Over the years, many event generators have appeared. A recent comprehensive overview is the Les Houches guidebook to Monte Carlo event generators [Dob04]. Surveys of generators for physics in general and LEP in particular may be found in [Kle89,Sjö89,Kno96,Lön96,Bam00], for high-energy hadron-hadron ( ) physics in [Ans90,Sjö92,Kno93,LHC00], and for physics in [HER92,HER99]. We refer the reader to those for additional details and references. In this particular report, the two closely connected programs JETSET and PYTHIA, now merged under the PYTHIA label, will be described.
JETSET has its roots in the efforts of the Lund group to understand the hadronization process, starting in the late seventies [And83]. The so-called string fragmentation model was developed as an explicit and detailed framework, within which the long-range confinement forces are allowed to distribute the energies and flavours of a parton configuration among a collection of primary hadrons, which subsequently may decay further. This model, known as the Lund string model, or `Lund' for short, contained a number of specific predictions, which were confirmed by data from annihilation around 30 GeV at PETRA and PEP, whence the model gained a widespread acceptance. The Lund string model is still today the most elaborate and widely used fragmentation model at our disposal. It remains at the heart of the PYTHIA program.
In order to predict the shape of events at PETRA/PEP, and to study the fragmentation process in detail, it was necessary to start out from the partonic configurations that were to fragment. The generation of complete hadronic events was therefore added, originally based on simple exchange and first-order QCD matrix elements, later extended to full exchange with first-order initial-state QED radiation and second-order QCD matrix elements. A number of utility routines were also provided early on, for everything from event listing to jet finding.
By the mid-eighties it was clear that the pure matrix-element approach had reached the limit of its usefulness, in the sense that it could not fully describe the exclusive multijet topologies of the data. (It is still useful for inclusive descriptions, like the optimized perturbation theory discussed in section , and in combination with renormalon contributions [Dok97].) Therefore a parton-shower description was developed [Ben87a] as an alternative to the higher-order matrix-element one. (Or rather as a complement, since the trend over the years has been towards the development of methods to marry the two approaches.) The combination of parton showers and string fragmentation has been very successful, and has formed the main approach to the description of hadronic events.
This way, the JETSET code came to cover the four main areas of fragmentation, final-state parton showers, event generation and general utilities.
The successes of string fragmentation in made it interesting to try to extend this framework to other processes, and explore possible physics consequences. Therefore a number of other programs were written, which combined a process-specific description of the hard interactions with the general fragmentation framework of JETSET. The PYTHIA program evolved out of early studies on fixed-target proton-proton processes, addressed mainly at issues related to string drawing.
With time, the interest shifted towards hadron collisions at higher energies, first to the SPS collider, and later to the Tevatron, SSC and LHC, in the context of a number of workshops in the USA and Europe. Parton showers were added, for final-state radiation by making use of the JETSET routine, for initial-state one by the development of the concept of `backwards evolution', specifically for PYTHIA [Sjö85]. Also a framework was developed for minimum-bias and underlying events [Sjö87a].
Another main change was the introduction of an increasing number of hard processes, within the Standard Model and beyond. A special emphasis was put on the search for the Standard Model Higgs, in different mass ranges and in different channels, with due respect to possible background processes.
The bulk of the machinery developed for hard processes actually depended little on the choice of initial state, as long as the appropriate parton distributions were there for the incoming partons and particles. It therefore made sense to extend the program from being only a generator to working also for and . This process was completed in 1991, again spurred on by physics workshop activities. Currently PYTHIA should therefore work well for a selection of different possible incoming beam particles.
An effort independent of the Lund group activities got going to include supersymmetric event simulation in PYTHIA. This resulted in the SPYTHIA program [Mre97].
While JETSET was independent of PYTHIA until 1996, their ties had grown much stronger over the years, and the border-line between the two programs had become more and more artificial. It was therefore decided to merge the two, and also include the SPYTHIA extensions, starting from PYTHIA 6.1. The different origins in part still are reflected in this manual, but the striving is towards a seamless merger.
Among the most recent developments, primarily intended for Tevatron and LHC physics studies, is the introduction of `interleaved evolution' in PYTHIA 6.3, with new -ordered parton showers and a more sophisticated framework for minimum-bias and underlying events [Sjö04,Sjö04a]. The possibilities for studying physics beyond the Standard Model have also been extended significantly, to include supersymmetric models with -parity violation, Technicolor models, models, as well as models with (Randall-Sundrum) extra dimensions. This still only includes the models available internally in PYTHIA. Versatility is further enhanced by the addition of an interface to external user processes, according to the Les Houches Accord (LHA) standard [Boo01], and by interfaces to SUSY RGE and decay packages via the SUSY Les Houches Accord (SLHA) [Ska03].
The tasks of including new processes, and of improving the simulation of parton showers and other aspects of already present processes, are never-ending. Work therefore continues apace.