Diffraction in Pythia

The PYTHIA program can be used to generate high-energy-physics 'events' with sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide a representation, as accurate as possible, of event properties in a wide range of reactions. One such reaction, that is not well understood is Diffraction. Among the several alternative approaches that have been proposed, in PYTHIA, we follow a fairly conventional Pomeron based one, but fully integrated to use the standard PYTHIA machinery for multiple interactions, parton showers and hadronization. This note reports the development in PYTHIA in the way diffraction is modeled without providing specific details for usage. Results are compared with an alternative event generator called PHOJET. The code and further information may be found on the Pythia web page: http://home.thep.lu.se/~torbjorn/Pythia.html.