From high $p_\perp$ theory and data to inferring anisotropy of Quark-Gluon Plasma

TL;DR

This paper proposes a novel method using high transverse momentum data and a dynamical energy loss formalism to reliably extract the spatial anisotropy of the Quark-Gluon Plasma in heavy-ion collisions, enhancing QGP property inference.

Contribution

It introduces a new observable ratio of $R_{AA}$ and $v_2$ for direct extraction of initial state anisotropy from high $p_ot$ data, supported by analytical and numerical validation.

Findings

The $v_2/(1-R_{AA})$ ratio correlates with the anisotropy coefficient $oldsymbol{_{2L}}$.

Analytical estimates agree well with numerical calculations.

Current data accuracy limits anisotropy extraction, but future LHC improvements will enable it.

Abstract

High $p_\perp$ theory and data are commonly used to study high $p_\perp$ parton interactions with QGP, while low $p_\perp$ data and corresponding models are employed to infer QGP bulk properties. On the other hand, with a proper description of high $p_\perp$ parton-medium interactions, high $p_\perp$ probes become also powerful tomography tools, since they are sensitive to global QGP features, such as different temperature profiles or initial conditions. This tomographic role of high $p_\perp$ probes can be utilized to assess the spatial anisotropy of the QCD matter. With our dynamical energy loss formalism, we show that a (modified) ratio of $R_{AA}$ and $v_2$ presents a reliable and robust observable for straightforward extraction of initial state anisotropy. We analytically estimated the proportionality between the $v_2/(1-R_{AA})$ and anisotropy coefficient $\epsilon_{2L}$, and found surprisingly good agreement with full-fledged numerical calculations. Within the current error bars, the extraction of the anisotropy from the existing data using this approach is still inaccessible. However, with the expected accuracy improvement in the upcoming LHC runs, the anisotropy of the QGP formed in heavy ion collisions can be straightforwardly derived from the data. Such a data-based anisotropy parameter would present an important test to models describing the initial stages of heavy-ion collision and formation of QGP, and demonstrate the usefulness of high $p_\perp$ theory and data in obtaining QGP properties.