### Speaker

Dr
Fernando Gardim
(USP)

### Description

Fluctuations in the initial geometry of a nucleus-nucleus
collision have been recently shown to produce the correlation
structures known as ``ridge" and ``shoulder". These event-by-event
fluctuations result in new types of anisotropic flow, such as
triangular flow $v_3$ and a new type of directed flow $v_1$,
which, unlike the usual directed flow, is also present at
midrapidity. The anisotropic flows due to the fluctuations in the
initial density profile result in different reference angles
$\Psi_n$ for every harmonic $n$, which are not necessarily
correlated with the event plane angle $\Psi_2$ (the elliptic flow
reference angle), used by the experimentalists to measure the
anisotropic flows, as $v_1$, $v_2$ and $v_4$. Unlike triangular
flow, this new $v_1$ has not been studied in a hydrodynamic
framework.
This work is based on the first quantitative predictions for this
new $v_1$ in Au-Au collisions at the top RHIC energy, using the
hydrodynamic code NEXSPheRIO. NEXSPheRIO solves the relativistic
ideal hydrodynamics using initial conditions provided by the event
generator NeXus, providing good description for several
observables, like elliptic flow. Shear viscosity is not
implemented in this computation, though its effect should be
smaller than higher harmonics, for instance $v_2$.
First, we compute this new $v_1$ versus transverse momentum and
centrality for Au-Au collisions at RHIC using the hydrodynamic
code NeXSPheRIO. Even without dedicated analysis of this new
$v_1$, indirect evidence has been obtained from recent STAR
correlation data, and we compared our results with those inferred
data, finding remarkable agreement.
As the fluctuations in the initial geometry break the symmetry of
the initial density profile, there will be one direction where the
profile is steepest. This effect can be quantified by the
magnitude dipole asymmetry $\varepsilon_1$, and by the steepest
direction for a smooth profile $\Phi_1$. For smooth initial
conditions, one expects $\Psi_1=\Phi_1$ and $v_1\propto
\varepsilon_1$ in each event. We compute those features for our
bumpy initial conditions and compare with the directed flow
quantities. We find that the event plane of $v_1$ is correlated
with the angle of the initial dipole of the distribution, as
predicted, though with a large dispersion, but it is uncorrelated
with the reaction plane. This shows that the dipole asymmetry is
indeed the mechanism to create $v_1$.
Reference: arXiv:1103.4605

### Primary author

Dr
Fernando Gardim
(USP)

### Co-authors

Prof.
Frederique Grassi
(USP)
Prof.
Jean-Yves Ollitrault
(Saclay)
Dr
Matthew Luzum
(Saclay)
Prof.
Yogiro Hama
(USP)