# 5.3. ISR parameters¶

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

BUNCHES

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.

ISR_SMIN/ISR_SMAX

This parameter specifies the minimum fraction of cms energy squared after ISR. The reference value is the total centre of mass energy squared of the collision, not the centre of mass energy after eventual Beamstrahlung. The parameter can be specified using the internal interpreter, see Interpreter, e.g. as ISR_SMIN: sqr(20/E_CMS).

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

PDF_LIBRARY

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

LHAPDFSherpa

Use PDF’s from LHAPDF [B+11].

CT14Sherpa

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

CT12Sherpa

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

CT10Sherpa

Built-in library for some PDF sets from the CTEQ collaboration, cf. [LGH+10].

CTEQ6Sherpa

Built-in library for some PDF sets from the CTEQ collaboration, cf. [N+08].

NNPDF30Sherpa

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

MSTW08Sherpa

Built-in library for PDF sets from the MSTW group, cf. [MSTW09].

MRST04QEDSherpa

Built-in library for photon PDF sets from the MRST group, cf. [MRST05].

MRST01LOSherpa

Built-in library for the 2001 leading-order PDF set from the MRST group, cf. [MRST02].

MRST99Sherpa

Built-in library for the 1999 PDF sets from the MRST group, cf. [MRST00].

GRVSherpa

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

GRSSherpa

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

SALSherpa

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

SASGSherpa

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

PDFESherpa

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).

None

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.

PDF_SET

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).

PDF_SET_VERSIONS

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.