How to create PDF sets with flavor-number variations

The default PDF fits are generated assuming a nf=5 flavor scheme. It is possible to use a different flavour scheme for the evolution, given an existing fit at the initial scale.

After performing a fit, the varflavours script can be used to create a PDF evolved with a different maximum number of flavors. Instead of running evolven3fit to evolve the fit with the theory used in the predictions, one would call varflavours as follows

varflavors fit_folder max_replicas THEORYID --new_name=[name of output fit]

where max replicas are the maximum number of replicas evolved by evolven3vfit, THEORYID is again the theoryID of the theory used to do the DGLAP evolution, and new_name is an optional argument setting the new name for the fit.

To finalize the PDF set, postfit should be run in the usual manner.


Note that the resulting PDFs will not correspond to the best PDF in that flavour scheme (for which we would need to do an actual fit yielding different initial scale PDFs).

Implementation details

Under the hood, the varflavors script uses the --theory_id flag of evolven3fit which allows for the possibility to use a different theory during evolution than was used during the fit itself.

As such, one can perform a fit using a theory corresponding to the nf=5 flavor scheme, and then perform the evolution by running

evolven3fit --theory_id=[THEORYID] [configuration folder] [number of replicas]

where THEORYID corresponds to a theory with a flavor number scheme that differs from the nf=5 flavor scheme used during the fit.


In case it is desired to create a PDF with flavor-number differing from the fit, while also keeping the original nf=5 version, one can use vp-fitrename or vp-pdfrename to create two PDF sets with different names that are based on the same input fit.

If we do this, the values for AlphaS_MZ and MZ in the .info file need to be changed to take the alphas and Qref values from the theory used to do the evolution.