Simple Showers
Overview of recent changes
PYTHIA comes with a complete parton-shower machinery, but also
allows external shower programs to be linked in to it, see the
Implement New Showers
page. Notably the VINCIA and DIRE codes have been structured
to make use of this functionality. Currently these codes are
distributed separately, but the intention is to integrate them
into the PYTHIA distribution in the future.
Originally the TimeShower
and SpaceShower
classes implemented the default PYTHIA showers, but also acted as
base classes from which the external showers derived. This has some
disadvantages, so the two aspects are now split. The
TimeShower
and SpaceShower
classes remain
as simple base classes from which the actual showers are derived.
The physics code has been moved to the new derived
SimpleTimeShower
and SimpleSpaceShower
.
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 of course derive from the new
classes and then reuse relevant code in them.
Settings names have been retained, again for reasons of backwards
compatibility of user code, e.g. in command files. Thus setting names
beginning with TimeShower:
, SpaceShower:
,
WeakShower:
or UncertaintyBands:
refer
uniquely to the current baseline "simple" ones. In the future some
of them may become common with VINCIA and DIRE, notably the
uncertainty bands ones, whereas ones specific to those two programs
will have names that spell it out.
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
- 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
higher-order information.
- 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.
- 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.
- 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
interactions.
- 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.
Shower components
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.
- 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
Hidden Valleys
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
showers.
- Tunes that include ISR and FSR parameters are described on the
Tunes page.
- The shower evolution can be interrupted or modified with the
help of User Hooks.