The PYTHIA Facility

TL;DR

PYTHIA is a long-standing, widely used Monte Carlo event generator that underpins major high-energy physics experiments and faces sustainability challenges as particle physics advances towards future large-scale facilities.

Contribution

This paper presents PYTHIA as a Swedish contribution to big science, analyzing its development, user base, integration, and sustainability challenges in the context of future physics facilities.

Findings

PYTHIA is central to international high-energy physics experiments.

It has a broad user base and integration across various frameworks.

Sustainability of long-lived research software is a key challenge.

Abstract

The development and operation of large-scale particle physics facilities rely not only on accelerators and detectors, but also on sustained, high-precision simulation infrastructure. Originating in Lund in the late 1970s and continuously developed in Sweden for nearly five decades, PYTHIA has evolved into one of the most widely used Monte Carlo event generators in high-energy physics. Today it functions as a facility-scale software infrastructure underpinning the physics programmes of major international experiments, including those at the Large Hadron Collider, and plays a central role in validation, tuning, and uncertainty evaluation. In this article, we present PYTHIA as a Swedish contribution to big science facilities. We outline its historical development, analyze its contemporary user base through citation and text-based studies, and map its integration across experimental frameworks, generator ecosystems, validation infrastructures, and emerging machine-learning workflows. These analyses show that PYTHIA We discuss the operational model and sustainability challenges associated with maintaining long-lived research software at facility scale. As particle physics moves toward the High-Luminosity LHC era and future facilities such as the EIC and FCC, continued investment in robust, interoperable simulation infrastructure remains essential. We discuss the operational model and sustainability challenges associated with maintaining long-lived research software at facility scale. As particle physics moves toward the High-Luminosity LHC era and future facilities such as the EIC and FCC, continued investment in robust, interoperable simulation infrastructure remains essential.