Parton shower evolution in a 3-d hydrodynamical medium

TL;DR

This paper develops a Monte Carlo simulation of parton showers in a 3D hydrodynamical medium, revealing how medium interactions modify jet fragmentation and suppress high-PT hadron spectra in heavy-ion collisions.

Contribution

It introduces a novel simulation framework combining pQCD shower evolution with 3D hydrodynamics to study medium effects on jet fragmentation.

Findings

Medium-modified fragmentation functions are consistent with observed hadron suppression.

The transport coefficient qhat can be extracted from data using this model.

Soft momentum physics may involve different mechanisms not captured by the model.

Abstract

We present a Monte Carlo simulation of the perturbative Quantum Chromodynamics (pQCD) shower developing after a hard process embedded in a heavy-ion collision. The main assumption is that the cascade of branching partons traverses a medium which (consistent with standard radiative energy loss pictures) is characterized by a local transport coefficient qhat which measures the virtuality per unit length transferred to a parton which propagates in this medium. This increase in parton virtuality alters the development of the shower and in essence leads to extra induced radiation and hence a softening of the momentum distribution in the shower. After hadronization, this leads to the concept of a medium-modified fragmentation function. On the level of observables, this is manifest as the suppression of high transverse momentum (PT) hadron spectra. We simulate the soft medium created in heavy-ion collisions by a 3-d hydrodynamical evolution and average the medium-modified fragmentation function over this evolution in order to compare with data on single inclusive hadron suppression and extract the qhat which characterizes the medium. Finally, we discuss possible uncertainties of the model formulation and argue that the data in a soft momentum show evidence of qualitatively different physics which presumably cannot be described by a medium-modified parton shower.