About this Report

As we see, JETSET and PYTHIA started out as very ideologically motivated programs, developed to study specific physics questions in enough detail that explicit predictions could be made for experimental quantities. As it was recognized that experimental imperfections could distort the basic predictions, the programs were made available for general use by experimentalists. It thus became feasible to explore the models in more detail than would otherwise have been possible. As time went by, the emphasis came to shift somewhat, away from the original strong coupling to a specific fragmentation model, towards a description of high-energy multiparticle production processes in general. Correspondingly, the use expanded from being one of just comparing data with specific model predictions, to one of extensive use for the understanding of detector performance, for the derivation of acceptance correction factors, for the prediction of physics at future high-energy accelerators, and for the design of related detectors.

While the ideology may be less apparent, it is still there, however. This is not something unique to the programs discussed here, but inherent in any event generator, or at least any generator that attempts to go beyond the simple parton level skeleton description of a hard process. Do not accept the myth that everything available in Monte Carlo form represents ages-old common knowledge, tested and true. Ideology is present by commissions or omissions in any number of details. A programs like PYTHIA represents a major amount of original physics research, often on complicated topics where no simple answers are available. As a (potential) program user you must be aware of this, so that you can form your own opinion, not just about what to trust and what not to trust, but also how much to trust a given prediction, i.e. how uncertain it is likely to be. PYTHIA is particularly well endowed in this respect, since a number of publications exist where most of the relevant physics is explained in considerable detail. In fact, the problem may rather be the opposite, to find the relevant information among all the possible places. One main objective of the current report is therefore to collect much of this information in one single place. Not all the material found in specialized papers is reproduced, by a wide margin, but at least enough should be found here to understand the general picture and to know where to go for details.

The current report is therefore intended to update and extend the previous round of published physics descriptions and program manuals [Sjö86,Sjö87,Ben87,Sjö94,Mre97,Sjö01]. Make all references to the most recent published one in [Sjö01]. Further specification could include a statement of the type `We use PYTHIA version X.xxx'. (If you are a L<sup>A</sup>TEX fan, you may want to know that the program name in this report has been generated by the command \textsc{Pythia}.) Kindly do not refer to PYTHIA as `unpublished', `private communication' or `in preparation': such phrases are incorrect and only create unnecessary confusion.

In addition, remember that many of the individual physics components are documented in separate publications. If some of these contain ideas that are useful to you, there is every reason to cite them. A reasonable selection would vary as a function of the physics you are studying. The criterion for which to pick should be simple: imagine that a Monte Carlo implementation had not been available. Would you then have cited a given paper on the grounds of its physics contents alone? If so, do not punish the extra effort of turning these ideas into publicly available software. (Monte Carlo manuals are good for nothing in the eyes of many theorists, so often only the acceptance of `mainstream' publications counts.) Here follows a list of some main areas where the $\textsc{Pythia}$ programs contain original research:

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The string fragmentation model [And83,And98].

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The string effect [And80].

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Baryon production (diquark/popcorn) [And82,And85,Edé97].

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Small-mass string fragmentation [Nor98].

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Fragmentation of multiparton systems [Sjö84].

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Colour rearrangement [Sjö94a] and Bose-Einstein effects [Lön95].

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Fragmentation effects on $\alpha_{\mathrm{s}}$ determinations [Sjö84a].

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Initial-state parton showers [Sjö85,Miu99].

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Final-state parton showers [Ben87a,Nor01].

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Photon radiation from quarks [Sjö92c]

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Deeply Inelastic Scattering [And81a,Ben88].

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Photoproduction [Sch93a], $\gamma\gamma$ [Sch94a] and $\gamma^*\mathrm{p}/\gamma^*\gamma/\gamma^*\gamma^*$ [Fri00] physics.

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Parton distributions of the photon [Sch95,Sch96].

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Colour flow in hard scatterings [Ben84].

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Elastic and diffractive cross sections [Sch94].

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Minijets (multiple parton-parton interactions) [Sjö87a].

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Rapidity gaps [Dok92].

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Jet clustering in $k_{\perp}$ [Sjö83].

In addition to a physics survey, the current report also contains a complete manual for the program. Such manuals have always been updated and distributed jointly with the programs, but have grown in size with time. A word of warning may therefore be in place. The program description is fairly lengthy, and certainly could not be absorbed in one sitting. This is not even necessary, since all switches and parameters are provided with sensible default values, based on our best understanding (of the physics, and of what you expect to happen if you do not specify any options). As a new user, you can therefore disregard all the fancy options, and just run the program with a minimum ado. Later on, as you gain experience, the options that seem useful can be tried out. No single user is ever likely to find need for more than a fraction of the total number of possibilities available, yet many of them have been added to meet specific user requests.

In some instances, not even this report will provide you with all the information you desire. You may wish to find out about recent versions of the program, know about related software, pick up a few sample main programs to get going, or get hold of related physics papers. Some such material can be found on the PYTHIA web page:
http://www.thep.lu.se/$\sim$torbjorn/Pythia.html .