5.10. Parton showers¶
The following parameters are used to steer the shower setup.
There are two shower generators in Sherpa,
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
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
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.
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
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_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_IS_AS_FAC. They multiply the ordering parameter, which
is given in units of GeV^2.
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,
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
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
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
The cut-off behaviour of the strong coupling for scales at which its value
would formally exceed one can be set using
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.