Tuning PYTHIA 8.1: the Monash 2013 tune

Abstract

We present an updated set of parameters for the PYTHIA 8 event generator. We reevaluate the constraints imposed by LEP and SLD on hadronization, in particular with regard to heavy-quark fragmentation and strangeness production. For hadron collisions, we combine the updated fragmentation parameters with the new NNPDF2.3 LO PDF set. We use minimum-bias, Drell-Yan, and underlying-event data from the LHC to constrain the initial-state-radiation and multi-parton-interaction parameters, combined with data from SPS and the Tevatron to constrain the energy scaling. Several distributions show significant improvements with respect to the current defaults, for both ee and pp collisions, though we emphasize that interesting discrepancies remain in particular for strange particles and baryons. The updated parameters are available as an option starting from PYTHIA 8.185, by setting Tune:ee = 7 and Tune:pp = 14. # FSR: IR cutoff TimeShower:pTmin = 0.50 ! for QCD radiation TimeShower:pTminChgQ = 0.50 ! for QED radiation off quarks # FSR: Spin Correlations TimeShower:phiPolAsym = on ! approximate FSR polarization effects