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 CSS_KIN_SCHEME: 0 corresponds to using the recoil scheme proposed in [HSS10], while CSS_KIN_SCHEME: 1 (default) enables the original recoil strategy. The lower cutoff of the shower evolution can be set via CSS_FS_PT2MIN and CSS_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 CSS_FS_AS_FAC and CSS_IS_AS_FAC. They multiply the ordering parameter, which is given in units of GeV^2.

Setting CSS_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.

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 CSS_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 CSS_EVOLUTION_SCHEME. Two options are currently implemented, which correspond to transverse momentum ordering (option 0) and modified transverse momentum ordering (option 1). In addition, modified versions of these options (option 2 and option 3) are implemented, which take parton masses into account where applicable. The scale at which the strong coupling for gluon splitting into quarks is evaluated can be chosen with CSS_SCALE_SCHEME, where 0 (default) corresponds to the ordering parameter and 1 corresponds to invariant mass. 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 CSS_MASS_THRESHOLD.

The cut-off behaviour of the strong coupling for scales at which its value would formally exceed one can be set using CSS_ALPHAS_FREEZE_MODE. Setting it to 0 (which is the default) means that the strong coupling evaluates to zero at such scales (hard cut-off). Setting it to 1 lets it evaluate to one instead (freeze-in). A freeze-in behaviour is preferable when reweighting shower emissions, but it has not been used in the default Sherpa tune, and should therefore not be used when comparing particle-level predictions to data.