Parton Distribution Functions¶

LHAPDF¶

To be able to use ​LHAPDF instead of the built-in PDF sets, you need to specify the installation path of LHAPDF when configuring ThePEG

--with-lhapdf=/path/to/lhapdf


this is done by default by the bootstrap script.

To use a set from LHAPDF instead you should need to create a new PDF object, for example for one of the NNPDF sets

cd /Herwig/Partons
create ThePEG::LHAPDF myPDFset ThePEGLHAPDF.so
set myPDFset:PDFName NNPDF30_nlo_as_0118


Herwig allows the use of different PDFs for different parts of the simulation. The normal location is the PDF member of the beam particle, e.g.

set /Herwig/Particles/p+:PDF myPDFset
set /Herwig/Particles/pbar-:PDF myPDFset


The PDFs used in the calculation of the hard process are set using

set /Herwig/Partons/QCDExtractor:FirstPDF myPDFset
set /Herwig/Partons/QCDExtractor:SecondPDF myPDFset


and should be evaluated at the same order in perturbation theory as the hard process. The PDF used in the shower can be set using

set /Herwig/Shower/ShowerHandler:PDFA myPDFset
set /Herwig/Shower/ShowerHandler:PDFB myPDFset

set /Herwig/DipoleShower/DipoleShowerHandler:PDFA myPDFset
set /Herwig/DipoleShower/DipoleShowerHandler:PDFB myPDFset


In general we would recommend using the same PDF as used for the hard process, particularly if you are using MC@NLO or POWHEG matching.

However we would always recommend that you use a leading-order PDF for both the remnant handling

set /Herwig/Shower/ShowerHandler:PDFARemnant myPDFset
set /Herwig/Shower/ShowerHandler:PDFBRemnant myPDFset
set /Herwig/DipoleShower/DipoleShowerHandler:PDFARemnant myPDFset
set /Herwig/DipoleShower/DipoleShowerHandler:PDFBRemnant myPDFset


and multiple scattering

set /Herwig/Partons/MPIExtractor:FirstPDF myPDFset
set /Herwig/Partons/MPIExtractor:SecondPDF myPDFset