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 GRS photon PDF [GRS99].
Built-in library for the SAL photon PDF [SAL06].
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_SCHEMEallows to set the scheme of respecting non-leading terms. Possible options are 0 (“mixed choice”), 1 (“eta choice”), or 2 (“beta choice”, default).
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.
Specifies the PDF set for hadronic bunch particles. All sets available in the chosen
PDF_LIBRARYcan 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
Defaultcan 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.