1,657 research outputs found
Effect of flow fluctuations and nonflow on elliptic flow methods
We discuss how the different estimates of elliptic flow are influenced by
flow fluctuations and nonflow effects. It is explained why the event-plane
method yields estimates between the two-particle correlation methods and the
multiparticle correlation methods. It is argued that nonflow effects and
fluctuations cannot be disentangled without other assumptions. However, we
provide equations where, with reasonable assumptions about fluctuations and
nonflow, all measured values of elliptic flow converge to a unique mean
v_{2,PP} elliptic flow in the participant plane and, with a Gaussian assumption
on eccentricity fluctuations, can be converted to the mean v_{2,RP} in the
reaction plane. Thus, the 20% spread in observed elliptic flow measurements
from different analysis methods is no longer mysterious.Comment: one typo in Table I correcte
, , , : nonlinear hydrodynamic response versus LHC data
Higher harmonics of anisotropic flow ( with ) in heavy-ion
collisions can be measured either with respect to their own plane, or with
respect to a plane constructed using lower-order harmonics. We explain how such
measurements are related to event-plane correlations. We show that CMS data on
and are compatible with ATLAS data on event-plane correlations. If
one assumes that higher harmonics are the superposition of non-linear and
linear responses, then the linear and non-linear parts can be isolated under
fairly general assumptions. By combining analyses of higher harmonics with
analyses of and , one can eliminate the uncertainty from initial
conditions and define quantities that only involve nonlinear hydrodynamic
response coefficients. Experimental data on , and are in good
agreement with hydrodynamic calculations. We argue that can be measured
with respect to elliptic and triangular flow. We present predictions for
versus centrality in Pb-Pb collisions at the LHC.Comment: 7 pages and 3 figure
Universal fluctuation-driven eccentricities in proton-proton, proton-nucleus and nucleus-nucleus collisions
We show that the statistics of fluctuation-driven initial-state anisotropies
in proton-proton, proton-nucleus and nucleus-nucleus collisions is to a large
extent universal. We propose a simple parametrization for the probability
distribution of the Fourier coefficient in harmonic , which
is in good agreement with Monte-Carlo simulations. Our results provide a simple
explanation for the 4-particle cumulant of triangular flow measured in Pb-Pb
collisions, and for the 4-particle cumulant of elliptic flow recently measured
in p-Pb collisions. Both arise as natural consequences of the condition that
initial anisotropies are bounded by unity. We argue that the initial rms
anisotropy in harmonic can be directly extracted from the measured ratio
: this gives direct access to a property of the initial
density profile from experimental data. We also make quantitative predictions
for the small lifting of degeneracy between , and
. If confirmed by future experiments, they will support the picture
that long-range correlations observed in p-Pb collisions at the LHC originate
from collective flow proportional to the initial anisotropy.Comment: Data file of Fig.3 adde
Constraining the viscous freeze-out distribution function with data obtained at the BNL Relativistic Heavy Ion Collider (RHIC)
We investigate the form of the viscous correction to the equilibrium
distribution function in the context of a Cooper-Frye freeze out prescription
for viscous hydrodynamic simulations of relativistic heavy ion collisions. The
standard quadratic ansatz used by all groups for the case of shear viscosity is
found to be disfavored by experimental data for v4/(v2)^2 at the Relativistic
Heavy Ion Collider, and is unlikely to be correct for the hadron resonance gas
present at freeze out. Instead, data for v2(pt) along with v4/(v2)^2 favor a
momentum dependence between linear and quadratic.Comment: 5 pages, 4 figures; v2: version accepted for publication (minor
changes
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