Tuning Monte Carlo generators: The Perugia tunes

We present 9 new tunes of the ${p}_{\ensuremath{\perp}}$-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 ${Z}^{0}$ decays at LEP, Tevatron min-bias data at 630, 1800, and 1960 GeV, Tevatron Drell-Yan 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.

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