The PYTHIA program aims to deliver a full description of all aspects of collisions between particles such as electrons, protons, photons and heavy nuclei at high energies. These high energies can be obtained at accelerator complexes such as CERN, Brookhaven National Laboratory, and other particle colliders around the world. To describe these collisions, many different types of calculations must be applied in succession. In the Theoretical Particle Physics group at Lund University, there are experts working on modelling most aspects of these collisions, for example:

• The initial interaction between the quarks and gluons (constituents of the proton).
• Additional collisions between multiple partons in composite systems such as protons or heavy nuclei.
• The parton shower (additional radiation of quarks and gluons).
• The final transition of quarks and gluons into hadrons.

The hadrons formed in the last step is what our experimental colleagues measure in detectors.

The transition of quarks and gluon into hadrons is the cornerstone of the PYTHIA event generator, and is based on a model first developed at Lund University since the 1970s, known as the Lund string model (or often just the Lund model).

You can read more about the historical development of PYTHIA in this publication:

Torbjörn Sjöstrand. The PYTHIA Event Generator: Past, Present and Future.
Comput. Phys. Comm. 246 (2020) 106910
DOI: 10.1016/j.cpc.2019.106910
Preprint link: arXiv:1907.09874v1 [hep-ph]