<chapter name="Parton Showers"> 
 
<h2>Parton Showers</h2> 
 
<h3>Shower Model Selection</h3> 
 
As of version 8.301, PYTHIA integrates different complete 
parton-shower frameworks: the original "simple" showers, the VINCIA 
antenna showers and the Dire dipole showers. Thereby comparisons between 
different shower approaches become easier. 
Starting with version 8.316, the Dire dipole shower has been retired 
and will no longer be actively supported. 
 
<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 code originally was structured to make use of this 
functionality, but was developed and distributed as separate code. 
Starting with version 8.300, VINCIA is now fully integrated into 
the PYTHIA distribution, and appears 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="2"> 
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. An option for EW showers is also 
available, based on collinear splitting kernels. A few longstanding 
hallmarks of VINCIA, such as automated uncertainty variations and 
(iterated) matrix-element corrections, are not yet available in this 
version. 
</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>While both 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. VINCIA therefore comes 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, tree-level 2&rarr;4 branchings and NLO-corrected 2&rarr; 
3 ones, automated uncertainty variations, and the ROOT-based 
<code>LiveDisplays</code> 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. 
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>. 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> 
