Triangularity and Dipole Asymmetry in Heavy Ion Collisions

TL;DR

This paper introduces a cumulant expansion to describe initial conditions in heavy ion collisions, analyzing their effects on flow coefficients and proposing new measurements to better understand the initial asymmetries.

Contribution

The paper develops a systematic cumulant expansion approach to characterize initial geometries and their hydrodynamic response in heavy ion collisions, including novel predictions for dipole asymmetry effects.

Findings

Cumulant expansion converges and reproduces Glauber initial conditions.

Dipole asymmetry orientation prefers out-of-plane in mid-peripheral collisions.

Hydrodynamic response links initial asymmetries to flow coefficients v1 and v3.

Abstract

We introduce a cumulant expansion to parameterize possible initial conditions in relativistic heavy ion collisions. We show that the cumulant expansion converges and that it can systematically reproduce the results of Glauber type initial conditions. At third order in the gradient expansion, the cumulants characterize the triangularity $<r^3 \cos3(\phi - \psi_{3,3})>$ and the dipole asymmetry $<r^3 \cos(\phi- \psi_{1,3})>$ of the initial entropy distribution. We show that for mid-peripheral collisions the orientation angle of the dipole asymmetry $\psi_{1,3}$ has a $20%$ preference out of plane. This leads to a small net $v_1$ out of plane. In peripheral and mid-central collisions the orientation angles $\psi_{1,3}$ and $\psi_{3,3}$ are strongly correlated. We study the ideal hydrodynamic response to these cumulants and determine the associated $v_1/\epsilon_1$ and $v_3/\epsilon_3$ for a massless ideal gas equation of state. $v_1$ and $v_3$ develop towards the edge of the nucleus, and consequently the final spectra are more sensitive to the viscous dynamics of freezeout. The hydrodynamic calculations for $v_3$ are compared to Alver and Roland fit of two particle correlation functions. Finally, we propose to measure the $v_1$ associated with the dipole asymmetry and the correlations between $\psi_{1,3}$ and $\psi_{3,3}$ by measuring a two particle correlation with respect to the participant plane, $<\cos(\phi_a - 3\phi_b + 2\Psi_{PP})>$. The hydrodynamic prediction for this correlation function is several times larger than a correlation currently measured by the STAR collaboration, $<\cos(\phi_a + \phi_\b - 2\Psi_{PP})>$.