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 [WBG]. The interface is only available if Sherpa has been compiled with support for LHAPDF, see Installation.
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].
NNPDF30NLO
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
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 switchISR_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 parameterSHOW_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 valueDefault
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]