Preface

The PYTHIA program is frequently used for event generation in high-energy physics. The emphasis is on multiparticle production in collisions between elementary particles. This in particular means hard interactions in $\mathrm{e}^+\mathrm{e}^-$, $\mathrm{p}\mathrm{p}$ and $\mathrm{e}\mathrm{p}$ colliders, although also other applications are envisaged. The program is intended to generate complete events, in as much detail as experimentally observable ones, within the bounds of our current understanding of the underlying physics. Many of the components of the program represents original research, in the sense that models have been developed and implemented for a number of aspects not covered by standard theory.

Historically, the family of event generators from the Lund group was begun with JETSET in 1978. The PYTHIA program followed a few years later. With time, the two programs so often had to be used together that it made sense to merge them. Therefore PYTHIA 5.7 and JETSET 7.4 were the last versions to appear individually; as of PYTHIA 6.1 all the code is collected under the PYTHIA heading. At the same time, the SPYTHIA sideline of PYTHIA was reintegrated. Both programs have a long history, and several manuals have come out. The most recent one is
T. Sjöstrand, P. Edén, C. Friberg, L. Lönnblad, G. Miu, S. Mrenna and E. Norrbin,
Computer Physics Commun. 135 (2001) 238,
so please use this for all official references. Additionally remember to cite the original literature on the physics topics of particular relevance for your studies. (There is no reason to omit references to good physics papers simply because some of their contents have also been made available as program code.)

Event generators often have a reputation for being `black boxes'; if nothing else, this report should provide you with a glimpse of what goes on inside the program. Some such understanding may be of special interest for new users, who have no background in the field. An attempt has been made to structure the report sufficiently well so that many of the sections can be read independently of each other, so you can pick the sections that interest you. We have tried to keep together the physics and the manual sections on specific topics, where practicable.

A large number of persons should be thanked for their contributions. Bo Andersson and Gösta Gustafson are the originators of the Lund model, and strongly influenced the early development of the programs. Hans-Uno Bengtsson is the originator of the PYTHIA program. Mats Bengtsson is the main author of the final-state parton-shower algorithm. Patrik Edén has contributed an improved popcorn scenario for baryon production. Christer Friberg has helped develop the expanded photon physics machinery, Emanuel Norrbin the new matrix-element matching of the final-state parton shower algorithm and the handling of low-mass strings, and Gabriela Miu the matching of initial-state showers.

Further comments on the programs and smaller pieces of code have been obtained from users too numerous to be mentioned here, but who are all gratefully acknowledged. To write programs of this size and complexity would be impossible without a strong support and user feedback. So, if you find errors, please let us know.

The moral responsibility for any remaining errors clearly rests with the authors. However, kindly note that this is a `University World' product, distributed `as is', free of charge, without any binding guarantees. And always remember that the program does not represent a dead collection of established truths, but rather one of many possible approaches to the problem of multiparticle production in high-energy physics, at the frontline of current research. Be critical!