Utilizing high-$p_\perp$ theory and data to constrain the initial stages of quark-gluon plasma

TL;DR

This paper investigates how high-$p_\perp$ probes can provide complementary insights into the initial stages of quark-gluon plasma, revealing limitations of current methods and emphasizing the need for combined observables for better constraints.

Contribution

It introduces a framework to analyze high-$p_\perp$ energy loss sensitivity to initial plasma conditions and critiques existing parameter fitting procedures.

Findings

$v_2$ is insensitive to initial stages.

$R_{AA}$ shows some sensitivity but lacks current experimental resolution.

Fitting energy loss parameters can obscure physical interpretations.

Abstract

The scarce knowledge of the initial stages of quark-gluon plasma before the thermalization is mostly inferred through the low-$p_\perp$ sector. We propose a complementary approach in this report - the use of high-$p_\perp$ probes' energy loss. We study the effects of four commonly assumed initial stages, whose temperature profiles differ only before the thermalization, on high-$p_\perp$ $R_{AA}$ and $v_2$ predictions. The predictions are based on our Dynamical Radiative and Elastic ENergy-loss Approach (DREENA) framework. We report insensitivity of $v_2$ to the initial stages, making it unable to distinguish between different cases. $R_{AA}$ displays sensitivity to the presumed initial stages, but current experimental precision does not allow resolution between these cases. We further revise the commonly accepted procedure of fitting the energy loss parameters, for each individual initial stage, to the measured $R_{AA}$. We show that the sensitivity of $v_2$ to various initial stages obtained through such procedure is mostly a consequence of fitting procedure, which may obscure the physical interpretations. Overall, the simultaneous study of high-$p_\perp$ observables, with unchanged energy loss parametrization and restrained temperature profiles, is crucial for future constraints on initial stages.