Tuning Monte Carlo generators: The Perugia tunes

We present 9 new tunes of the p?-ordered shower and underlying-event model in PYTHIA 6.4. These ‘‘Perugia’’ tunes update and supersede the older ‘‘S0’’ family. The data sets used to constrain the models include hadronic Z decays at LEP, Tevatron min-bias data at 630, 1800, and 1960 GeV, Tevatron DrellYan data at 1800 and 1960 GeV, and SPS min-bias data at 200, 546, and 900 GeV. In addition to the central parameter set, called ‘‘Perugia 0,’’ we introduce a set of 8 related ‘‘Perugia variations’’ that attempt to systematically explore soft, hard, parton density, and color structure variations in the theoretical parameters. Based on these variations, a best-guess prediction of the charged track multiplicity in inelastic, nondiffractive minimum-bias events at the LHC is made. Note that these tunes can only be used with PYTHIA 6, not with PYTHIA 8.