<chapter name="Simple Showers"> 
 
<h2>The Simple Shower</h2> 
 
<h3>Introduction</h3> 
 
Up until the end of the PYTHIA 8.2 version these simple shower codes 
were the only ones distributed as part of the PYTHIA package. 
They were originally only called <code>TimeShower</code> and 
<code>SpaceShower</code>, with no further qualification necessary. 
In version 8.240 (December 2018) a transformation was begun where 
this physics code was transferred to <code>SimpleTimeShower</code> 
and <code>SimpleSpaceShower</code>, while <code>TimeShower</code> 
and <code>SpaceShower</code> remained as bare-bones base classes. 
This paved the way for the inclusion of the VINCIA and Dire showers 
as core parts of the PYTHIA 8.3 distribution. 
 
<p/> 
The prepending of "simple" was a minimalistic choice under the 
circumstances; more fancy names could have been chosen. What it 
refers to is that showers like VINCIA and Dire aim higher, in 
striving to achieve full NLL accuracy, whereas the simple ones 
operate in an improved LL approximation. In other respects the 
simple showers can do more different physics than the other two, 
at least currently. Some examples of the broad approach are 
<ul> 
<li>Matrix elements corrections for the first ("hardest") gluon 
emission in most two-body resonance decays, effectively making the 
FSR in these decays NLO accurate.</li> 
<li>There is no corresponding NLO accuracy for ISR in any processes, 
but several examples where reasonably accurate kinematics spectra 
are available over the full phase space, by input of partial 
higher-order information.</li> 
<li>The default dipole-recoil scheme for FSR can be switched to a 
global-recoil option for the first few emissions, in order to simplify 
matching and merging to higher-order calculations (e.g. as done 
in MadGraph_aMC@NLO).</li> 
<li>The default global-recoil scheme for ISR can be replaced by a 
dipole-recoil scheme, where the other colour dipole end may be in 
the final state (as needed e.g. for DIS studies).</li> 
<li>Showers off massive objects, within and beyond the Standard 
Model, including e.g. octet onium states.</li> 
<li>Showers interleaved with multiparton interactions, and set up 
to handle <aloc href="SecondHardProcess">two predefined hard 
interactions</aloc>.</li> 
<li>QED showers, where photons can be emitted and then branch 
into fermion pairs that shower further.</li> 
<li>Weak radiation of <ei>W^+-</ei> and <ei>Z^0</ei> off fermions.</li> 
<li>Radiation also in some hadronic decays.</li> 
<li>Possibility to handle both abelian and nonabelian showers in a 
hidden valley sector, where relevant fully interleaved with normal 
QCD and QED radiation.</li> 
<li>A wide selection of further switches and parameters to vary shower 
assumptions: running of <ei>alpha_s</ei>, <ei>p_Tmin</ei> value, 
scale choices, gluon polarization effects, mass effects in 
<ei> g &rarr; q qbar</ei>, etc.</li> 
<li>Most existing tunes to LEP and LHC data are based on this choice 
of showers.</li> 
</ul> 
 
<p/> 
When the <code>Time/SpaceShower</code> became 
<code>SimpleTime/SpaceShower</code> the related settings names 
were retained, for reasons of backwards compatibility of user code, 
e.g. in command files. Thus shower setting names beginning with 
<code>TimeShower:</code>, <code>SpaceShower:</code>, 
<code>WeakShower:</code>, <code>UncertaintyBands:</code> 
or <code>HiddenValley:</code> refer to the current baseline "simple" 
showers. 
 
<h3>Shower components</h3> 
 
The <aloc href="MasterSwitches">Master Switches</aloc> for ISR and FSR 
in general, and a switch for QED radiation in 
<aloc href="ParticleDecays">Particle Decays</aloc> to leptons, 
are intended to be common for all shower programs, where applicable. 
 
<p/> 
The full description of settings in the Simple Shower framework 
is spread across several pages: 
<ul> 
<li>The final-state <aloc href="TimelikeShowers">Timelike Showers</aloc> 
cover all aspects of QCD and QED FSR.</li> 
<li>The initial-state <aloc href="SpacelikeShowers">Spacelike Showers</aloc> 
cover all aspects of QCD and QED ISR.</li> 
<li>While the main switches for weak radiation of <ei>W^+-</ei> and 
<ei>Z^0</ei> are found in the two previous FSR and ISR pages, 
there also a few common technical 
<aloc href="WeakShowers">Weak Showers</aloc> settings.</li> 
<li>There is a special framework to produce uncertainty bands from 
<aloc href="Variations">Automated Variations</aloc> of basic 
parameters, such as factorization and renormalization scales, or 
choice of parton distributions.</li> 
<li>The settings for final-state showers in a 
<aloc href="HiddenValleyProcesses">Hidden Valleys</aloc> 
are stored along with the switches for such hard processes.</li> 
<li>There is a wide selection of 
<aloc href="MatchingAndMerging">Matching and Merging</aloc> 
approaches that have been implemented so as to work well with these 
showers.</li> 
<li>Tunes that include ISR and FSR parameters are described on the 
<aloc href="Tunes">Tunes</aloc> page.</li> 
<li>The shower evolution can be interrupted or modified with the 
help of <aloc href="UserHooks">User Hooks</aloc>.</li> 
</ul> 
 
</chapter> 
 
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