$\pi^{0}$ Azimuthal Anisotropy in Au+Au Collisions at $\sqrt{s_{NN}}=39-200$ GeV from PHENIX: Collision Energy and Path-Length Dependence of Jet-Quenching and the Role of Initial Geometry

TL;DR

This paper investigates how the azimuthal anisotropy of high transverse momentum particles in heavy ion collisions varies with collision energy and initial geometry, providing insights into jet quenching mechanisms.

Contribution

It presents new measurements of $b6^{0}$ azimuthal anisotropy at different energies, enhancing understanding of energy loss dependence on path length and initial collision geometry.

Findings

$b6^{0}$ $v_{2}$ exceeds pQCD predictions at 200 GeV

Provides new high $p_T$ measurements at 62 and 39 GeV

Constraints on jet quenching onset and energy dependence

Abstract

The azimuthal anisotropy of high $p_{T}$ particle production in heavy ion collisions is a sensitive probe of the jet quenching mechanism. Recent PHENIX measurements for Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV show a $\pi^{0}$ $v_{2}$ signal that exceeds the pQCD energy loss calculations up to $p_{T} \sim 10$ GeV/c, challenging the traditional perturbative picture of the energy loss process. Here, we present an update and details of that measurement, as well as new high $p_T$ measurements at $\sqrt{s_{NN}}=62$ and 39 GeV. These measurements not only provide an important constraint for understanding the path-length dependence of jet energy loss and the role of initial collision geometry, but also allow a search for the onset of jet quenching as $\sqrt{s_{NN}}$ is varied.