<chapter name="Parton Showers"> 
 
<h2>Parton Showers</h2> 
 
<h3>Shower Model Selection</h3> 
 
As of version 8.3, PYTHIA comes with three different complete 
parton-shower frameworks: the original "simple" showers, the VINCIA 
antenna showers and the Dire dipole showers. Thereby comparisons between 
these different shower approaches become easier. A warning, however, 
is that the integration of VINCIA and Dire is still ongoing, so one 
may expect some adjustments to occur in the next few releases, 
over and above the level of normal evolution. 
 
<p/> 
Since early days PYTHIA 8 has allowed external shower programs 
to be linked, thereby replacing the internal simple ones, see the 
<aloc href="ImplementNewShowers">Implement New Showers</aloc> page. 
The VINCIA and Dire codes originally were structured to make use of this 
functionality, but were developed and distributed as separate codes. 
Starting with version 8.300, these two programs now are fully 
integrated into the PYTHIA distributions, and appear on equal footing 
with the old simple showers. Which of the internal shower frameworks 
that will be used in a run is determined by the following switch. 
 
<modepick name="PartonShowers:model" default="1" min="1" max="3"> 
Choice of which shower machinery that will be used in PYTHIA (when not 
linking an external shower). 
<option value="1"><aloc href="SimpleShowers">Simple Showers</aloc>. 
This is the "old" shower framework that has its roots in PYTHIA 6 and 
has been distributed with PYTHIA 8 since the beginning. It is a less 
ambitious project than the other two, but for that reason also more 
mature and stable, which is a reason why it for now remains as default. 
It also has some special features that the other two don't. 
</option> 
<option value="2"><aloc href="Vincia">VINCIA 
Showers</aloc>. Based on sequences of pT-ordered 2&rarr;3 
branchings, the VINCIA shower model is similar to that of 
ARIADNE, which it resembles strongly for final-state evolution while 
VINCIA 
adopts a different (backwards-evolution) picture for initial-state 
radiation. The branching kernels, known as antenna 
functions, treat coherent sums of parton pairs without requiring a 
separation into "radiators" and "spectators". The current PYTHIA 
implementation includes QCD and QED 2&rarr;3 branchings with full 
mass dependence and, for the latter, multipole interference effects. A 
few longstanding hallmarks of 
VINCIA, such as 
automated uncertainty variations and (iterated) matrix-element 
corrections, are not yet available in this version. 
</option> 
<option value="3"><aloc href="DireShowers">Dire Showers</aloc>. 
Dire (short for Dipole resummation) implements a transverse-momentum ordered 
dipole shower in which radiator-spectator particle pairs evolve 
simultaneously. The emission phase space is fully symmetric between radiator 
and spectator, while the overall emission probability is separated into two 
pieces corresponding that are enhanced (suppressed) in region collinear 
(anti-collinear) to the radiator or the spectator, respectively. 
Dire includes QCD and QED emissions, a detailed treatment 
of (quark/lepton) mass effects, and is set up to include higher-order 
corrections, such as triple-collinear or double-soft parton emissions. 
</option> 
<br/>Further webpages, as linked above (and in the Parton Showers 
section of the left-column index), provide more detailed information 
on the respective framework. 
</modepick> 
 
<p/> 
There are some differences between the showers to be aware of 
<ul> 
<li>The Dire shower comes with a nontrivial variable weight. It is 
therefore important that results for each event are weighted by the 
event weight in <code>pythia.info.weight()</code>, e.g. when filled 
in a histogram. The simple and VINCIA showers by default come with 
unit weight, so do not have that issue. (Non-unit weights can still 
come from the hard-process cross section in some cases, so it is 
prudent always to apply the event weight.)</li> 
<li>While all three shower models do ordinary QCD and QED radiation, 
beyond that the capabilities vary.</li> 
<li> Tuned parameter values for PYTHIA's modeling of 
nonperturbative physics are normally only valid for 
the specific shower model they were tuned with, 
due to differences in perturbative regularisations between the 
three shower models and a nontrivial 
interplay between perturbative and nonperturbative physics modeling 
in general. Both VINCIA 
and Dire therefore come with their own dedicated default tune 
parameters which are activated in different ways, see the respective 
documentation pages. 
</li> 
<li>As mentioned above, some of the hallmark features of VINCIA 
during its standalone development have not yet been carried over to 
the PYTHIA implementation. This includes (iterated) matrix-element 
corrections, automated uncertainty variations, tree-level 2&rarr;4 
branchings and NLO corrected 2&rarr; 3 ones, sector showers, 
facilities for matching and merging, and the 
VinciaRoot tool for live histogram displays. 
This is work in progress 
and we expect most of these features to reappear in future 
releases of PYTHIA 8.3. 
</li> 
</ul> 
 
<h3>Some details</h3> 
 
Originally the <code>TimeShower</code> and <code>SpaceShower</code> 
classes implemented the simple shower, but also acted as base classes 
from which the external showers derived. This had some disadvantages 
for the purpose of the tighter integration of VINCIA and Dire, so the 
two aspects are now split. The <code>TimeShower</code> and 
<code>SpaceShower</code> classes remain as bare-bones base classes, 
from which the actual showers are derived. The code for the simple 
showers has been moved to the new derived <code>SimpleTimeShower</code> 
and <code>SimpleSpaceShower</code>. An external shower that does not 
use any of the existing shower algorithms will therefore work as before, 
which would be the normal case, but alternatively a shower could derive 
from the new classes and then reuse relevant code in them. 
 
<p/> 
Settings names for the simple shower have been retained through this 
transformation, again for reasons of backwards compatibility of user 
code, e.g. in command files. Thus setting names beginning with 
<code>TimeShower:</code>, <code>SpaceShower:</code>, 
<code>WeakShower:</code> or <code>UncertaintyBands:</code> refer 
to the current baseline "simple" showers, and so do the 
<code>HiddenValley:</code> shower-related settings. 
As a rule, the corresponding VINCIA settings begin with 
<code>Vincia:</code> and the Dire ones with <code>Dire:</code>, 
<code>DireTimes:</code>, <code>DireSpace:</code> or 
<code>Variations:</code>. A few of the simple shower settings also 
apply to the other two, as documented on the respective page. 
It is possible that future evolution will standardize some settings, 
while others will be more clearly separated. 
 
</chapter> 
