Empirical Constraints on Parton Energy Loss in Nucleus-Nucleus Collisions at RHIC

TL;DR

This paper empirically investigates parton energy loss in nucleus-nucleus collisions at RHIC, revealing a proportional relationship with transverse momentum and a rapid medium expansion that diminishes path length dependence.

Contribution

It provides new empirical constraints on how partons lose energy in a dense, rapidly expanding medium created in heavy-ion collisions.

Findings

$R_{AA}$ is flat at high $p_{T}$, indicating proportional energy loss.

No strong path length dependence of energy loss was observed.

Rapid medium expansion reduces the effective interaction length.

Abstract

We present empirical features of parton energy loss in nucleus-nucleus collisions at RHIC through studies of the spectra and nuclear modification factors ($R_{AA}$) for charged hadrons, neutral pions ($\pi^0$) and non-photonic electrons. The flat distribution of $R_{AA}$ at high transverse momentum ($p_{T}$) for a given collision centrality is consistent with a scenario where parton energy loss $\Delta p_{T}$ is proportional to $p_{T}$. The centrality dependence of the parton energy loss indicates the absence of path length dependence in the magnitude of energy loss. The lack of strong path length dependence suggests a dynamical picture where the dense partonic medium undergoes rapid expansion and the density of the medium falls rapidly in the first a few Fermi interval, which may be much shorter than the full path length. Implications of the empirical constraints on the parton energy loss will also be discussed.