System size dependence of nuclear modification and azimuthal anisotropy of jet quenching

TL;DR

This paper studies how the size of the colliding nuclei affects jet quenching phenomena, using theoretical models and experimental data to understand energy loss mechanisms in quark-gluon plasma.

Contribution

It introduces a model for parton energy loss depending on formation time and path length, successfully reproducing nuclear modification factors and azimuthal asymmetries in heavy-ion collisions.

Findings

Energy loss per collision varies with formation time assumptions.

Model reproduces nuclear modification factor $R_{AA}$ across centralities.

Azimuthal asymmetry predictions agree within a factor of two with data.

Abstract

We investigate the system size dependence of jet-quenching by analyzing transverse momentum spectra of neutral pions in Au+Au and Cu+Cu collisions at $\sqrt{s_{\textrm{NN}}}$ =200 GeV for different centralities. The fast partons are assumed to lose energy by radiating gluons as they traverse the plasma and undergo multiple collisions. The energy loss per collision, $\epsilon$, is taken as proportional to $E$(where $E$ is the energy of the parton), proportional to $\sqrt{E}$, or a constant depending on whether the formation time of the gluon is less than the mean path, greater than the mean free path but less than the path length, or greater than the path length of the partons, respectively. NLO pQCD is used to evaluate pion production by modifying the fragmentation function to account for the energy loss. We reproduce the nuclear modification factor $R_\textrm{AA}$ by treating $\epsilon$ as the only free parameter, depending on the centrality and the mechanism of energy loss. These values are seen to explain the nuclear modification of prompt photons, caused by the energy lost by final state quarks before they fragment into photons. These also reproduce the azimuthal asymmetry of transverse momentum distribution for pions within a factor of two and for prompt photons in a fair agreement with experimental data.