Q-PYTHIA: a medium-modified implementation of final state radiation

TL;DR

This work addresses the challenge of modeling jet quenching with differential, energy-conserving dynamics by introducing Q-PYTHIA, a Monte Carlo that embeds medium-induced final-state radiation into the PYSHOW shower via an additive Medium modification to the splitting functions driven by the BDMS/BDMPS spectrum. The approach uses medium parameters $L$ and $\\hat{q}$ (with $\\omega_c=\\hat{q}L^2/2$) and a hybrid virtuality-ordered evolution with finite formation-time corrections, enabling realistic exploration of fragmentation functions, jet shapes, and intra-jet multiplicities. Results show expected jet-quenching signatures: leading-particle energy loss, enhanced intra-jet multiplicities, and jet broadening, with hadronization diminishing soft-region effects but preserving core modifications; comparisons with PQM-based $R_{AA}$ reproduce qualitative RHIC-like suppression. The tool, publicly released as Q-PYTHIA, provides a practical and flexible framework for jet analyses in heavy-ion environments, bridging theory and experimental jet measurements and enabling future refinements such as non-eikonal corrections and energy-flow studies. The work thus offers a robust Monte Carlo platform for studying medium-modified jets in current and upcoming collider experiments.

Abstract

We present a Monte Carlo implementation, within PYTHIA, of medium-induced gluon radiation in the final state branching process. Medium effects are introduced through an additive term in the splitting functions computed in the multiple-soft scattering approximation. The observable effects of this modification are studied for different quantities as fragmentation functions and the hump-backed plateau, and transverse momentum and angular distributions. The anticipated increase of intra-jet multiplicities, energy loss of the leading particle and jet broadening are observed as well as modifications of naive expectations based solely on analytical calculations. This shows the adequacy of a Monte Carlo simulator for jet analyses. Effects of hadronization are found to wash out medium effects in the soft region, while the main features remain. To show the performance of the implementation and the feasibility of our approach in realistic experimental situations we provide some examples: fragmentation functions, nuclear suppression factors, jet shapes and jet multiplicities. The package containing the modified routines is available for public use. This code, which is not an official PYTHIA release, is called Q-PYTHIA. We also include a short manual to perform the simulations of jet quenching.