<chapter name="DIRE Parton Showers"> 
 
<h2>Dire Parton Showers</h2> 
 
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 
 
<ul> 
<li> 
QCD and QED showers with automatic uncertainties and the possiblity to 
enhance individual splittings. 
</li> 
<li> 
Inclusive next-to-leading order DGLAP corrections to the shower evolution. 
</li> 
<li> 
Next-to-leading order final-state showering for lepton colliders. 
</li> 
<li> 
Iterated Matrix-Element corrections for splittings within the parton-shower 
phase space. 
</li> 
</ul> 
 
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 <code>pythia.info.weight()</code>. 
 
<p/> 
The following discusses some basic settings that might be of interest. Some 
convenience features are discussed <aloc href="DireWeights">here</aloc>, while 
expert features (mainly aimed at comparisons of Dire to analytic calculations) 
are documented <aloc href="DireExpert">here</aloc>. 
 
<modepick name="DireTimes:kernelOrder" default="1" min="-1" max="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 value="-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> 
<option value="0"> 
Leading order evolution kernels, as defined in the 
original Dire publication. 
</option> 
<option value="1"> 
Leading order evolution kernels, and additional 
rescaling of the soft pieces to incorporate two-loop cusp efects 
(similar to the CMW scheme). 
</option> 
<option value="2"> 
Leading order evolution kernels, and additional 
rescaling of the soft pieces to incorporate two- and three-loop 
cusp efects. 
</option> 
<option value="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. 
</option> 
</modepick> 
 
<modepick name="DireSpace:kernelOrder" default="1" min="-1" max="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 value="-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> 
<option value="0"> 
Leading order evolution kernels, as defined in the 
original Dire publication. 
</option> 
<option value="1"> 
Leading order evolution kernels, and additional 
rescaling of the soft pieces to incorporate two-loop cusp efects 
(similar to the CMW scheme). 
</option> 
<option value="2"> 
Leading order evolution kernels, and additional 
rescaling of the soft pieces to incorporate two- and three-loop 
cusp efects. 
</option> 
<option value="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. 
</option> 
</modepick> 
 
<modeopen name="DireTimes:kernelOrderMPI" default="1" min="-1" max="4"> 
Same as 
<b>DireTimes:kernelOrder = n</b>, but for showers from secondary 
scatterings or hadron decays. 
</modeopen> 
 
<modeopen name="DireSpace:kernelOrderMPI" default="1" min="-1" max="4"> 
Same as 
<b>DireSpace:kernelOrder = n</b>, but for showers from secondary 
scatterings or hadron decays. 
</modeopen> 
 
<parm name="DireTimes:pTrecombine" default="2.0"> 
The minimal pT value <i>t</i> (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 <i>pT&lt;t</i>, the probabilities for <i>q->qg</i> 
and <i>q->gq</i> are combined into a single probability, as are 
<i>g->qqbar</i> and <i>g->qbarq</i> and the two color structures for 
<i>g->gg</i>. 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. 
</parm> 
 
<h3>Settings related to the evaluation of running (QCD) couplings</h3> 
 
<flag name="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. 
</flag> 
 
<h3>Settings to perform variations to gauge shower uncertainties</h3> 
 
<flag name="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. 
</flag> 
 
<parm name="Variations:muRisrDown" default="1.0"> 
The (double) value with which the 
(GeV<sup>2</sup>-valued) argument of &alpha;<sub>s</sub> in 
initial-state evolution should be rescaled to produce a smaller value 
of the renormalization scale in the context of automatic variations. 
</parm> 
 
<parm name="Variations:muRisrUp" default="1.0"> 
The (double) value with which the 
(GeV<sup>2</sup>-valued) argument of &alpha;<sub>s</sub> in 
initial-state evolution should be rescaled to produce a larger value 
of the renormalization scale in the context of automatic variations. 
</parm> 
 
<parm name="Variations:muRfsrDown" default="1.0"> 
The (double) value with which the 
(GeV<sup>2</sup>-valued) argument of &alpha;<sub>s</sub> in 
final-state evolution should be rescaled to produce a smaller value 
of the renormalization scale in the context of automatic variations. 
</parm> 
 
<parm name="Variations:muRfsrUp" default="1.0"> 
The (double) value with which the 
(GeV<sup>2</sup>-valued) argument of &alpha;<sub>s</sub> in 
final-state evolution should be rescaled to produce a larger value 
of the renormalization scale in the context of automatic variations. 
</parm> 
 
<parm name="Variations:pTmin" default="-1.0"> 
The minimal pT value <i>t</i> (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. 
</parm> 
 
<p> 
The following settings are currently under investigation and not used 
in the code. 
</p> 
 
<modeopen name="Variations:PDFmemberMin" default="-1"> 
</modeopen> 
<modeopen name="Variations:PDFmemberMax" default="-1"> 
</modeopen> 
<flag name="Variations:PDFup" default="off"> 
</flag> 
<flag name="Variations:PDFdown" default="off"> 
</flag> 
 
 
<h3>Settings related to parton distribution functions and quark masses</h3> 
 
<flag name="ShowerPDF:usePDFmasses" default="on"> 
Turned on, quark masses will be directly taken from the PDF 
set interfaced through LHAPDF6. 
</flag> 
 
<flag name="ShowerPDF:usePDF" default="on"> 
</flag> 
 
<flag name="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> 
 
<flag name="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> 
 
<flag name="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. 
</flag> 
 
<h3>Settings related to the tune of Pythia 8 + Dire</h3> 
 
<modeopen name="Dire:Tune" default="1" min="-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. 
</modeopen> 
 
<h3>Settings for debugging or educational purposes</h3> 
 
<modeopen name="DireSpace:nFinalMax" default="-10" min="0"> 
The spacelike showers will stop if this number of 
final state particles is reached. 
</modeopen> 
 
<modeopen name="DireTimes:nFinalMax" default="-10" min="0"> 
The timelike showers will stop if this number of 
final state particles is reached. 
</modeopen> 
 
</chapter> 
 
<!-- Copyright (C) 2020 Stefan Prestel --> 
