCD transition region has the largest impact on the collective flow of
the system. We also find that the centrality dependence of the elliptic flow is
sensitive to the temperature dependence of .Comment: 13 pages, 20 figure
On the formation of Hubble flow in Little Bangs
A dynamical appearance of scaling solutions in the relativistic hydrodynamics
applied to describe ultra-relativistic heavy-ion collisions is studied. We
consider the boost-invariant cylindrically symmetric systems and the effects of
the phase transition are taken into account by using a temperature dependent
sound velocity inferred from the lattice simulations of QCD. We find that the
transverse flow acquires the scaling form r/t within the short evolution times,
10 - 15 fm, only if the initial transverse flow originating from the
pre-equilibrium collective behavior is present at the initial stage of the
hydrodynamic evolution. The amount of such pre-equilibrium flow is correlated
with the initial pressure gradient; larger gradients require smaller initial
flow. The results of the numerical calculations support the phenomenological
parameterizations used in the Blast-Wave, Buda-Lund, and Cracow models of the
freeze-out process.Comment: 11 page
v4: A small, but sensitive observable for heavy ion collisions
Higher order Fourier coefficients of the azimuthally dependent single
particle spectra resulting from noncentral heavy ion collisions are
investigated. For intermediate to large transverse momenta, these anisotropies
are expected to become as large as 5 %, and should be clearly measurable. The
physics content of these observables is discussed from two different extreme
but complementary viewpoints, hydrodynamics and the geometric limit with
extreme energy loss.Comment: as published: typos corrected, Fig. 3 slightly improved in numerics
and presentatio
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