Energy loss predicts no $v_2$ in small systems

TL;DR

This paper uses a QCD-based energy loss model with hydrodynamics to predict high-$p_T$ particle suppression and anisotropy in small systems, finding no significant $v_2$ signal at high-$p_T$ in such collisions.

Contribution

It introduces a comprehensive energy loss model incorporating event-by-event hydrodynamics and small system corrections, and predicts negligible high-$p_T$ $v_2$ in small systems, aligning with recent experimental data.

Findings

The model accurately describes large system $R_{AA}$ and $v_2$ data.

Extrapolation matches recent measurements in small systems.

Predicts $v_2 ightarrow 0$ at high-$p_T$ for small systems.

Abstract

We present high-$p_T$ $R_{AB}$ and $v_2$ from a perturbative quantum chromodynamics-based energy loss model that includes event-by-event hydrodynamic evolution of the medium and small system size corrections to the energy loss. The model is calibrated on, and describes well, large system $R_{AA}$ and $v_2$ experimental data. The extrapolation of our model to $\mathrm{Ne}+\mathrm{Ne}$ and $\mathrm{O}+\mathrm{O}$ agrees quantitatively with recent experimental measurements of $R_{AA}$. Surprisingly, at high-$p_T$ our energy loss model predicts $v_2\approx0$ for all symmetric and asymmetric small systems when extracted using either hard-hard or hard-soft two-particle correlations. We argue that all energy loss models will in general predict $v_2\approx0$ when extracted using hard-soft correlations, which is the usual experimental method for measuring anisotropy in hadronic collisions, due to a generic geometric decorrelation between the hard and soft sector participant planes.