Directed Flow in event-by-event hydrodynamics

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Théâtre National (Centre Bonlieu)

Théâtre National

Centre Bonlieu

Board: 5
Poster Global and collective dynamics


Dr Fernando Gardim (USP)


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


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

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