Torbjorn Sjostrand
The Development of MPI Modelling in PYTHIA


Many of the basic ideas in multiparton interaction (MPI) phenomenology were first developed in the context of the PYTHIA event generator, and MPIs have been central in its modelling of both minimum-bias and underlying-event physics in one unified framework. This chapter traces the evolution towards an increasingly sophisticated description of MPIs in PYTHIA, including topics such as the ordering of MPIs, the regularization of the divergent QCD cross section, the impact-parameter picture, colour reconnection, multiparton PDFs and beam remnants, interleaved and intertwined evolution, and diffraction.

LU TP 17-22