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
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.
Note
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.