5.10. Parton showers

The following parameters are used to steer the shower setup.


There are two shower generators in Sherpa, Dire (default) and CSS. See the module summaries in Basic structure for details about these showers.

Other shower modules are in principle supported and more choices will be provided by Sherpa in the near future. To list all available shower modules, the tag SHOW_SHOWER_GENERATORS: 1 can be specified on the command line.

SHOWER_GENERATOR: None switches parton showering off completely. However, even in the case of strict fixed order calculations, this might not be the desired behaviour as, for example, then neither the METS scale setter, cf. SCALES, nor Sudakov rejection weights can be employed. To circumvent when using the Dire or CS Shower see Sherpa Shower options.


This option uses the value for SHOWER_GENERATOR as its default. Correspondingly, the only natively supported options in Sherpa are CSS and Dire. The corresponding jet criterion is described in [HKSS09]. A custom jet criterion, tailored to a specific experimental analysis, can be supplied using Sherpa’s plugin mechanism.

5.10.3. MASSIVE_PS

This option instructs Sherpa to treat certain partons as massive in the shower, which have been considered massless by the matrix element. The argument is a list of parton flavours, for example MASSIVE_PS: [4, 5], if both c- and b-quarks are to be treated as massive.


When hard decays are used, Sherpa treats all flavours as massive in the parton shower. This option instructs Sherpa to treat certain partons as massless in the shower nonetheless. The argument is a list of parton flavours, for example MASSLESS_PS: [1, 2, 3], if u-, d- and s-quarks are to be treated as massless.

5.10.5. Sherpa Shower options

Sherpa’s default shower module is based on [SK08b]. A new ordering parameter for initial state splitters was introduced in [HKSS09] and a novel recoil strategy for initial state splittings was proposed in [HSS10]. While the ordering variable is fixed, the recoil strategy for dipoles with initial-state emitter and final-state spectator can be changed for systematics studies. Setting SHOWER:KIN_SCHEME: 0 corresponds to using the recoil scheme proposed in [HSS10], while SHOWER:KIN_SCHEME: 1 (default) enables the original recoil strategy. The lower cutoff of the shower evolution can be set via SHOWER:FS_PT2MIN and SHOWER:IS_PT2MIN for final and initial state shower, respectively. Note that this value is specified in GeV^2. Scale factors for the evaluation of the strong coupling in the parton shower are given by SHOWER:FS_AS_FAC and SHOWER:IS_AS_FAC. They multiply the ordering parameter, which is given in units of GeV^2.

Setting SHOWER:MAXEM: forces the CS Shower to truncate its evolution at the Nth emission. Note that in this case not all of the Sudakov weights might be computed correctly. On the other hand, the use of CS Shower in the METS scale setter is not affected, cf. SCALES.

The parton shower coupling scales, PDF scales and PDF themselves can be varied on-the-fly, along with the on-the-fly variations of the corresponding matrix element parameters. See On-the-fly event weight variations to find out how specify the variations and enable them in the shower.

Most parton showers available in Sherpa allow the same options. These options are specified as follows:

  KIN_SCHEME: <scheme>
  IS_AS_FAC: <factor>
  # other shower settings ...

When the parton shower is used for MC@NLO matching, the options can be set differently. They are then specified as follows:

  IS_AS_FAC: <factor>
  # other shower settings ...

5.10.6. CS Shower options

By default, only QCD splitting functions are enabled in the CS shower. If you also want to allow for photon splittings, you can enable them by using SHOWER:EW_MODE: true. Note, that if you have leptons in your matrix-element final state, they are by default treated by a soft photon resummation as explained in QED corrections. To avoid double counting, this has to be disabled as explained in that section.

The evolution variable of the CS shower can be changed using SHOWER:EVOLUTION_SCHEME. Several options are currently implemented:


transverse momentum ordering


modified transverse momentum ordering.


like 0 but parton masses taken into account


like 1 but parton masses taken into account


like 0 but parton masses taken into account only for g->QQ


like 1 but parton masses taken into account only for g->QQ

The scale can be set differently for final- and initial-state shower. The two values are combined as FS+100*IS, where FS is the choice for the final state, and IS is the choice for the initial state. The scale at which the strong coupling for shower splittings is evaluated can be chosen with SHOWER:SCALE_SCHEME:

The default is to evaluate the strong coupling at the transverse momentum in the parton splitting. Gluon splittings into quarks in the final state are evaluated at the virtuality of the gluon, as are branchings into a soft t-channel gluon in the initial state. Options are additive.




evaluate final-state gluon splitting into quarks at the transverse momentum


evaluate initial-state quark to gluon splittings at the transverse momentum


evaluate initial-state gluon splitting into soft t-channel gluons at the transverse momentum

Additionally, the CS shower allows to disable splittings at scales below the on-shell mass of heavy quarks. The upper limit for the corresponding heavy quark mass is set using SHOWER:MASS_THRESHOLD.

Likewise, by default the CS shower forces heavy quarks to be produced from gluon splittings below their mass threshold. This behaviour can be steered using SHOWER:FORCED_IS_QUARK_SPLITTING. Its precise kinematics are governed by SHOWER:FORCED_SPLITTING_GLUON_SCALING.