Dire Parton Showers
- Settings related to the evaluation of running (QCD) couplings
- Settings to perform variations to gauge shower uncertainties
- Settings related to parton distribution functions and quark masses
- Settings related to the tune of Pythia 8 + Dire
- Settings for debugging or educational purposes
The Dire parton shower combines dipole-shower ideas in the spirit of Ariadne
with the careful handling of collinear enhancements in classical parton
showers. It includes a detailed treatment of mass effects and various
higher-order corrections. The Dire parton shower is developed jointly in
Pythia and Sherpa to allow maximal validation.
The Dire physics publication is arxiv:1506.05057 [hep-ph]. Please cite this
article (and of course the Pythia 8 reference) if you use Pythia 8 + Dire for
your research.
Dire is a complete replacement for the default showers in Pythia, and is
automatically interleaved with Pythia's multiple interaction model.
At present, Dire offers
-
QCD and QED showers with automatic uncertainties and the possiblity to
enhance individual splittings.
-
Inclusive next-to-leading order DGLAP corrections to the shower evolution.
-
Next-to-leading order final-state showering for lepton colliders.
-
Iterated Matrix-Element corrections for splittings within the parton-shower
phase space.
Built-in CKKW-L and UNLOPS merging are in preparation. It is worth
noting that Dire is constructed as as weighted parton shower, meaning
that all events that are produced with Dire acquire a weight close to,
but not identical to, unity. These weights need to be included when
filling histograms or storing events. These weights can, conveniently
combined with other weights (due to weighted LHEF inputs, merging
etc.), be retrieved from pythia.info.weight().
The following discusses some basic settings that might be of interest. Some
convenience features are discussed here, while
expert features (mainly aimed at comparisons of Dire to analytic calculations)
are documented here.
mode DireTimes:kernelOrder
(default = 1; minimum = -1; maximum = 4)
This setting defines which higher-order
corrections are applied to the parton-shower splitting functions used
for timelike (i.e. final state) evolution:
option -1 :
Double-logarithmic kernels only, i.e. only gluon emissions, and no collinear
terms or gluon-to-quark branchings. This setting is only intended to compare
the parton shower to leading-logarithmic analytical resummation.
option 0 :
Leading order evolution kernels, as defined in the
original Dire publication.
option 1 :
Leading order evolution kernels, and additional
rescaling of the soft pieces to incorporate two-loop cusp efects
(similar to the CMW scheme).
option 2 :
Leading order evolution kernels, and additional
rescaling of the soft pieces to incorporate two- and three-loop
cusp efects.
option 3 :
Leading order evolution kernels, additional rescaling
of the soft pieces to incorporate two- and three-loop cusp
efects, and NLO corrections to collinear evolution from NLO DGLAP
kernels.
mode DireSpace:kernelOrder
(default = 1; minimum = -1; maximum = 4)
This setting defines which higher-order
corrections are applied to the parton-shower splitting functions used
for spacelike (i.e. initial state) evolution:
option -1 :
Double-logarithmic kernels only, i.e. only gluon emissions, and no collinear
terms or gluon-to-quark branchings. This setting is only intended to compare
the parton shower to leading-logarithmic analytical resummation.
option 0 :
Leading order evolution kernels, as defined in the
original Dire publication.
option 1 :
Leading order evolution kernels, and additional
rescaling of the soft pieces to incorporate two-loop cusp efects
(similar to the CMW scheme).
option 2 :
Leading order evolution kernels, and additional
rescaling of the soft pieces to incorporate two- and three-loop
cusp efects.
option 3 :
Leading order evolution kernels, additional rescaling
of the soft pieces to incorporate two- and three-loop cusp
efects, and NLO corrections to collinear evolution from NLO DGLAP
kernels.
mode DireTimes:kernelOrderMPI
(default = 1; minimum = -1; maximum = 4)
Same as
DireTimes:kernelOrder = n, but for showers from secondary
scatterings or hadron decays.
mode DireSpace:kernelOrderMPI
(default = 1; minimum = -1; maximum = 4)
Same as
DireSpace:kernelOrder = n, but for showers from secondary
scatterings or hadron decays.
parm DireTimes:pTrecombine
(default = 2.0)
The minimal pT value t (in
GeV) of final-state emissions below which the splitting probabilities
of some kernels are combined into a single splitting probability.
To be more precise, for pT<t, the probabilities for q->qg
and q->gq are combined into a single probability, as are
g->qqbar and g->qbarq and the two color structures for
g->gg. This can help improve the numerical stability of the
Sudakov veto algorithm, however at the expense accuracy. Negative
values mean that variations are performed for all parton shower
emissions. Default value is 2.0 GeV.
Settings related to the evaluation of running (QCD) couplings
flag ShowerPDF:usePDFalphas
(default = off)
This switch is turned off
by default. Turned on, the alphaS running and thresholds will be
directly taken from the PDF set interfaced through LHAPDF6. This can be
helpful when validating the code, but will lead to a longer run time.
Settings to perform variations to gauge shower uncertainties
flag Variations:doVariations
(default = off)
Turned on, this means that the timelike and spacelike showers
are allowed to perform on-the-fly variations of the renormalization
scale. Renormalization scale uncertainties are an important part of the
perturbative evolution. Note that currently, these variations will also
apply to showers off (soft) secondary scatterings. The range of
renormalization scale variations is given by the next four parameters.
parm Variations:muRisrDown
(default = 1.0)
The (double) value with which the
(GeV2-valued) argument of αs in
initial-state evolution should be rescaled to produce a smaller value
of the renormalization scale in the context of automatic variations.
parm Variations:muRisrUp
(default = 1.0)
The (double) value with which the
(GeV2-valued) argument of αs in
initial-state evolution should be rescaled to produce a larger value
of the renormalization scale in the context of automatic variations.
parm Variations:muRfsrDown
(default = 1.0)
The (double) value with which the
(GeV2-valued) argument of αs in
final-state evolution should be rescaled to produce a smaller value
of the renormalization scale in the context of automatic variations.
parm Variations:muRfsrUp
(default = 1.0)
The (double) value with which the
(GeV2-valued) argument of αs in
final-state evolution should be rescaled to produce a larger value
of the renormalization scale in the context of automatic variations.
parm Variations:pTmin
(default = -1.0)
The minimal pT value t (in GeV)
of emissions below which no parton shower variations are considered.
Negative values mean that variations may be performed for emissions at
any pT. Default value is -1.0 GeV.
The following settings are currently under investigation and not used
in the code.
mode Variations:PDFmemberMin
(default = -1)
mode Variations:PDFmemberMax
(default = -1)
flag Variations:PDFup
(default = off)
flag Variations:PDFdown
(default = off)
Settings related to parton distribution functions and quark masses
flag ShowerPDF:usePDFmasses
(default = on)
Turned on, quark masses will be directly taken from the PDF
set interfaced through LHAPDF6.
flag ShowerPDF:usePDF
(default = on)
flag ShowerPDF:useSummedPDF
(default = on)
Turned on, this means that the PDF ratios that are used in the
evolution once an initial state parton partakes in a branching include
both sea and valence quark contributions (if applicable).
flag DireSpace:useGlobalMapIF
(default = off)
Turned on, this means that the phase space of intial state
emissions with a final state spectator is setup such that all final
state particles share the momentum recoil of the emission.
flag DireSpace:forceMassiveMap
(default = off)
Turned on, this means that initial state emissions are
allowed masses. This means the shower produces the awkward situation
that incoming quarks are massless, but yield a assive final state
quark upon conversion to an incoming gluon.
Settings related to the tune of Pythia 8 + Dire
mode Dire:Tune
(default = 1; minimum = -1)
If set to one, this enables the default tune of
Pythia8 + Dire. Currently, all other values mean that no Pythia
parameters are automatically overwritten by Dire.
Settings for debugging or educational purposes
mode DireSpace:nFinalMax
(default = -10; minimum = 0)
The spacelike showers will stop if this number of
final state particles is reached.
mode DireTimes:nFinalMax
(default = -10; minimum = 0)
The timelike showers will stop if this number of
final state particles is reached.