 
<chapter name="Scale and Scheme Variations"> 
 
<h2>Scale and Scheme Variations in CKKW-L Based Merging 
</h2> 
 
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 <ref>Gel20</ref>. 
 
<h3>Renormalization Scale Variations</h3> 
 
Renormalization scale variations are automatically performed when multi-jet 
merging according to one of the follwing schemes is used: CKKW-L, UMEPS, 
NL<sup>3</sup> 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 name="Merging:muRfactors" 
default="{0.5, 2.0}"> 
Vector of renormalization scale variation factors to be applied in 
CKKW-L based merging schemes. 
</pvec> 
 
<h3>UNLOPS Merging Scheme Variations</h3> 
 
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 name="Merging:doSchemeVariation" default="on"> 
Perform UNLOPS scheme variations. 
</flag> 
 
</chapter> 
 
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