Introduction to PYTHIA

PYTHIA is a program for the generation of high-energy physics events, i.e. for the description of collisions at high energies between elementary particles such as e+, e-, p and pbar in various combinations. It contains theory and models for a number of physics aspects, including hard and soft interactions, parton distributions, initial- and final-state parton showers, multiparton interactions, fragmentation and decay. It is largely based on original research, but also borrows many formulae and other knowledge from the literature.

Development of JETSET, the first member of the "Lund Monte Carlo" family, was begun by members of the Lund theory group in 1978, and has continued since then, on and off. A number of people have contributed to this and other programs based on it. The most extensive of these is PYTHIA. Over the years, these two programs have more and more come to be maintained in common. In 1997 they were therefore merged to one, under the PYTHIA label. Up until some years ago the main version was PYTHIA 6.4.

In 2004 a rewriting from Fortran 77 to C++ was begun, and with the release of PYTHIA 8.1 in 2007 the new code became the official main version. In reality the experimental community continued to rely largely on the 6.4 version for LHC run 1, but by now the transition to PYTHIA 8 is almost complete. In 2012 the development of PYTHIA 6.4 was definitely stopped, and it is now reduced to the status of legacy code. The release of PYTHIA 8.2 in 2014 underlined the nature of a by-now mature product with a (reasonably) smooth evolution path.

The current PYTHIA 8.2 release is found here.

Other programs in the Lund family and beyond

In addition to PYTHIA and JETSET, several other programs have been written that are closely associated with them, and in particular make use of the hadronization machinery.