Implementation of a medium-modified parton shower algorithm

TL;DR

This paper introduces a Monte Carlo implementation of a medium-modified parton shower algorithm within PYTHIA, accounting for medium effects on gluon radiation and resulting distributions in high-energy collisions.

Contribution

It presents a novel implementation of medium-induced gluon radiation in PYTHIA through additive modifications to splitting functions, enabling detailed simulation of medium effects.

Findings

Medium effects increase low-momentum partons and broaden angular distributions.

Medium density correlates with increased small-$x$ partons and angular broadening.

Transverse momentum distributions show enhancement at low/intermediate $p_T$ and suppression at high $p_T$.

Abstract

We present a Monte Carlo implementation of medium-induced gluon radiation in the final-state branching process. Medium effects are introduced through an additive term in the splitting functions. We have implemented such modification within PYTHIA. We show the medium effects on the hump-backed plateau, and the transverse momentum and angular distributions with respect to the parent parton. As expected, with increasing medium densities there is an increase (decrease) of partons with small (large) momentum fraction, and angular broadening is observed. The effects on the transverse-momentum distributions are more involved, with an enhancement of low- and intermediate-$p_T$ partons and a decrease at large $p_T$, which is related to energy conservation, and to the lack of momentum exchange with the medium in our approach.