Minimum bias and underlying event data from the LHC and Tevatron have been used to tune the PYTHIA. We tune Multiple Parton Interaction (MPI) parameters including three Lambda parameters. It is shown that these tunings describe selected distributions quite well, especially pt distribution at three collision energies, 0.9 TeV , 1.9 TeV and 7 TeV provided that charge particles with pT > 500MeV are used. Results are presented using three types of Parton Density Functions (PDFs) of proton : leading order (LO), modied leading order(LO*) and next-to-leading order (NLO). We found that all three types of PDFs can describe minimum bias and underlying event distributions equally well.
A parameter study of PYTHIA6 Multiple Parton Interaction (MPI) using best fit to the data published by ATLAS Collaboration is presented. Altogether thirteen parameters are investigated to find the most sensitive parameters to the selected data. This type of study helps in better understanding of different parameters needed to be used in PYTHIA6 tunes.