5.3. ISR parameters

The following parameters are used to steer the setup of beam substructure and initial state radiation (ISR).


Specify the PDG ID of the first (left) and second (right) bunch particle (or both if only one value is provided), i.e. the particle after eventual Beamstrahlung specified through the beam parameters, see Beam parameters. Per default these are taken to be identical to the values set using BEAMS, assuming the default beam spectrum is Monochromatic. In case the Simple Compton or Laser Backscattering spectra are enabled the bunch particles would have to be set to 22, the PDG code of the photon.

Sherpa provides access to a variety of structure functions. They can be configured with the following parameters.


This parameter takes the list of PDF interfaces to load. The following options are distributed with Sherpa:


Use PDF’s from LHAPDF [B+11]. This is the default.


Built-in library for some PDF sets from the CTEQ collaboration, cf. [D+].


Built-in library for PDF sets from the NNPDF group, cf. [B+].


Built-in library for the GRV photon PDF [GRV92b], [GRV92a].


Built-in library for the GRS photon PDF [GRS99].


Built-in library for the SAL photon PDF [SAL06].


Built-in library for the CJK photon PDF [CJKL03], [CJK04b], [CJK04c], [CJK04a].


Built-in library for the SaSgam photon PDF [SS95], [SS96].


Built-in library for the electron structure function. The perturbative order of the fine structure constant can be set using the parameter ISR_E_ORDER (default: 1). The switch ISR_E_SCHEME allows to set the scheme of respecting non-leading terms. Possible options are 0 (“mixed choice”), 1 (“eta choice”), or 2 (“beta choice”, default).


Built-in library for the pomeron PDF from the 2006 Diffractive PDF fit by the H1 collaboration [A+06b]. Has the PDF sets FitA and FitB.


No PDF. Fixed beam energy.

Furthermore it is simple to build an external interface to an arbitrary PDF and load that dynamically in the Sherpa run. See External PDF for instructions.

By default, Sherpa will try to install with the LHAPDF interface enabled. If this is not desired, for example in lepton-lepton collisions where LHAPDF is not used, the user can disable the interface with the cmake option -DCMAKE_ENABLE_LHAPDF=OFF. Sherpa will then use the internal PDF_LIBRARY for hadronic collisions, with the default set being NNPDF31_nnlo_as_0118_mc. Note that PDF variations and the evolution of ALPHAS, ALPHAS: {USE_PDF: 1}, can only be used with LHAPDF enabled.


Specifies the PDF set for hadronic bunch particles. All sets available in the chosen PDF_LIBRARY can be figured by running Sherpa with the parameter SHOW_PDF_SETS: 1, e.g.:

$ Sherpa 'PDF_LIBRARY: CTEQ6Sherpa' 'SHOW_PDF_SETS: 1'

If the two colliding beams are of different type, e.g. protons and electrons or photons and electrons, it is possible to specify two different PDF sets by providing two values: PDF_SET: [pdf1, pdf2]. The special value Default can be used as a placeholder for letting Sherpa choose the appropriate PDF set (or none).


This parameter allows to select a specific version (member) within the chosen PDF set. It is possible to specify two different PDF sets using PDF_SET_VERSIONS: [version1, version2]

See On-the-fly event weight variations to find out how to vary PDF sets and version on-the-fly, both in the matrix element and in the parton shower.