The Simple Shower
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
SpaceShower, with no further qualification necessary.
In version 8.240 (December 2018) a transformation was begun where
this physics code was transferred to
SpaceShower 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.
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 better coherence and higher-logarithmic 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
- Matrix elements corrections for the first ("hardest") gluon
emission in most two-body resonance decays, effectively making the
FSR in these decays NLO accurate.
- 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
- 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
- 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).
- Showers off massive objects, within and beyond the Standard
Model, including e.g. octet onium states.
- Showers interleaved with multiparton interactions, and set up
to handle two predefined hard
- QED showers, where photons can be emitted and then branch
into fermion pairs that shower further.
- Weak radiation of W^+- and Z^0 off fermions.
- Radiation also in some hadronic decays.
- Possibility to handle both abelian and nonabelian showers in a
hidden valley sector, where relevant fully interleaved with normal
QCD and QED radiation.
- A wide selection of further switches and parameters to vary shower
assumptions: running of alpha_s, p_Tmin value,
scale choices, gluon polarization effects, mass effects in
g → q qbar, etc.
- Most existing tunes to LEP and LHC data are based on this choice
SimpleTime/SpaceShower 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
HiddenValley: refer to the current baseline "simple"
The Master Switches for ISR and FSR
in general, and a switch for QED radiation in
Particle Decays to leptons,
are intended to be common for all shower programs, where applicable.
The full description of settings in the Simple Shower framework
is spread across several pages:
- The final-state Timelike Showers
cover all aspects of QCD and QED FSR, including also options for
interleaved resonance decays.
- The initial-state Spacelike Showers
cover all aspects of QCD and QED ISR.
- While the main switches for weak radiation of W^+- and
Z^0 are found in the two previous FSR and ISR pages,
there also a few common technical
Weak Showers settings.
- There is a special framework to produce uncertainty bands from
Automated Variations of basic
parameters, such as factorization and renormalization scales, or
choice of parton distributions.
- The settings for final-state showers in a
are stored along with the switches for such hard processes.
- There is a wide selection of
Matching and Merging
approaches that have been implemented so as to work well with these
- Tunes that include ISR and FSR parameters are described on the
- The shower evolution can be interrupted or modified with the
help of User Hooks.