Dissecting the role of initial collision geometry for jet quenching observables in relativistic heavy ion collisions

TL;DR

This paper investigates how initial collision geometry influences jet quenching observables, especially the azimuthal anisotropy $v_2$, in relativistic heavy ion collisions, highlighting the importance of geometry modeling for data interpretation.

Contribution

It demonstrates that the $v_2$ at high transverse momentum is highly sensitive to the collision geometry and energy loss path length dependence, proposing higher power dependencies to match experimental data.

Findings

$v_2$ is sensitive to collision geometry and multiplicity centrality.

Higher power path length dependence improves $v_2$ data fit.

Early time dynamics significantly affect $v_2$ calculations.

Abstract

The observation of large azimuthal anisotropy or $v_2$ for hadrons above $p_T>5$ GeV/$c$ in Au+Au collisions at $\sqrt{s_{\rm nn}}=200$ GeV has been a longstanding challenge for jet quenching models based on perturbative QCD (pQCD). Using a simple jet absorption model, we seek to clarify the situation by exploring in detail how the calculated $v_2$ varies with choices of the collision geometry as well as choices of the path length dependence and thermalization time $\tau_0$ in the energy loss formula. Besides the change of eccentricity due to distortion from gluon saturation or event-by-event fluctuation, we find that the $v_2$ is also sensitive to the centrality dependence of multiplicity and the relative size between the matter profile and the jet profile. We find that the $v_2$ calculated for the naive quadratic path length dependence of energy loss, even including eccentricity fluctuation and the gluon saturation, is not enough to describe the experimental data at high $p_T$ ($\sim$ 6 GeV/$c$) in Au+Au collisions. However, it can match the full centrality dependence of $v_2$ data if higher power path length dependence of energy loss is allowed. We also find that the calculated $v_2$ is sensitive to the assumption of the early time dynamics but generally increases with $\tau_0$, opposite to what one expects for elliptic flow. This study attests to the importance of confining the initial geometry, possibly by combining jet quenching $v_2$ with elliptic flow and other jet quenching observables, for proper interpretation of the experimental data.