## 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.