Small systems and the single-hit approximation in the AMY parton cascade ALPACA

TL;DR

This paper introduces the Alpaca parton cascade model, which simulates small collision systems using the AMY kinetic theory, comparing it to the single-hit approximation and analyzing the impact of interaction range on flow observables.

Contribution

The paper presents Alpaca, a new parton cascade that accurately encodes the AMY kinetic theory and compares its results to the single-hit approximation in small systems.

Findings

Alpaca reproduces known single-hit approximation results at weak coupling.

Differences between cascade and Boltzmann approaches increase at larger coupling.

Finite interaction distance affects the elliptic flow coefficient in small systems.

Abstract

Understanding how momentum anisotropies arise in small collision systems is important for a quantitative understanding of collectivity in terms of QCD dynamics in small and large collision systems. In this letter we present results for small collision systems from the newly developed parton cascade \textsc{Alpaca}, which faithfully encodes the AMY effective kinetic theory. \textsc{Alpaca} reproduces quantitatively previously know results from a calculation in the single-hit approximation for small values of the coupling. We discuss in detail how such a comparison is to be carried out. Particularly at larger coupling a generic differences between the two approaches becomes apparent, namely that in parton cascades particles interact over a finite distance while in direct integrations of the Boltzmann equation the interactions are local. This leads to quantitative differences in the extracted values for the elliptic flow coefficient. These discrepancies appear in situations where the mean free path is not large compared to the interaction time and the applicability of kinetic theory is thus questionable.