Scale and Scheme Variations

Scale and Scheme Variations in CKKW-L Based Merging

Pythia allowes automated renormalization scale variations in CKKW-L based merging schemes, i.e., CKKW-L, UMEPS and NLO merging. Furthermore, automated merging scheme variations are available for UNLOPS merging. The details are presented in [Gel20].

Renormalization Scale Variations

Renormalization scale variations are automatically performed when multi-jet merging according to one of the follwing schemes is used: CKKW-L, UMEPS, NL³ or UNLOPS. NLO LHEF input files need to include the required scale variation weights, which are detected and used by Pythia. Leading order LHEF input files do not need to include scale variation weights. The renormalization scale (in GeV) will be varied by the factors in the following vector.

pvec Merging:muRfactors (default = {0.5, 2.0})
Vector of renormalization scale variation factors to be applied in CKKW-L based merging schemes.

UNLOPS Merging Scheme Variations

Three alternative UNLOPS merging schemes have been implemented and can be applied simultaneously when running Pythia. The fixed order and parton shower accuracy is identical in all schemes. Differences arise from subleading contributions in the weighting of NLO events. The following flag enables the automatic scheme variations.

flag Merging:doSchemeVariation (default = on)
Perform UNLOPS scheme variations.