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

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.

arXiv commentexpanded discussion, version accepted for publication

References in corpus (18)

• Evidence for collectivity in pp collisions at the LHC
• Isotropization and hydrodynamization in weakly coupled heavy-ion collisions
• Small System Collectivity in Relativistic Hadron and Nuclear Collisions
• Novel Collective Phenomena in High-Energy Proton-Proton and Proton-Nucleus Collisions
• Multi-particle azimuthal correlations in p-Pb and Pb-Pb collisions at the CERN Large Hadron Collider
• Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at sqrt(s_NN)=5.02 TeV
• Anisotropic parton escape is the dominant source of azimuthal anisotropy in transport models
• Anisotropic parton escape is the dominant source of azimuthal anisotropy in transport models
• Mass ordering of spectra from fragmentation of saturated gluon states in high multiplicity proton-proton collisions
• UV Cascade in Classical Yang-Mills via Kinetic Theory
• Chemical Equilibration in Weakly Coupled QCD
• Jet-Medium Interactions at NLO in a Weakly-Coupled Quark-Gluon Plasma
• Gluon spectrum in the glasma from JIMWLK evolution
• Measurements of multiparticle correlations in $d$$+$Au collisions at 200, 62.4, 39, and 19.6 GeV and $p$$+$Au collisions at 200 GeV and implications for collective behavior
• Nearly isentropic flow at sizeable $\eta /s$
• Opacity dependence of elliptic flow in kinetic theory
• Development of transverse flow at small and large opacities in conformal kinetic theory
• A Poincare-Covariant Parton Cascade Model for Ultrarelativistic Heavy-Ion Reactions